GHC/Hs/Expr.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
    -}

    {-# LANGUAGE CPP, DeriveDataTypeable, ScopedTypeVariables #-}
    {-# LANGUAGE StandaloneDeriving #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE UndecidableInstances #-} -- Note [Pass sensitive types]
                                          -- in module GHC.Hs.PlaceHolder
    {-# LANGUAGE ConstraintKinds #-}
    {-# LANGUAGE ExistentialQuantification #-}
    {-# LANGUAGE DeriveFunctor #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE ViewPatterns #-}

    -- | Abstract Haskell syntax for expressions.
    module GHC.Hs.Expr where

    #include "HsVersions.h"

    -- friends:
    import GhcPrelude

    import GHC.Hs.Decls
    import GHC.Hs.Pat
    import GHC.Hs.Lit
    import GHC.Hs.PlaceHolder ( NameOrRdrName )
    import GHC.Hs.Extension
    import GHC.Hs.Types
    import GHC.Hs.Binds

    -- others:
    import TcEvidence
    import CoreSyn
    import DynFlags ( gopt, GeneralFlag(Opt_PrintExplicitCoercions) )
    import Name
    import NameSet
    import BasicTypes
    import ConLike
    import SrcLoc
    import Util
    import Outputable
    import FastString
    import Type
    import TysWiredIn (mkTupleStr)
    import TcType (TcType)
    import {-# SOURCE #-} TcRnTypes (TcLclEnv)

    -- libraries:
    import Data.Data hiding (Fixity(..))
    import qualified Data.Data as Data (Fixity(..))
    import Data.Maybe (isNothing)

    import GHCi.RemoteTypes ( ForeignRef )
    import qualified Language.Haskell.TH as TH (Q)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Expressions proper}
    *                                                                      *
    ************************************************************************
    -}

    -- * Expressions proper

    -- | Located Haskell Expression
    type LHsExpr p = Located (HsExpr p)
      -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnComma' when
      --   in a list

      -- For details on above see note [Api annotations] in ApiAnnotation

    -------------------------
    -- | Post-Type checking Expression
    --
    -- PostTcExpr is an evidence expression attached to the syntax tree by the
    -- type checker (c.f. postTcType).
    type PostTcExpr  = HsExpr GhcTc

    -- | Post-Type checking Table
    --
    -- We use a PostTcTable where there are a bunch of pieces of evidence, more
    -- than is convenient to keep individually.
    type PostTcTable = [(Name, PostTcExpr)]

    -------------------------
    -- | Syntax Expression
    --
    -- SyntaxExpr is like 'PostTcExpr', but it's filled in a little earlier,
    -- by the renamer.  It's used for rebindable syntax.
    --
    -- E.g. @(>>=)@ is filled in before the renamer by the appropriate 'Name' for
    --      @(>>=)@, and then instantiated by the type checker with its type args
    --      etc
    --
    -- This should desugar to
    --
    -- > syn_res_wrap $ syn_expr (syn_arg_wraps[0] arg0)
    -- >                         (syn_arg_wraps[1] arg1) ...
    --
    -- where the actual arguments come from elsewhere in the AST.
    -- This could be defined using @GhcPass p@ and such, but it's
    -- harder to get it all to work out that way. ('noSyntaxExpr' is hard to
    -- write, for example.)
    data SyntaxExpr p = SyntaxExpr { syn_expr      :: HsExpr p
                                   , syn_arg_wraps :: [HsWrapper]
                                   , syn_res_wrap  :: HsWrapper }

    -- | This is used for rebindable-syntax pieces that are too polymorphic
    -- for tcSyntaxOp (trS_fmap and the mzip in ParStmt)
    noExpr :: HsExpr (GhcPass p)
    noExpr = HsLit noExtField (HsString (SourceText  "noExpr") (fsLit "noExpr"))

    noSyntaxExpr :: SyntaxExpr (GhcPass p)
                                  -- Before renaming, and sometimes after,
                                  -- (if the syntax slot makes no sense)
    noSyntaxExpr = SyntaxExpr { syn_expr      = HsLit noExtField
                                                      (HsString NoSourceText
                                                      (fsLit "noSyntaxExpr"))
                              , syn_arg_wraps = []
                              , syn_res_wrap  = WpHole }

    -- | Make a 'SyntaxExpr (HsExpr _)', missing its HsWrappers.
    mkSyntaxExpr :: HsExpr (GhcPass p) -> SyntaxExpr (GhcPass p)
    mkSyntaxExpr expr = SyntaxExpr { syn_expr      = expr
                                   , syn_arg_wraps = []
                                   , syn_res_wrap  = WpHole }

    -- | Make a 'SyntaxExpr Name' (the "rn" is because this is used in the
    -- renamer), missing its HsWrappers.
    mkRnSyntaxExpr :: Name -> SyntaxExpr GhcRn
    mkRnSyntaxExpr name = mkSyntaxExpr $ HsVar noExtField $ noLoc name
      -- don't care about filling in syn_arg_wraps because we're clearly
      -- not past the typechecker

    instance OutputableBndrId p
           => Outputable (SyntaxExpr (GhcPass p)) where
      ppr (SyntaxExpr { syn_expr      = expr
                      , syn_arg_wraps = arg_wraps
                      , syn_res_wrap  = res_wrap })
        = sdocWithDynFlags $ \ dflags ->
          getPprStyle $ \s ->
          if debugStyle s || gopt Opt_PrintExplicitCoercions dflags
          then ppr expr <> braces (pprWithCommas ppr arg_wraps)
                        <> braces (ppr res_wrap)
          else ppr expr

    -- | Command Syntax Table (for Arrow syntax)
    type CmdSyntaxTable p = [(Name, HsExpr p)]
    -- See Note [CmdSyntaxTable]

    {-
    Note [CmdSyntaxtable]
    ~~~~~~~~~~~~~~~~~~~~~
    Used only for arrow-syntax stuff (HsCmdTop), the CmdSyntaxTable keeps
    track of the methods needed for a Cmd.

    * Before the renamer, this list is an empty list

    * After the renamer, it takes the form @[(std_name, HsVar actual_name)]@
      For example, for the 'arr' method
       * normal case:            (GHC.Control.Arrow.arr, HsVar GHC.Control.Arrow.arr)
       * with rebindable syntax: (GHC.Control.Arrow.arr, arr_22)
                 where @arr_22@ is whatever 'arr' is in scope

    * After the type checker, it takes the form [(std_name, <expression>)]
      where <expression> is the evidence for the method.  This evidence is
      instantiated with the class, but is still polymorphic in everything
      else.  For example, in the case of 'arr', the evidence has type
             forall b c. (b->c) -> a b c
      where 'a' is the ambient type of the arrow.  This polymorphism is
      important because the desugarer uses the same evidence at multiple
      different types.

    This is Less Cool than what we normally do for rebindable syntax, which is to
    make fully-instantiated piece of evidence at every use site.  The Cmd way
    is Less Cool because
      * The renamer has to predict which methods are needed.
        See the tedious RnExpr.methodNamesCmd.

      * The desugarer has to know the polymorphic type of the instantiated
        method. This is checked by Inst.tcSyntaxName, but is less flexible
        than the rest of rebindable syntax, where the type is less
        pre-ordained.  (And this flexibility is useful; for example we can
        typecheck do-notation with (>>=) :: m1 a -> (a -> m2 b) -> m2 b.)
    -}

    -- | A Haskell expression.
    data HsExpr p
      = HsVar     (XVar p)
                  (Located (IdP p)) -- ^ Variable

                                 -- See Note [Located RdrNames]

      | HsUnboundVar (XUnboundVar p)
                     OccName     -- ^ Unbound variable; also used for "holes"
                                 --   (_ or _x).
                                 -- Turned from HsVar to HsUnboundVar by the
                                 --   renamer, when it finds an out-of-scope
                                 --   variable or hole.
                                 -- Turned into HsVar by type checker, to support
                                 --   deferred type errors.

      | HsConLikeOut (XConLikeOut p)
                     ConLike     -- ^ After typechecker only; must be different
                                 -- HsVar for pretty printing

      | HsRecFld  (XRecFld p)
                  (AmbiguousFieldOcc p) -- ^ Variable pointing to record selector
                                        -- Not in use after typechecking

      | HsOverLabel (XOverLabel p)
                    (Maybe (IdP p)) FastString
         -- ^ Overloaded label (Note [Overloaded labels] in GHC.OverloadedLabels)
         --   @Just id@ means @RebindableSyntax@ is in use, and gives the id of the
         --   in-scope 'fromLabel'.
         --   NB: Not in use after typechecking

      | HsIPVar   (XIPVar p)
                  HsIPName   -- ^ Implicit parameter (not in use after typechecking)
      | HsOverLit (XOverLitE p)
                  (HsOverLit p)  -- ^ Overloaded literals

      | HsLit     (XLitE p)
                  (HsLit p)      -- ^ Simple (non-overloaded) literals

      | HsLam     (XLam p)
                  (MatchGroup p (LHsExpr p))
                           -- ^ Lambda abstraction. Currently always a single match
           --
           -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
           --       'ApiAnnotation.AnnRarrow',

           -- For details on above see note [Api annotations] in ApiAnnotation

      | HsLamCase (XLamCase p) (MatchGroup p (LHsExpr p)) -- ^ Lambda-case
           --
           -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
           --           'ApiAnnotation.AnnCase','ApiAnnotation.AnnOpen',
           --           'ApiAnnotation.AnnClose'

           -- For details on above see note [Api annotations] in ApiAnnotation

      | HsApp     (XApp p) (LHsExpr p) (LHsExpr p) -- ^ Application

      | HsAppType (XAppTypeE p) (LHsExpr p) (LHsWcType (NoGhcTc p))  -- ^ Visible type application
           --
           -- Explicit type argument; e.g  f @Int x y
           -- NB: Has wildcards, but no implicit quantification
           --
           -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnAt',

      -- | Operator applications:
      -- NB Bracketed ops such as (+) come out as Vars.

      -- NB We need an expr for the operator in an OpApp/Section since
      -- the typechecker may need to apply the operator to a few types.

      | OpApp       (XOpApp p)
                    (LHsExpr p)       -- left operand
                    (LHsExpr p)       -- operator
                    (LHsExpr p)       -- right operand

      -- | Negation operator. Contains the negated expression and the name
      -- of 'negate'
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnMinus'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | NegApp      (XNegApp p)
                    (LHsExpr p)
                    (SyntaxExpr p)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'('@,
      --             'ApiAnnotation.AnnClose' @')'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsPar       (XPar p)
                    (LHsExpr p)  -- ^ Parenthesised expr; see Note [Parens in HsSyn]

      | SectionL    (XSectionL p)
                    (LHsExpr p)    -- operand; see Note [Sections in HsSyn]
                    (LHsExpr p)    -- operator
      | SectionR    (XSectionR p)
                    (LHsExpr p)    -- operator; see Note [Sections in HsSyn]
                    (LHsExpr p)    -- operand

      -- | Used for explicit tuples and sections thereof
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --         'ApiAnnotation.AnnClose'

      -- For details on above see note [Api annotations] in ApiAnnotation
      -- Note [ExplicitTuple]
      | ExplicitTuple
            (XExplicitTuple p)
            [LHsTupArg p]
            Boxity

      -- | Used for unboxed sum types
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'(#'@,
      --          'ApiAnnotation.AnnVbar', 'ApiAnnotation.AnnClose' @'#)'@,
      --
      --  There will be multiple 'ApiAnnotation.AnnVbar', (1 - alternative) before
      --  the expression, (arity - alternative) after it
      | ExplicitSum
              (XExplicitSum p)
              ConTag --  Alternative (one-based)
              Arity  --  Sum arity
              (LHsExpr p)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnCase',
      --       'ApiAnnotation.AnnOf','ApiAnnotation.AnnOpen' @'{'@,
      --       'ApiAnnotation.AnnClose' @'}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsCase      (XCase p)
                    (LHsExpr p)
                    (MatchGroup p (LHsExpr p))

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnIf',
      --       'ApiAnnotation.AnnSemi',
      --       'ApiAnnotation.AnnThen','ApiAnnotation.AnnSemi',
      --       'ApiAnnotation.AnnElse',

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsIf        (XIf p)
                    (Maybe (SyntaxExpr p)) -- cond function
                                            -- Nothing => use the built-in 'if'
                                            -- See Note [Rebindable if]
                    (LHsExpr p)    --  predicate
                    (LHsExpr p)    --  then part
                    (LHsExpr p)    --  else part

      -- | Multi-way if
      --
      -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnIf'
      --       'ApiAnnotation.AnnOpen','ApiAnnotation.AnnClose',

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsMultiIf   (XMultiIf p) [LGRHS p (LHsExpr p)]

      -- | let(rec)
      --
      -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLet',
      --       'ApiAnnotation.AnnOpen' @'{'@,
      --       'ApiAnnotation.AnnClose' @'}'@,'ApiAnnotation.AnnIn'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsLet       (XLet p)
                    (LHsLocalBinds p)
                    (LHsExpr  p)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDo',
      --             'ApiAnnotation.AnnOpen', 'ApiAnnotation.AnnSemi',
      --             'ApiAnnotation.AnnVbar',
      --             'ApiAnnotation.AnnClose'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsDo        (XDo p)                  -- Type of the whole expression
                    (HsStmtContext Name)     -- The parameterisation is unimportant
                                             -- because in this context we never use
                                             -- the PatGuard or ParStmt variant
                    (Located [ExprLStmt p]) -- "do":one or more stmts

      -- | Syntactic list: [a,b,c,...]
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'['@,
      --              'ApiAnnotation.AnnClose' @']'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      -- See Note [Empty lists]
      | ExplicitList
                    (XExplicitList p)  -- Gives type of components of list
                    (Maybe (SyntaxExpr p))
                                       -- For OverloadedLists, the fromListN witness
                    [LHsExpr p]

      -- | Record construction
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{'@,
      --         'ApiAnnotation.AnnDotdot','ApiAnnotation.AnnClose' @'}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | RecordCon
          { rcon_ext      :: XRecordCon p
          , rcon_con_name :: Located (IdP p)    -- The constructor name;
                                                --  not used after type checking
          , rcon_flds     :: HsRecordBinds p }  -- The fields

      -- | Record update
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{'@,
      --         'ApiAnnotation.AnnDotdot','ApiAnnotation.AnnClose' @'}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | RecordUpd
          { rupd_ext  :: XRecordUpd p
          , rupd_expr :: LHsExpr p
          , rupd_flds :: [LHsRecUpdField p]
          }
      -- For a type family, the arg types are of the *instance* tycon,
      -- not the family tycon

      -- | Expression with an explicit type signature. @e :: type@
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDcolon'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ExprWithTySig
                    (XExprWithTySig p)

                    (LHsExpr p)
                    (LHsSigWcType (NoGhcTc p))

      -- | Arithmetic sequence
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'['@,
      --              'ApiAnnotation.AnnComma','ApiAnnotation.AnnDotdot',
      --              'ApiAnnotation.AnnClose' @']'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | ArithSeq
                    (XArithSeq p)
                    (Maybe (SyntaxExpr p))
                                      -- For OverloadedLists, the fromList witness
                    (ArithSeqInfo p)

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsSCC       (XSCC p)
                    SourceText            -- Note [Pragma source text] in BasicTypes
                    StringLiteral         -- "set cost centre" SCC pragma
                    (LHsExpr p)           -- expr whose cost is to be measured

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{-\# CORE'@,
      --             'ApiAnnotation.AnnVal', 'ApiAnnotation.AnnClose' @'\#-}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsCoreAnn   (XCoreAnn p)
                    SourceText            -- Note [Pragma source text] in BasicTypes
                    StringLiteral         -- hdaume: core annotation
                    (LHsExpr p)

      -----------------------------------------------------------
      -- MetaHaskell Extensions

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --         'ApiAnnotation.AnnOpenE','ApiAnnotation.AnnOpenEQ',
      --         'ApiAnnotation.AnnClose','ApiAnnotation.AnnCloseQ'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsBracket    (XBracket p) (HsBracket p)

        -- See Note [Pending Splices]
      | HsRnBracketOut
          (XRnBracketOut p)
          (HsBracket GhcRn)    -- Output of the renamer is the *original* renamed
                               -- expression, plus
          [PendingRnSplice]    -- _renamed_ splices to be type checked

      | HsTcBracketOut
          (XTcBracketOut p)
          (HsBracket GhcRn)    -- Output of the type checker is the *original*
                               -- renamed expression, plus
          [PendingTcSplice]    -- _typechecked_ splices to be
                               -- pasted back in by the desugarer

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --         'ApiAnnotation.AnnClose'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsSpliceE  (XSpliceE p) (HsSplice p)

      -----------------------------------------------------------
      -- Arrow notation extension

      -- | @proc@ notation for Arrows
      --
      --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnProc',
      --          'ApiAnnotation.AnnRarrow'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsProc      (XProc p)
                    (LPat p)               -- arrow abstraction, proc
                    (LHsCmdTop p)          -- body of the abstraction
                                           -- always has an empty stack

      ---------------------------------------
      -- static pointers extension
      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnStatic',

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsStatic (XStatic p) -- Free variables of the body
                 (LHsExpr p)        -- Body

      ---------------------------------------
      -- Haskell program coverage (Hpc) Support

      | HsTick
         (XTick p)
         (Tickish (IdP p))
         (LHsExpr p)                       -- sub-expression

      | HsBinTick
         (XBinTick p)
         Int                                -- module-local tick number for True
         Int                                -- module-local tick number for False
         (LHsExpr p)                        -- sub-expression

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
      --       'ApiAnnotation.AnnOpen' @'{-\# GENERATED'@,
      --       'ApiAnnotation.AnnVal','ApiAnnotation.AnnVal',
      --       'ApiAnnotation.AnnColon','ApiAnnotation.AnnVal',
      --       'ApiAnnotation.AnnMinus',
      --       'ApiAnnotation.AnnVal','ApiAnnotation.AnnColon',
      --       'ApiAnnotation.AnnVal',
      --       'ApiAnnotation.AnnClose' @'\#-}'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsTickPragma                      -- A pragma introduced tick
         (XTickPragma p)
         SourceText                       -- Note [Pragma source text] in BasicTypes
         (StringLiteral,(Int,Int),(Int,Int))
                                          -- external span for this tick
         ((SourceText,SourceText),(SourceText,SourceText))
            -- Source text for the four integers used in the span.
            -- See note [Pragma source text] in BasicTypes
         (LHsExpr p)

      ---------------------------------------
      -- Finally, HsWrap appears only in typechecker output
      -- The contained Expr is *NOT* itself an HsWrap.
      -- See Note [Detecting forced eta expansion] in DsExpr. This invariant
      -- is maintained by GHC.Hs.Utils.mkHsWrap.

      |  HsWrap     (XWrap p)
                    HsWrapper    -- TRANSLATION
                    (HsExpr p)

      | XExpr       (XXExpr p) -- Note [Trees that Grow] extension constructor

    -- | Extra data fields for a 'RecordCon', added by the type checker
    data RecordConTc = RecordConTc
          { rcon_con_like :: ConLike      -- The data constructor or pattern synonym
          , rcon_con_expr :: PostTcExpr   -- Instantiated constructor function
          }

    -- | Extra data fields for a 'RecordUpd', added by the type checker
    data RecordUpdTc = RecordUpdTc
          { rupd_cons :: [ConLike]
                    -- Filled in by the type checker to the
                    -- _non-empty_ list of DataCons that have
                    -- all the upd'd fields

          , rupd_in_tys  :: [Type] -- Argument types of *input* record type
          , rupd_out_tys :: [Type] --             and  *output* record type
                                   -- The original type can be reconstructed
                                   -- with conLikeResTy
          , rupd_wrap :: HsWrapper -- See note [Record Update HsWrapper]
          } deriving Data

    -- ---------------------------------------------------------------------

    type instance XVar           (GhcPass _) = NoExtField
    type instance XUnboundVar    (GhcPass _) = NoExtField
    type instance XConLikeOut    (GhcPass _) = NoExtField
    type instance XRecFld        (GhcPass _) = NoExtField
    type instance XOverLabel     (GhcPass _) = NoExtField
    type instance XIPVar         (GhcPass _) = NoExtField
    type instance XOverLitE      (GhcPass _) = NoExtField
    type instance XLitE          (GhcPass _) = NoExtField
    type instance XLam           (GhcPass _) = NoExtField
    type instance XLamCase       (GhcPass _) = NoExtField
    type instance XApp           (GhcPass _) = NoExtField

    type instance XAppTypeE      (GhcPass _) = NoExtField

    type instance XOpApp         GhcPs = NoExtField
    type instance XOpApp         GhcRn = Fixity
    type instance XOpApp         GhcTc = Fixity

    type instance XNegApp        (GhcPass _) = NoExtField
    type instance XPar           (GhcPass _) = NoExtField
    type instance XSectionL      (GhcPass _) = NoExtField
    type instance XSectionR      (GhcPass _) = NoExtField
    type instance XExplicitTuple (GhcPass _) = NoExtField

    type instance XExplicitSum   GhcPs = NoExtField
    type instance XExplicitSum   GhcRn = NoExtField
    type instance XExplicitSum   GhcTc = [Type]

    type instance XCase          (GhcPass _) = NoExtField
    type instance XIf            (GhcPass _) = NoExtField

    type instance XMultiIf       GhcPs = NoExtField
    type instance XMultiIf       GhcRn = NoExtField
    type instance XMultiIf       GhcTc = Type

    type instance XLet           (GhcPass _) = NoExtField

    type instance XDo            GhcPs = NoExtField
    type instance XDo            GhcRn = NoExtField
    type instance XDo            GhcTc = Type

    type instance XExplicitList  GhcPs = NoExtField
    type instance XExplicitList  GhcRn = NoExtField
    type instance XExplicitList  GhcTc = Type

    type instance XRecordCon     GhcPs = NoExtField
    type instance XRecordCon     GhcRn = NoExtField
    type instance XRecordCon     GhcTc = RecordConTc

    type instance XRecordUpd     GhcPs = NoExtField
    type instance XRecordUpd     GhcRn = NoExtField
    type instance XRecordUpd     GhcTc = RecordUpdTc

    type instance XExprWithTySig (GhcPass _) = NoExtField

    type instance XArithSeq      GhcPs = NoExtField
    type instance XArithSeq      GhcRn = NoExtField
    type instance XArithSeq      GhcTc = PostTcExpr

    type instance XSCC           (GhcPass _) = NoExtField
    type instance XCoreAnn       (GhcPass _) = NoExtField
    type instance XBracket       (GhcPass _) = NoExtField

    type instance XRnBracketOut  (GhcPass _) = NoExtField
    type instance XTcBracketOut  (GhcPass _) = NoExtField

    type instance XSpliceE       (GhcPass _) = NoExtField
    type instance XProc          (GhcPass _) = NoExtField

    type instance XStatic        GhcPs = NoExtField
    type instance XStatic        GhcRn = NameSet
    type instance XStatic        GhcTc = NameSet

    type instance XTick          (GhcPass _) = NoExtField
    type instance XBinTick       (GhcPass _) = NoExtField
    type instance XTickPragma    (GhcPass _) = NoExtField
    type instance XWrap          (GhcPass _) = NoExtField
    type instance XXExpr         (GhcPass _) = NoExtCon

    -- ---------------------------------------------------------------------

    -- | Located Haskell Tuple Argument
    --
    -- 'HsTupArg' is used for tuple sections
    -- @(,a,)@ is represented by
    -- @ExplicitTuple [Missing ty1, Present a, Missing ty3]@
    -- Which in turn stands for @(\x:ty1 \y:ty2. (x,a,y))@
    type LHsTupArg id = Located (HsTupArg id)
    -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnComma'

    -- For details on above see note [Api annotations] in ApiAnnotation

    -- | Haskell Tuple Argument
    data HsTupArg id
      = Present (XPresent id) (LHsExpr id)     -- ^ The argument
      | Missing (XMissing id)    -- ^ The argument is missing, but this is its type
      | XTupArg (XXTupArg id)    -- ^ Note [Trees that Grow] extension point

    type instance XPresent         (GhcPass _) = NoExtField

    type instance XMissing         GhcPs = NoExtField
    type instance XMissing         GhcRn = NoExtField
    type instance XMissing         GhcTc = Type

    type instance XXTupArg         (GhcPass _) = NoExtCon

    tupArgPresent :: LHsTupArg id -> Bool
    tupArgPresent (L _ (Present {})) = True
    tupArgPresent (L _ (Missing {})) = False
    tupArgPresent (L _ (XTupArg {})) = False

    {-
    Note [Parens in HsSyn]
    ~~~~~~~~~~~~~~~~~~~~~~
    HsPar (and ParPat in patterns, HsParTy in types) is used as follows

      * HsPar is required; the pretty printer does not add parens.

      * HsPars are respected when rearranging operator fixities.
        So   a * (b + c)  means what it says (where the parens are an HsPar)

      * For ParPat and HsParTy the pretty printer does add parens but this should be
        a no-op for ParsedSource, based on the pretty printer round trip feature
        introduced in
        https://phabricator.haskell.org/rGHC499e43824bda967546ebf95ee33ec1f84a114a7c

      * ParPat and HsParTy are pretty printed as '( .. )' regardless of whether or
        not they are strictly necessary. This should be addressed when #13238 is
        completed, to be treated the same as HsPar.

    Note [Sections in HsSyn]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Sections should always appear wrapped in an HsPar, thus
             HsPar (SectionR ...)
    The parser parses sections in a wider variety of situations
    (See Note [Parsing sections]), but the renamer checks for those
    parens.  This invariant makes pretty-printing easier; we don't need
    a special case for adding the parens round sections.

    Note [Rebindable if]
    ~~~~~~~~~~~~~~~~~~~~
    The rebindable syntax for 'if' is a bit special, because when
    rebindable syntax is *off* we do not want to treat
       (if c then t else e)
    as if it was an application (ifThenElse c t e).  Why not?
    Because we allow an 'if' to return *unboxed* results, thus
      if blah then 3# else 4#
    whereas that would not be possible using a all to a polymorphic function
    (because you can't call a polymorphic function at an unboxed type).

    So we use Nothing to mean "use the old built-in typing rule".

    Note [Record Update HsWrapper]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There is a wrapper in RecordUpd which is used for the *required*
    constraints for pattern synonyms. This wrapper is created in the
    typechecking and is then directly used in the desugaring without
    modification.

    For example, if we have the record pattern synonym P,
      pattern P :: (Show a) => a -> Maybe a
      pattern P{x} = Just x

      foo = (Just True) { x = False }
    then `foo` desugars to something like
      foo = case Just True of
              P x -> P False
    hence we need to provide the correct dictionaries to P's matcher on
    the RHS so that we can build the expression.

    Note [Located RdrNames]
    ~~~~~~~~~~~~~~~~~~~~~~~
    A number of syntax elements have seemingly redundant locations attached to them.
    This is deliberate, to allow transformations making use of the API Annotations
    to easily correlate a Located Name in the RenamedSource with a Located RdrName
    in the ParsedSource.

    There are unfortunately enough differences between the ParsedSource and the
    RenamedSource that the API Annotations cannot be used directly with
    RenamedSource, so this allows a simple mapping to be used based on the location.

    Note [ExplicitTuple]
    ~~~~~~~~~~~~~~~~~~~~
    An ExplicitTuple is never just a data constructor like (,,,).
    That is, the `[LHsTupArg p]` argument of `ExplicitTuple` has at least
    one `Present` member (and is thus never empty).

    A tuple data constructor like () or (,,,) is parsed as an `HsVar`, not an
    `ExplicitTuple`, and stays that way. This is important for two reasons:

      1. We don't need -XTupleSections for (,,,)
      2. The type variables in (,,,) can be instantiated with visible type application.
         That is,

           (,,)     :: forall a b c. a -> b -> c -> (a,b,c)
           (True,,) :: forall {b} {c}. b -> c -> (Bool,b,c)

         Note that the tuple section has *inferred* arguments, while the data
         constructor has *specified* ones.
         (See Note [Required, Specified, and Inferred for types] in TcTyClsDecls
         for background.)

    Sadly, the grammar for this is actually ambiguous, and it's only thanks to the
    preference of a shift in a shift/reduce conflict that the parser works as this
    Note details. Search for a reference to this Note in Parser.y for further
    explanation.

    Note [Empty lists]
    ~~~~~~~~~~~~~~~~~~
    An empty list could be considered either a data constructor (stored with
    HsVar) or an ExplicitList. This Note describes how empty lists flow through the
    various phases and why.

    Parsing
    -------
    An empty list is parsed by the sysdcon nonterminal. It thus comes to life via
    HsVar nilDataCon (defined in TysWiredIn). A freshly-parsed (HsExpr GhcPs) empty list
    is never a ExplicitList.

    Renaming
    --------
    If -XOverloadedLists is enabled, we must type-check the empty list as if it
    were a call to fromListN. (This is true regardless of the setting of
    -XRebindableSyntax.) This is very easy if the empty list is an ExplicitList,
    but an annoying special case if it's an HsVar. So the renamer changes a
    HsVar nilDataCon to an ExplicitList [], but only if -XOverloadedLists is on.
    (Why not always? Read on, dear friend.) This happens in the HsVar case of rnExpr.

    Type-checking
    -------------
    We want to accept an expression like [] @Int. To do this, we must infer that
    [] :: forall a. [a]. This is easy if [] is a HsVar with the right DataCon inside.
    However, the type-checking for explicit lists works differently: [x,y,z] is never
    polymorphic. Instead, we unify the types of x, y, and z together, and use the
    unified type as the argument to the cons and nil constructors. Thus, treating
    [] as an empty ExplicitList in the type-checker would prevent [] @Int from working.

    However, if -XOverloadedLists is on, then [] @Int really shouldn't be allowed:
    it's just like fromListN 0 [] @Int. Since
      fromListN :: forall list. IsList list => Int -> [Item list] -> list
    that expression really should be rejected. Thus, the renamer's behaviour is
    exactly what we want: treat [] as a datacon when -XNoOverloadedLists, and as
    an empty ExplicitList when -XOverloadedLists.

    See also #13680, which requested [] @Int to work.
    -}

    instance (OutputableBndrId p) => Outputable (HsExpr (GhcPass p)) where
        ppr expr = pprExpr expr

    -----------------------
    -- pprExpr, pprLExpr, pprBinds call pprDeeper;
    -- the underscore versions do not
    pprLExpr :: (OutputableBndrId p) => LHsExpr (GhcPass p) -> SDoc
    pprLExpr (L _ e) = pprExpr e

    pprExpr :: (OutputableBndrId p) => HsExpr (GhcPass p) -> SDoc
    pprExpr e | isAtomicHsExpr e || isQuietHsExpr e =            ppr_expr e
              | otherwise                           = pprDeeper (ppr_expr e)

    isQuietHsExpr :: HsExpr id -> Bool
    -- Parentheses do display something, but it gives little info and
    -- if we go deeper when we go inside them then we get ugly things
    -- like (...)
    isQuietHsExpr (HsPar {})        = True
    -- applications don't display anything themselves
    isQuietHsExpr (HsApp {})        = True
    isQuietHsExpr (HsAppType {})    = True
    isQuietHsExpr (OpApp {})        = True
    isQuietHsExpr _ = False

    pprBinds :: (OutputableBndrId idL, OutputableBndrId idR)
             => HsLocalBindsLR (GhcPass idL) (GhcPass idR) -> SDoc
    pprBinds b = pprDeeper (ppr b)

    -----------------------
    ppr_lexpr :: (OutputableBndrId p) => LHsExpr (GhcPass p) -> SDoc
    ppr_lexpr e = ppr_expr (unLoc e)

    ppr_expr :: forall p. (OutputableBndrId p)
             => HsExpr (GhcPass p) -> SDoc
    ppr_expr (HsVar _ (L _ v))  = pprPrefixOcc v
    ppr_expr (HsUnboundVar _ uv)= pprPrefixOcc uv
    ppr_expr (HsConLikeOut _ c) = pprPrefixOcc c
    ppr_expr (HsIPVar _ v)      = ppr v
    ppr_expr (HsOverLabel _ _ l)= char '#' <> ppr l
    ppr_expr (HsLit _ lit)      = ppr lit
    ppr_expr (HsOverLit _ lit)  = ppr lit
    ppr_expr (HsPar _ e)        = parens (ppr_lexpr e)

    ppr_expr (HsCoreAnn _ stc (StringLiteral sta s) e)
      = vcat [pprWithSourceText stc (text "{-# CORE")
              <+> pprWithSourceText sta (doubleQuotes $ ftext s) <+> text "#-}"
             , ppr_lexpr e]

    ppr_expr e@(HsApp {})        = ppr_apps e []
    ppr_expr e@(HsAppType {})    = ppr_apps e []

    ppr_expr (OpApp _ e1 op e2)
      | Just pp_op <- ppr_infix_expr (unLoc op)
      = pp_infixly pp_op
      | otherwise
      = pp_prefixly

      where
        pp_e1 = pprDebugParendExpr opPrec e1   -- In debug mode, add parens
        pp_e2 = pprDebugParendExpr opPrec e2   -- to make precedence clear

        pp_prefixly
          = hang (ppr op) 2 (sep [pp_e1, pp_e2])

        pp_infixly pp_op
          = hang pp_e1 2 (sep [pp_op, nest 2 pp_e2])

    ppr_expr (NegApp _ e _) = char '-' <+> pprDebugParendExpr appPrec e

    ppr_expr (SectionL _ expr op)
      | Just pp_op <- ppr_infix_expr (unLoc op)
      = pp_infixly pp_op
      | otherwise
      = pp_prefixly
      where
        pp_expr = pprDebugParendExpr opPrec expr

        pp_prefixly = hang (hsep [text " \\ x_ ->", ppr op])
                           4 (hsep [pp_expr, text "x_ )"])

        pp_infixly v = (sep [pp_expr, v])

    ppr_expr (SectionR _ op expr)
      | Just pp_op <- ppr_infix_expr (unLoc op)
      = pp_infixly pp_op
      | otherwise
      = pp_prefixly
      where
        pp_expr = pprDebugParendExpr opPrec expr

        pp_prefixly = hang (hsep [text "( \\ x_ ->", ppr op, text "x_"])
                           4 (pp_expr <> rparen)

        pp_infixly v = sep [v, pp_expr]

    ppr_expr (ExplicitTuple _ exprs boxity)
        -- Special-case unary boxed tuples so that they are pretty-printed as
        -- `Unit x`, not `(x)`
      | [dL -> L _ (Present _ expr)] <- exprs
      , Boxed <- boxity
      = hsep [text (mkTupleStr Boxed 1), ppr expr]
      | otherwise
      = tupleParens (boxityTupleSort boxity) (fcat (ppr_tup_args $ map unLoc exprs))
      where
        ppr_tup_args []               = []
        ppr_tup_args (Present _ e : es) = (ppr_lexpr e <> punc es) : ppr_tup_args es
        ppr_tup_args (Missing _   : es) = punc es : ppr_tup_args es
        ppr_tup_args (XTupArg x   : es) = (ppr x <> punc es) : ppr_tup_args es

        punc (Present {} : _) = comma <> space
        punc (Missing {} : _) = comma
        punc (XTupArg {} : _) = comma <> space
        punc []               = empty

    ppr_expr (ExplicitSum _ alt arity expr)
      = text "(#" <+> ppr_bars (alt - 1) <+> ppr expr <+> ppr_bars (arity - alt) <+> text "#)"
      where
        ppr_bars n = hsep (replicate n (char '|'))

    ppr_expr (HsLam _ matches)
      = pprMatches matches

    ppr_expr (HsLamCase _ matches)
      = sep [ sep [text "\\case"],
              nest 2 (pprMatches matches) ]

    ppr_expr (HsCase _ expr matches@(MG { mg_alts = L _ [_] }))
      = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of {")],
              nest 2 (pprMatches matches) <+> char '}']
    ppr_expr (HsCase _ expr matches)
      = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of")],
              nest 2 (pprMatches matches) ]

    ppr_expr (HsIf _ _ e1 e2 e3)
      = sep [hsep [text "if", nest 2 (ppr e1), ptext (sLit "then")],
             nest 4 (ppr e2),
             text "else",
             nest 4 (ppr e3)]

    ppr_expr (HsMultiIf _ alts)
      = hang (text "if") 3  (vcat (map ppr_alt alts))
      where ppr_alt (L _ (GRHS _ guards expr)) =
              hang vbar 2 (ppr_one one_alt)
              where
                ppr_one [] = panic "ppr_exp HsMultiIf"
                ppr_one (h:t) = hang h 2 (sep t)
                one_alt = [ interpp'SP guards
                          , text "->" <+> pprDeeper (ppr expr) ]
            ppr_alt (L _ (XGRHS x)) = ppr x

    -- special case: let ... in let ...
    ppr_expr (HsLet _ (L _ binds) expr@(L _ (HsLet _ _ _)))
      = sep [hang (text "let") 2 (hsep [pprBinds binds, ptext (sLit "in")]),
             ppr_lexpr expr]

    ppr_expr (HsLet _ (L _ binds) expr)
      = sep [hang (text "let") 2 (pprBinds binds),
             hang (text "in")  2 (ppr expr)]

    ppr_expr (HsDo _ do_or_list_comp (L _ stmts)) = pprDo do_or_list_comp stmts

    ppr_expr (ExplicitList _ _ exprs)
      = brackets (pprDeeperList fsep (punctuate comma (map ppr_lexpr exprs)))

    ppr_expr (RecordCon { rcon_con_name = con_id, rcon_flds = rbinds })
      = hang (ppr con_id) 2 (ppr rbinds)

    ppr_expr (RecordUpd { rupd_expr = L _ aexp, rupd_flds = rbinds })
      = hang (ppr aexp) 2 (braces (fsep (punctuate comma (map ppr rbinds))))

    ppr_expr (ExprWithTySig _ expr sig)
      = hang (nest 2 (ppr_lexpr expr) <+> dcolon)
             4 (ppr sig)

    ppr_expr (ArithSeq _ _ info) = brackets (ppr info)

    ppr_expr (HsSCC _ st (StringLiteral stl lbl) expr)
      = sep [ pprWithSourceText st (text "{-# SCC")
             -- no doublequotes if stl empty, for the case where the SCC was written
             -- without quotes.
              <+> pprWithSourceText stl (ftext lbl) <+> text "#-}",
              ppr expr ]

    ppr_expr (HsWrap _ co_fn e)
      = pprHsWrapper co_fn (\parens -> if parens then pprExpr e
                                                 else pprExpr e)

    ppr_expr (HsSpliceE _ s)         = pprSplice s
    ppr_expr (HsBracket _ b)         = pprHsBracket b
    ppr_expr (HsRnBracketOut _ e []) = ppr e
    ppr_expr (HsRnBracketOut _ e ps) = ppr e $$ text "pending(rn)" <+> ppr ps
    ppr_expr (HsTcBracketOut _ e []) = ppr e
    ppr_expr (HsTcBracketOut _ e ps) = ppr e $$ text "pending(tc)" <+> ppr ps

    ppr_expr (HsProc _ pat (L _ (HsCmdTop _ cmd)))
      = hsep [text "proc", ppr pat, ptext (sLit "->"), ppr cmd]
    ppr_expr (HsProc _ pat (L _ (XCmdTop x)))
      = hsep [text "proc", ppr pat, ptext (sLit "->"), ppr x]

    ppr_expr (HsStatic _ e)
      = hsep [text "static", ppr e]

    ppr_expr (HsTick _ tickish exp)
      = pprTicks (ppr exp) $
        ppr tickish <+> ppr_lexpr exp
    ppr_expr (HsBinTick _ tickIdTrue tickIdFalse exp)
      = pprTicks (ppr exp) $
        hcat [text "bintick<",
              ppr tickIdTrue,
              text ",",
              ppr tickIdFalse,
              text ">(",
              ppr exp, text ")"]
    ppr_expr (HsTickPragma _ _ externalSrcLoc _ exp)
      = pprTicks (ppr exp) $
        hcat [text "tickpragma<",
              pprExternalSrcLoc externalSrcLoc,
              text ">(",
              ppr exp,
              text ")"]

    ppr_expr (HsRecFld _ f) = ppr f
    ppr_expr (XExpr x) = ppr x

    ppr_infix_expr :: (OutputableBndrId p) => HsExpr (GhcPass p) -> Maybe SDoc
    ppr_infix_expr (HsVar _ (L _ v))    = Just (pprInfixOcc v)
    ppr_infix_expr (HsConLikeOut _ c)   = Just (pprInfixOcc (conLikeName c))
    ppr_infix_expr (HsRecFld _ f)       = Just (pprInfixOcc f)
    ppr_infix_expr (HsUnboundVar _ occ) = Just (pprInfixOcc occ)
    ppr_infix_expr (HsWrap _ _ e)       = ppr_infix_expr e
    ppr_infix_expr _                    = Nothing

    ppr_apps :: (OutputableBndrId p)
             => HsExpr (GhcPass p)
             -> [Either (LHsExpr (GhcPass p)) (LHsWcType (NoGhcTc (GhcPass p)))]
             -> SDoc
    ppr_apps (HsApp _ (L _ fun) arg)        args
      = ppr_apps fun (Left arg : args)
    ppr_apps (HsAppType _ (L _ fun) arg)    args
      = ppr_apps fun (Right arg : args)
    ppr_apps fun args = hang (ppr_expr fun) 2 (fsep (map pp args))
      where
        pp (Left arg)                             = ppr arg
        -- pp (Right (LHsWcTypeX (HsWC { hswc_body = L _ arg })))
        --   = char '@' <> pprHsType arg
        pp (Right arg)
          = char '@' <> ppr arg

    pprExternalSrcLoc :: (StringLiteral,(Int,Int),(Int,Int)) -> SDoc
    pprExternalSrcLoc (StringLiteral _ src,(n1,n2),(n3,n4))
      = ppr (src,(n1,n2),(n3,n4))

    {-
    HsSyn records exactly where the user put parens, with HsPar.
    So generally speaking we print without adding any parens.
    However, some code is internally generated, and in some places
    parens are absolutely required; so for these places we use
    pprParendLExpr (but don't print double parens of course).

    For operator applications we don't add parens, because the operator
    fixities should do the job, except in debug mode (-dppr-debug) so we
    can see the structure of the parse tree.
    -}

    pprDebugParendExpr :: (OutputableBndrId p)
                       => PprPrec -> LHsExpr (GhcPass p) -> SDoc
    pprDebugParendExpr p expr
      = getPprStyle (\sty ->
        if debugStyle sty then pprParendLExpr p expr
                          else pprLExpr      expr)

    pprParendLExpr :: (OutputableBndrId p)
                   => PprPrec -> LHsExpr (GhcPass p) -> SDoc
    pprParendLExpr p (L _ e) = pprParendExpr p e

    pprParendExpr :: (OutputableBndrId p)
                  => PprPrec -> HsExpr (GhcPass p) -> SDoc
    pprParendExpr p expr
      | hsExprNeedsParens p expr = parens (pprExpr expr)
      | otherwise                = pprExpr expr
            -- Using pprLExpr makes sure that we go 'deeper'
            -- I think that is usually (always?) right

    -- | @'hsExprNeedsParens' p e@ returns 'True' if the expression @e@ needs
    -- parentheses under precedence @p@.
    hsExprNeedsParens :: PprPrec -> HsExpr p -> Bool
    hsExprNeedsParens p = go
      where
        go (HsVar{})                      = False
        go (HsUnboundVar{})               = False
        go (HsConLikeOut{})               = False
        go (HsIPVar{})                    = False
        go (HsOverLabel{})                = False
        go (HsLit _ l)                    = hsLitNeedsParens p l
        go (HsOverLit _ ol)               = hsOverLitNeedsParens p ol
        go (HsPar{})                      = False
        go (HsCoreAnn _ _ _ (L _ e))      = go e
        go (HsApp{})                      = p >= appPrec
        go (HsAppType {})                 = p >= appPrec
        go (OpApp{})                      = p >= opPrec
        go (NegApp{})                     = p > topPrec
        go (SectionL{})                   = True
        go (SectionR{})                   = True
        go (ExplicitTuple{})              = False
        go (ExplicitSum{})                = False
        go (HsLam{})                      = p > topPrec
        go (HsLamCase{})                  = p > topPrec
        go (HsCase{})                     = p > topPrec
        go (HsIf{})                       = p > topPrec
        go (HsMultiIf{})                  = p > topPrec
        go (HsLet{})                      = p > topPrec
        go (HsDo _ sc _)
          | isComprehensionContext sc     = False
          | otherwise                     = p > topPrec
        go (ExplicitList{})               = False
        go (RecordUpd{})                  = False
        go (ExprWithTySig{})              = p >= sigPrec
        go (ArithSeq{})                   = False
        go (HsSCC{})                      = p >= appPrec
        go (HsWrap _ _ e)                 = go e
        go (HsSpliceE{})                  = False
        go (HsBracket{})                  = False
        go (HsRnBracketOut{})             = False
        go (HsTcBracketOut{})             = False
        go (HsProc{})                     = p > topPrec
        go (HsStatic{})                   = p >= appPrec
        go (HsTick _ _ (L _ e))           = go e
        go (HsBinTick _ _ _ (L _ e))      = go e
        go (HsTickPragma _ _ _ _ (L _ e)) = go e
        go (RecordCon{})                  = False
        go (HsRecFld{})                   = False
        go (XExpr{})                      = True

    -- | @'parenthesizeHsExpr' p e@ checks if @'hsExprNeedsParens' p e@ is true,
    -- and if so, surrounds @e@ with an 'HsPar'. Otherwise, it simply returns @e@.
    parenthesizeHsExpr :: PprPrec -> LHsExpr (GhcPass p) -> LHsExpr (GhcPass p)
    parenthesizeHsExpr p le@(L loc e)
      | hsExprNeedsParens p e = L loc (HsPar noExtField le)
      | otherwise             = le

    stripParensHsExpr :: LHsExpr (GhcPass p) -> LHsExpr (GhcPass p)
    stripParensHsExpr (L _ (HsPar _ e)) = stripParensHsExpr e
    stripParensHsExpr e = e

    isAtomicHsExpr :: HsExpr id -> Bool
    -- True of a single token
    isAtomicHsExpr (HsVar {})        = True
    isAtomicHsExpr (HsConLikeOut {}) = True
    isAtomicHsExpr (HsLit {})        = True
    isAtomicHsExpr (HsOverLit {})    = True
    isAtomicHsExpr (HsIPVar {})      = True
    isAtomicHsExpr (HsOverLabel {})  = True
    isAtomicHsExpr (HsUnboundVar {}) = True
    isAtomicHsExpr (HsWrap _ _ e)    = isAtomicHsExpr e
    isAtomicHsExpr (HsPar _ e)       = isAtomicHsExpr (unLoc e)
    isAtomicHsExpr (HsRecFld{})      = True
    isAtomicHsExpr _                 = False

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Commands (in arrow abstractions)}
    *                                                                      *
    ************************************************************************

    We re-use HsExpr to represent these.
    -}

    -- | Located Haskell Command (for arrow syntax)
    type LHsCmd id = Located (HsCmd id)

    -- | Haskell Command (e.g. a "statement" in an Arrow proc block)
    data HsCmd id
      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.Annlarrowtail',
      --          'ApiAnnotation.Annrarrowtail','ApiAnnotation.AnnLarrowtail',
      --          'ApiAnnotation.AnnRarrowtail'

      -- For details on above see note [Api annotations] in ApiAnnotation
      = HsCmdArrApp          -- Arrow tail, or arrow application (f -< arg)
            (XCmdArrApp id)  -- type of the arrow expressions f,
                             -- of the form a t t', where arg :: t
            (LHsExpr id)     -- arrow expression, f
            (LHsExpr id)     -- input expression, arg
            HsArrAppType     -- higher-order (-<<) or first-order (-<)
            Bool             -- True => right-to-left (f -< arg)
                             -- False => left-to-right (arg >- f)

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpenB' @'(|'@,
      --         'ApiAnnotation.AnnCloseB' @'|)'@

      -- For details on above see note [Api annotations] in ApiAnnotation
      | HsCmdArrForm         -- Command formation,  (| e cmd1 .. cmdn |)
            (XCmdArrForm id)
            (LHsExpr id)     -- The operator.
                             -- After type-checking, a type abstraction to be
                             -- applied to the type of the local environment tuple
            LexicalFixity    -- Whether the operator appeared prefix or infix when
                             -- parsed.
            (Maybe Fixity)   -- fixity (filled in by the renamer), for forms that
                             -- were converted from OpApp's by the renamer
            [LHsCmdTop id]   -- argument commands

      | HsCmdApp    (XCmdApp id)
                    (LHsCmd id)
                    (LHsExpr id)

      | HsCmdLam    (XCmdLam id)
                    (MatchGroup id (LHsCmd id))     -- kappa
           -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
           --       'ApiAnnotation.AnnRarrow',

           -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdPar    (XCmdPar id)
                    (LHsCmd id)                     -- parenthesised command
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'('@,
        --             'ApiAnnotation.AnnClose' @')'@

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdCase   (XCmdCase id)
                    (LHsExpr id)
                    (MatchGroup id (LHsCmd id))     -- bodies are HsCmd's
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnCase',
        --       'ApiAnnotation.AnnOf','ApiAnnotation.AnnOpen' @'{'@,
        --       'ApiAnnotation.AnnClose' @'}'@

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdIf     (XCmdIf id)
                    (Maybe (SyntaxExpr id))         -- cond function
                    (LHsExpr id)                    -- predicate
                    (LHsCmd id)                     -- then part
                    (LHsCmd id)                     -- else part
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnIf',
        --       'ApiAnnotation.AnnSemi',
        --       'ApiAnnotation.AnnThen','ApiAnnotation.AnnSemi',
        --       'ApiAnnotation.AnnElse',

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdLet    (XCmdLet id)
                    (LHsLocalBinds id)      -- let(rec)
                    (LHsCmd  id)
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLet',
        --       'ApiAnnotation.AnnOpen' @'{'@,
        --       'ApiAnnotation.AnnClose' @'}'@,'ApiAnnotation.AnnIn'

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdDo     (XCmdDo id)                     -- Type of the whole expression
                    (Located [CmdLStmt id])
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDo',
        --             'ApiAnnotation.AnnOpen', 'ApiAnnotation.AnnSemi',
        --             'ApiAnnotation.AnnVbar',
        --             'ApiAnnotation.AnnClose'

        -- For details on above see note [Api annotations] in ApiAnnotation

      | HsCmdWrap   (XCmdWrap id)
                    HsWrapper
                    (HsCmd id)     -- If   cmd :: arg1 --> res
                                   --      wrap :: arg1 "->" arg2
                                   -- Then (HsCmdWrap wrap cmd) :: arg2 --> res
      | XCmd        (XXCmd id)     -- Note [Trees that Grow] extension point

    type instance XCmdArrApp  GhcPs = NoExtField
    type instance XCmdArrApp  GhcRn = NoExtField
    type instance XCmdArrApp  GhcTc = Type

    type instance XCmdArrForm (GhcPass _) = NoExtField
    type instance XCmdApp     (GhcPass _) = NoExtField
    type instance XCmdLam     (GhcPass _) = NoExtField
    type instance XCmdPar     (GhcPass _) = NoExtField
    type instance XCmdCase    (GhcPass _) = NoExtField
    type instance XCmdIf      (GhcPass _) = NoExtField
    type instance XCmdLet     (GhcPass _) = NoExtField

    type instance XCmdDo      GhcPs = NoExtField
    type instance XCmdDo      GhcRn = NoExtField
    type instance XCmdDo      GhcTc = Type

    type instance XCmdWrap    (GhcPass _) = NoExtField
    type instance XXCmd       (GhcPass _) = NoExtCon

    -- | Haskell Array Application Type
    data HsArrAppType = HsHigherOrderApp | HsFirstOrderApp
      deriving Data

    {- | Top-level command, introducing a new arrow.
    This may occur inside a proc (where the stack is empty) or as an
    argument of a command-forming operator.
    -}

    -- | Located Haskell Top-level Command
    type LHsCmdTop p = Located (HsCmdTop p)

    -- | Haskell Top-level Command
    data HsCmdTop p
      = HsCmdTop (XCmdTop p)
                 (LHsCmd p)
      | XCmdTop (XXCmdTop p)        -- Note [Trees that Grow] extension point

    data CmdTopTc
      = CmdTopTc Type    -- Nested tuple of inputs on the command's stack
                 Type    -- return type of the command
                 (CmdSyntaxTable GhcTc) -- See Note [CmdSyntaxTable]

    type instance XCmdTop  GhcPs = NoExtField
    type instance XCmdTop  GhcRn = CmdSyntaxTable GhcRn -- See Note [CmdSyntaxTable]
    type instance XCmdTop  GhcTc = CmdTopTc

    type instance XXCmdTop (GhcPass _) = NoExtCon

    instance (OutputableBndrId p) => Outputable (HsCmd (GhcPass p)) where
        ppr cmd = pprCmd cmd

    -----------------------
    -- pprCmd and pprLCmd call pprDeeper;
    -- the underscore versions do not
    pprLCmd :: (OutputableBndrId p) => LHsCmd (GhcPass p) -> SDoc
    pprLCmd (L _ c) = pprCmd c

    pprCmd :: (OutputableBndrId p) => HsCmd (GhcPass p) -> SDoc
    pprCmd c | isQuietHsCmd c =            ppr_cmd c
             | otherwise      = pprDeeper (ppr_cmd c)

    isQuietHsCmd :: HsCmd id -> Bool
    -- Parentheses do display something, but it gives little info and
    -- if we go deeper when we go inside them then we get ugly things
    -- like (...)
    isQuietHsCmd (HsCmdPar {}) = True
    -- applications don't display anything themselves
    isQuietHsCmd (HsCmdApp {}) = True
    isQuietHsCmd _ = False

    -----------------------
    ppr_lcmd :: (OutputableBndrId p) => LHsCmd (GhcPass p) -> SDoc
    ppr_lcmd c = ppr_cmd (unLoc c)

    ppr_cmd :: forall p. (OutputableBndrId p) => HsCmd (GhcPass p) -> SDoc
    ppr_cmd (HsCmdPar _ c) = parens (ppr_lcmd c)

    ppr_cmd (HsCmdApp _ c e)
      = let (fun, args) = collect_args c [e] in
        hang (ppr_lcmd fun) 2 (sep (map ppr args))
      where
        collect_args (L _ (HsCmdApp _ fun arg)) args = collect_args fun (arg:args)
        collect_args fun args = (fun, args)

    ppr_cmd (HsCmdLam _ matches)
      = pprMatches matches

    ppr_cmd (HsCmdCase _ expr matches)
      = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of")],
              nest 2 (pprMatches matches) ]

    ppr_cmd (HsCmdIf _ _ e ct ce)
      = sep [hsep [text "if", nest 2 (ppr e), ptext (sLit "then")],
             nest 4 (ppr ct),
             text "else",
             nest 4 (ppr ce)]

    -- special case: let ... in let ...
    ppr_cmd (HsCmdLet _ (L _ binds) cmd@(L _ (HsCmdLet {})))
      = sep [hang (text "let") 2 (hsep [pprBinds binds, ptext (sLit "in")]),
             ppr_lcmd cmd]

    ppr_cmd (HsCmdLet _ (L _ binds) cmd)
      = sep [hang (text "let") 2 (pprBinds binds),
             hang (text "in")  2 (ppr cmd)]

    ppr_cmd (HsCmdDo _ (L _ stmts))  = pprDo ArrowExpr stmts

    ppr_cmd (HsCmdWrap _ w cmd)
      = pprHsWrapper w (\_ -> parens (ppr_cmd cmd))
    ppr_cmd (HsCmdArrApp _ arrow arg HsFirstOrderApp True)
      = hsep [ppr_lexpr arrow, larrowt, ppr_lexpr arg]
    ppr_cmd (HsCmdArrApp _ arrow arg HsFirstOrderApp False)
      = hsep [ppr_lexpr arg, arrowt, ppr_lexpr arrow]
    ppr_cmd (HsCmdArrApp _ arrow arg HsHigherOrderApp True)
      = hsep [ppr_lexpr arrow, larrowtt, ppr_lexpr arg]
    ppr_cmd (HsCmdArrApp _ arrow arg HsHigherOrderApp False)
      = hsep [ppr_lexpr arg, arrowtt, ppr_lexpr arrow]

    ppr_cmd (HsCmdArrForm _ (L _ (HsVar _ (L _ v))) _ (Just _) [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc v
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm _ (L _ (HsVar _ (L _ v))) Infix _    [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc v
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm _ (L _ (HsConLikeOut _ c)) _ (Just _) [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc (conLikeName c)
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm _ (L _ (HsConLikeOut _ c)) Infix _    [arg1, arg2])
      = hang (pprCmdArg (unLoc arg1)) 4 (sep [ pprInfixOcc (conLikeName c)
                                             , pprCmdArg (unLoc arg2)])
    ppr_cmd (HsCmdArrForm _ op _ _ args)
      = hang (text "(|" <+> ppr_lexpr op)
             4 (sep (map (pprCmdArg.unLoc) args) <+> text "|)")
    ppr_cmd (XCmd x) = ppr x

    pprCmdArg :: (OutputableBndrId p) => HsCmdTop (GhcPass p) -> SDoc
    pprCmdArg (HsCmdTop _ cmd)
      = ppr_lcmd cmd
    pprCmdArg (XCmdTop x) = ppr x

    instance (OutputableBndrId p) => Outputable (HsCmdTop (GhcPass p)) where
        ppr = pprCmdArg

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Record binds}
    *                                                                      *
    ************************************************************************
    -}

    -- | Haskell Record Bindings
    type HsRecordBinds p = HsRecFields p (LHsExpr p)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{@Match@, @GRHSs@, and @GRHS@ datatypes}
    *                                                                      *
    ************************************************************************

    @Match@es are sets of pattern bindings and right hand sides for
    functions, patterns or case branches. For example, if a function @g@
    is defined as:
    \begin{verbatim}
    g (x,y) = y
    g ((x:ys),y) = y+1,
    \end{verbatim}
    then \tr{g} has two @Match@es: @(x,y) = y@ and @((x:ys),y) = y+1@.

    It is always the case that each element of an @[Match]@ list has the
    same number of @pats@s inside it.  This corresponds to saying that
    a function defined by pattern matching must have the same number of
    patterns in each equation.
    -}

    data MatchGroup p body
      = MG { mg_ext     :: XMG p body -- Post-typechecker, types of args and result
           , mg_alts    :: Located [LMatch p body]  -- The alternatives
           , mg_origin  :: Origin }
         -- The type is the type of the entire group
         --      t1 -> ... -> tn -> tr
         -- where there are n patterns
      | XMatchGroup (XXMatchGroup p body)

    data MatchGroupTc
      = MatchGroupTc
           { mg_arg_tys :: [Type]  -- Types of the arguments, t1..tn
           , mg_res_ty  :: Type    -- Type of the result, tr
           } deriving Data

    type instance XMG         GhcPs b = NoExtField
    type instance XMG         GhcRn b = NoExtField
    type instance XMG         GhcTc b = MatchGroupTc

    type instance XXMatchGroup (GhcPass _) b = NoExtCon

    -- | Located Match
    type LMatch id body = Located (Match id body)
    -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnSemi' when in a
    --   list

    -- For details on above see note [Api annotations] in ApiAnnotation
    data Match p body
      = Match {
            m_ext :: XCMatch p body,
            m_ctxt :: HsMatchContext (NameOrRdrName (IdP p)),
              -- See note [m_ctxt in Match]
            m_pats :: [LPat p], -- The patterns
            m_grhss :: (GRHSs p body)
      }
      | XMatch (XXMatch p body)

    type instance XCMatch (GhcPass _) b = NoExtField
    type instance XXMatch (GhcPass _) b = NoExtCon

    instance (OutputableBndrId pr, Outputable body)
                => Outputable (Match (GhcPass pr) body) where
      ppr = pprMatch

    {-
    Note [m_ctxt in Match]
    ~~~~~~~~~~~~~~~~~~~~~~

    A Match can occur in a number of contexts, such as a FunBind, HsCase, HsLam and
    so on.

    In order to simplify tooling processing and pretty print output, the provenance
    is captured in an HsMatchContext.

    This is particularly important for the API Annotations for a multi-equation
    FunBind.

    The parser initially creates a FunBind with a single Match in it for
    every function definition it sees.

    These are then grouped together by getMonoBind into a single FunBind,
    where all the Matches are combined.

    In the process, all the original FunBind fun_id's bar one are
    discarded, including the locations.

    This causes a problem for source to source conversions via API
    Annotations, so the original fun_ids and infix flags are preserved in
    the Match, when it originates from a FunBind.

    Example infix function definition requiring individual API Annotations

        (&&&  ) [] [] =  []
        xs    &&&   [] =  xs
        (  &&&  ) [] ys =  ys

    -}

    isInfixMatch :: Match id body -> Bool
    isInfixMatch match = case m_ctxt match of
      FunRhs {mc_fixity = Infix} -> True
      _                          -> False

    isEmptyMatchGroup :: MatchGroup id body -> Bool
    isEmptyMatchGroup (MG { mg_alts = ms }) = null $ unLoc ms
    isEmptyMatchGroup (XMatchGroup {})      = False

    -- | Is there only one RHS in this list of matches?
    isSingletonMatchGroup :: [LMatch id body] -> Bool
    isSingletonMatchGroup matches
      | [L _ match] <- matches
      , Match { m_grhss = GRHSs { grhssGRHSs = [_] } } <- match
      = True
      | otherwise
      = False

    matchGroupArity :: MatchGroup (GhcPass id) body -> Arity
    -- Precondition: MatchGroup is non-empty
    -- This is called before type checking, when mg_arg_tys is not set
    matchGroupArity (MG { mg_alts = alts })
      | L _ (alt1:_) <- alts = length (hsLMatchPats alt1)
      | otherwise        = panic "matchGroupArity"
    matchGroupArity (XMatchGroup nec) = noExtCon nec

    hsLMatchPats :: LMatch (GhcPass id) body -> [LPat (GhcPass id)]
    hsLMatchPats (L _ (Match { m_pats = pats })) = pats
    hsLMatchPats (L _ (XMatch nec)) = noExtCon nec

    -- | Guarded Right-Hand Sides
    --
    -- GRHSs are used both for pattern bindings and for Matches
    --
    --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnVbar',
    --        'ApiAnnotation.AnnEqual','ApiAnnotation.AnnWhere',
    --        'ApiAnnotation.AnnOpen','ApiAnnotation.AnnClose'
    --        'ApiAnnotation.AnnRarrow','ApiAnnotation.AnnSemi'

    -- For details on above see note [Api annotations] in ApiAnnotation
    data GRHSs p body
      = GRHSs {
          grhssExt :: XCGRHSs p body,
          grhssGRHSs :: [LGRHS p body],      -- ^ Guarded RHSs
          grhssLocalBinds :: LHsLocalBinds p -- ^ The where clause
        }
      | XGRHSs (XXGRHSs p body)

    type instance XCGRHSs (GhcPass _) b = NoExtField
    type instance XXGRHSs (GhcPass _) b = NoExtCon

    -- | Located Guarded Right-Hand Side
    type LGRHS id body = Located (GRHS id body)

    -- | Guarded Right Hand Side.
    data GRHS p body = GRHS (XCGRHS p body)
                            [GuardLStmt p] -- Guards
                            body           -- Right hand side
                      | XGRHS (XXGRHS p body)

    type instance XCGRHS (GhcPass _) b = NoExtField
    type instance XXGRHS (GhcPass _) b = NoExtCon

    -- We know the list must have at least one @Match@ in it.

    pprMatches :: (OutputableBndrId idR, Outputable body)
               => MatchGroup (GhcPass idR) body -> SDoc
    pprMatches MG { mg_alts = matches }
        = vcat (map pprMatch (map unLoc (unLoc matches)))
          -- Don't print the type; it's only a place-holder before typechecking
    pprMatches (XMatchGroup x) = ppr x

    -- Exported to GHC.Hs.Binds, which can't see the defn of HsMatchContext
    pprFunBind :: (OutputableBndrId idR, Outputable body)
               => MatchGroup (GhcPass idR) body -> SDoc
    pprFunBind matches = pprMatches matches

    -- Exported to GHC.Hs.Binds, which can't see the defn of HsMatchContext
    pprPatBind :: forall bndr p body. (OutputableBndrId bndr,
                                       OutputableBndrId p,
                                       Outputable body)
               => LPat (GhcPass bndr) -> GRHSs (GhcPass p) body -> SDoc
    pprPatBind pat (grhss)
     = sep [ppr pat,
           nest 2 (pprGRHSs (PatBindRhs :: HsMatchContext (IdP (GhcPass p))) grhss)]

    pprMatch :: (OutputableBndrId idR, Outputable body)
             => Match (GhcPass idR) body -> SDoc
    pprMatch match
      = sep [ sep (herald : map (nest 2 . pprParendLPat appPrec) other_pats)
            , nest 2 (pprGRHSs ctxt (m_grhss match)) ]
      where
        ctxt = m_ctxt match
        (herald, other_pats)
            = case ctxt of
                FunRhs {mc_fun=L _ fun, mc_fixity=fixity, mc_strictness=strictness}
                    | strictness == SrcStrict -> ASSERT(null $ m_pats match)
                                                 (char '!'<>pprPrefixOcc fun, m_pats match)
                            -- a strict variable binding
                    | fixity == Prefix -> (pprPrefixOcc fun, m_pats match)
                            -- f x y z = e
                            -- Not pprBndr; the AbsBinds will
                            -- have printed the signature

                    | null pats2 -> (pp_infix, [])
                            -- x &&& y = e

                    | otherwise -> (parens pp_infix, pats2)
                            -- (x &&& y) z = e
                    where
                      pp_infix = pprParendLPat opPrec pat1
                             <+> pprInfixOcc fun
                             <+> pprParendLPat opPrec pat2

                LambdaExpr -> (char '\\', m_pats match)

                _  -> if null (m_pats match)
                         then (empty, [])
                         else ASSERT2( null pats1, ppr ctxt $$ ppr pat1 $$ ppr pats1 )
                              (ppr pat1, [])        -- No parens around the single pat

        (pat1:pats1) = m_pats match
        (pat2:pats2) = pats1

    pprGRHSs :: (OutputableBndrId idR, Outputable body)
             => HsMatchContext idL -> GRHSs (GhcPass idR) body -> SDoc
    pprGRHSs ctxt (GRHSs _ grhss (L _ binds))
      = vcat (map (pprGRHS ctxt . unLoc) grhss)
      -- Print the "where" even if the contents of the binds is empty. Only
      -- EmptyLocalBinds means no "where" keyword
     $$ ppUnless (eqEmptyLocalBinds binds)
          (text "where" $$ nest 4 (pprBinds binds))
    pprGRHSs _ (XGRHSs x) = ppr x

    pprGRHS :: (OutputableBndrId idR, Outputable body)
            => HsMatchContext idL -> GRHS (GhcPass idR) body -> SDoc
    pprGRHS ctxt (GRHS _ [] body)
     =  pp_rhs ctxt body

    pprGRHS ctxt (GRHS _ guards body)
     = sep [vbar <+> interpp'SP guards, pp_rhs ctxt body]

    pprGRHS _ (XGRHS x) = ppr x

    pp_rhs :: Outputable body => HsMatchContext idL -> body -> SDoc
    pp_rhs ctxt rhs = matchSeparator ctxt <+> pprDeeper (ppr rhs)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Do stmts and list comprehensions}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located @do@ block Statement
    type LStmt id body = Located (StmtLR id id body)

    -- | Located Statement with separate Left and Right id's
    type LStmtLR idL idR body = Located (StmtLR idL idR body)

    -- | @do@ block Statement
    type Stmt id body = StmtLR id id body

    -- | Command Located Statement
    type CmdLStmt   id = LStmt id (LHsCmd  id)

    -- | Command Statement
    type CmdStmt    id = Stmt  id (LHsCmd  id)

    -- | Expression Located Statement
    type ExprLStmt  id = LStmt id (LHsExpr id)

    -- | Expression Statement
    type ExprStmt   id = Stmt  id (LHsExpr id)

    -- | Guard Located Statement
    type GuardLStmt id = LStmt id (LHsExpr id)

    -- | Guard Statement
    type GuardStmt  id = Stmt  id (LHsExpr id)

    -- | Ghci Located Statement
    type GhciLStmt  id = LStmt id (LHsExpr id)

    -- | Ghci Statement
    type GhciStmt   id = Stmt  id (LHsExpr id)

    -- The SyntaxExprs in here are used *only* for do-notation and monad
    -- comprehensions, which have rebindable syntax. Otherwise they are unused.
    -- | API Annotations when in qualifier lists or guards
    --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnVbar',
    --         'ApiAnnotation.AnnComma','ApiAnnotation.AnnThen',
    --         'ApiAnnotation.AnnBy','ApiAnnotation.AnnBy',
    --         'ApiAnnotation.AnnGroup','ApiAnnotation.AnnUsing'

    -- For details on above see note [Api annotations] in ApiAnnotation
    data StmtLR idL idR body -- body should always be (LHs**** idR)
      = LastStmt  -- Always the last Stmt in ListComp, MonadComp,
                  -- and (after the renamer, see RnExpr.checkLastStmt) DoExpr, MDoExpr
                  -- Not used for GhciStmtCtxt, PatGuard, which scope over other stuff
              (XLastStmt idL idR body)
              body
              Bool               -- True <=> return was stripped by ApplicativeDo
              (SyntaxExpr idR)   -- The return operator
                -- The return operator is used only for MonadComp
                -- For ListComp we use the baked-in 'return'
                -- For DoExpr, MDoExpr, we don't apply a 'return' at all
                -- See Note [Monad Comprehensions]
                -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLarrow'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | BindStmt (XBindStmt idL idR body) -- Post typechecking,
                                    -- result type of the function passed to bind;
                                    -- that is, S in (>>=) :: Q -> (R -> S) -> T
                 (LPat idL)
                 body
                 (SyntaxExpr idR) -- The (>>=) operator; see Note [The type of bind in Stmts]
                 (SyntaxExpr idR) -- The fail operator
                 -- The fail operator is noSyntaxExpr
                 -- if the pattern match can't fail

      -- | 'ApplicativeStmt' represents an applicative expression built with
      -- '<$>' and '<*>'.  It is generated by the renamer, and is desugared into the
      -- appropriate applicative expression by the desugarer, but it is intended
      -- to be invisible in error messages.
      --
      -- For full details, see Note [ApplicativeDo] in RnExpr
      --
      | ApplicativeStmt
                 (XApplicativeStmt idL idR body) -- Post typecheck, Type of the body
                 [ ( SyntaxExpr idR
                   , ApplicativeArg idL) ]
                          -- [(<$>, e1), (<*>, e2), ..., (<*>, en)]
                 (Maybe (SyntaxExpr idR))  -- 'join', if necessary

      | BodyStmt (XBodyStmt idL idR body) -- Post typecheck, element type
                                          -- of the RHS (used for arrows)
                 body              -- See Note [BodyStmt]
                 (SyntaxExpr idR)  -- The (>>) operator
                 (SyntaxExpr idR)  -- The `guard` operator; used only in MonadComp
                                   -- See notes [Monad Comprehensions]

      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLet'
      --          'ApiAnnotation.AnnOpen' @'{'@,'ApiAnnotation.AnnClose' @'}'@,

      -- For details on above see note [Api annotations] in ApiAnnotation
      | LetStmt  (XLetStmt idL idR body) (LHsLocalBindsLR idL idR)

      -- ParStmts only occur in a list/monad comprehension
      | ParStmt  (XParStmt idL idR body)    -- Post typecheck,
                                            -- S in (>>=) :: Q -> (R -> S) -> T
                 [ParStmtBlock idL idR]
                 (HsExpr idR)               -- Polymorphic `mzip` for monad comprehensions
                 (SyntaxExpr idR)           -- The `>>=` operator
                                            -- See notes [Monad Comprehensions]
                -- After renaming, the ids are the binders
                -- bound by the stmts and used after themp

      | TransStmt {
          trS_ext   :: XTransStmt idL idR body, -- Post typecheck,
                                                -- R in (>>=) :: Q -> (R -> S) -> T
          trS_form  :: TransForm,
          trS_stmts :: [ExprLStmt idL],   -- Stmts to the *left* of the 'group'
                                          -- which generates the tuples to be grouped

          trS_bndrs :: [(IdP idR, IdP idR)], -- See Note [TransStmt binder map]

          trS_using :: LHsExpr idR,
          trS_by :: Maybe (LHsExpr idR),  -- "by e" (optional)
            -- Invariant: if trS_form = GroupBy, then grp_by = Just e

          trS_ret :: SyntaxExpr idR,      -- The monomorphic 'return' function for
                                          -- the inner monad comprehensions
          trS_bind :: SyntaxExpr idR,     -- The '(>>=)' operator
          trS_fmap :: HsExpr idR          -- The polymorphic 'fmap' function for desugaring
                                          -- Only for 'group' forms
                                          -- Just a simple HsExpr, because it's
                                          -- too polymorphic for tcSyntaxOp
        }                                 -- See Note [Monad Comprehensions]

      -- Recursive statement (see Note [How RecStmt works] below)
      -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnRec'

      -- For details on above see note [Api annotations] in ApiAnnotation
      | RecStmt
         { recS_ext :: XRecStmt idL idR body
         , recS_stmts :: [LStmtLR idL idR body]

            -- The next two fields are only valid after renaming
         , recS_later_ids :: [IdP idR]
                             -- The ids are a subset of the variables bound by the
                             -- stmts that are used in stmts that follow the RecStmt

         , recS_rec_ids :: [IdP idR]
                             -- Ditto, but these variables are the "recursive" ones,
                             -- that are used before they are bound in the stmts of
                             -- the RecStmt.
            -- An Id can be in both groups
            -- Both sets of Ids are (now) treated monomorphically
            -- See Note [How RecStmt works] for why they are separate

            -- Rebindable syntax
         , recS_bind_fn :: SyntaxExpr idR -- The bind function
         , recS_ret_fn  :: SyntaxExpr idR -- The return function
         , recS_mfix_fn :: SyntaxExpr idR -- The mfix function
          }
      | XStmtLR (XXStmtLR idL idR body)

    -- Extra fields available post typechecking for RecStmt.
    data RecStmtTc =
      RecStmtTc
         { recS_bind_ty :: Type       -- S in (>>=) :: Q -> (R -> S) -> T
         , recS_later_rets :: [PostTcExpr] -- (only used in the arrow version)
         , recS_rec_rets :: [PostTcExpr] -- These expressions correspond 1-to-1
                                      -- with recS_later_ids and recS_rec_ids,
                                      -- and are the expressions that should be
                                      -- returned by the recursion.
                                      -- They may not quite be the Ids themselves,
                                      -- because the Id may be *polymorphic*, but
                                      -- the returned thing has to be *monomorphic*,
                                      -- so they may be type applications

          , recS_ret_ty :: Type        -- The type of
                                       -- do { stmts; return (a,b,c) }
                                       -- With rebindable syntax the type might not
                                       -- be quite as simple as (m (tya, tyb, tyc)).
          }

    type instance XLastStmt        (GhcPass _) (GhcPass _) b = NoExtField

    type instance XBindStmt        (GhcPass _) GhcPs b = NoExtField
    type instance XBindStmt        (GhcPass _) GhcRn b = NoExtField
    type instance XBindStmt        (GhcPass _) GhcTc b = Type

    type instance XApplicativeStmt (GhcPass _) GhcPs b = NoExtField
    type instance XApplicativeStmt (GhcPass _) GhcRn b = NoExtField
    type instance XApplicativeStmt (GhcPass _) GhcTc b = Type

    type instance XBodyStmt        (GhcPass _) GhcPs b = NoExtField
    type instance XBodyStmt        (GhcPass _) GhcRn b = NoExtField
    type instance XBodyStmt        (GhcPass _) GhcTc b = Type

    type instance XLetStmt         (GhcPass _) (GhcPass _) b = NoExtField

    type instance XParStmt         (GhcPass _) GhcPs b = NoExtField
    type instance XParStmt         (GhcPass _) GhcRn b = NoExtField
    type instance XParStmt         (GhcPass _) GhcTc b = Type

    type instance XTransStmt       (GhcPass _) GhcPs b = NoExtField
    type instance XTransStmt       (GhcPass _) GhcRn b = NoExtField
    type instance XTransStmt       (GhcPass _) GhcTc b = Type

    type instance XRecStmt         (GhcPass _) GhcPs b = NoExtField
    type instance XRecStmt         (GhcPass _) GhcRn b = NoExtField
    type instance XRecStmt         (GhcPass _) GhcTc b = RecStmtTc

    type instance XXStmtLR         (GhcPass _) (GhcPass _) b = NoExtCon

    data TransForm   -- The 'f' below is the 'using' function, 'e' is the by function
      = ThenForm     -- then f               or    then f by e             (depending on trS_by)
      | GroupForm    -- then group using f   or    then group by e using f (depending on trS_by)
      deriving Data

    -- | Parenthesised Statement Block
    data ParStmtBlock idL idR
      = ParStmtBlock
            (XParStmtBlock idL idR)
            [ExprLStmt idL]
            [IdP idR]          -- The variables to be returned
            (SyntaxExpr idR)   -- The return operator
      | XParStmtBlock (XXParStmtBlock idL idR)

    type instance XParStmtBlock  (GhcPass pL) (GhcPass pR) = NoExtField
    type instance XXParStmtBlock (GhcPass pL) (GhcPass pR) = NoExtCon

    -- | Applicative Argument
    data ApplicativeArg idL
      = ApplicativeArgOne      -- A single statement (BindStmt or BodyStmt)
        { xarg_app_arg_one  :: (XApplicativeArgOne idL)
        , app_arg_pattern   :: (LPat idL) -- WildPat if it was a BodyStmt (see below)
        , arg_expr          :: (LHsExpr idL)
        , is_body_stmt      :: Bool -- True <=> was a BodyStmt
                                  -- False <=> was a BindStmt
                                  -- See Note [Applicative BodyStmt]
        , fail_operator     :: (SyntaxExpr idL) -- The fail operator
                             -- The fail operator is needed if this is a BindStmt
                             -- where the pattern can fail. E.g.:
                             -- (Just a) <- stmt
                             -- The fail operator will be invoked if the pattern
                             -- match fails.
                             -- The fail operator is noSyntaxExpr
                             -- if the pattern match can't fail
        }
      | ApplicativeArgMany     -- do { stmts; return vars }
        { xarg_app_arg_many :: (XApplicativeArgMany idL)
        , app_stmts         :: [ExprLStmt idL] -- stmts
        , final_expr        :: (HsExpr idL)    -- return (v1,..,vn), or just (v1,..,vn)
        , bv_pattern        :: (LPat idL)      -- (v1,...,vn)
        }
      | XApplicativeArg (XXApplicativeArg idL)

    type instance XApplicativeArgOne  (GhcPass _) = NoExtField
    type instance XApplicativeArgMany (GhcPass _) = NoExtField
    type instance XXApplicativeArg    (GhcPass _) = NoExtCon

    {-
    Note [The type of bind in Stmts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Some Stmts, notably BindStmt, keep the (>>=) bind operator.
    We do NOT assume that it has type
        (>>=) :: m a -> (a -> m b) -> m b
    In some cases (see #303, #1537) it might have a more
    exotic type, such as
        (>>=) :: m i j a -> (a -> m j k b) -> m i k b
    So we must be careful not to make assumptions about the type.
    In particular, the monad may not be uniform throughout.

    Note [TransStmt binder map]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The [(idR,idR)] in a TransStmt behaves as follows:

      * Before renaming: []

      * After renaming:
              [ (x27,x27), ..., (z35,z35) ]
        These are the variables
           bound by the stmts to the left of the 'group'
           and used either in the 'by' clause,
                    or     in the stmts following the 'group'
        Each item is a pair of identical variables.

      * After typechecking:
              [ (x27:Int, x27:[Int]), ..., (z35:Bool, z35:[Bool]) ]
        Each pair has the same unique, but different *types*.

    Note [BodyStmt]
    ~~~~~~~~~~~~~~~
    BodyStmts are a bit tricky, because what they mean
    depends on the context.  Consider the following contexts:

            A do expression of type (m res_ty)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E any_ty:   do { ....; E; ... }
                    E :: m any_ty
              Translation: E >> ...

            A list comprehensions of type [elt_ty]
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E Bool:   [ .. | .... E ]
                            [ .. | ..., E, ... ]
                            [ .. | .... | ..., E | ... ]
                    E :: Bool
              Translation: if E then fail else ...

            A guard list, guarding a RHS of type rhs_ty
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E BooParStmtBlockl:   f x | ..., E, ... = ...rhs...
                    E :: Bool
              Translation: if E then fail else ...

            A monad comprehension of type (m res_ty)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            * BodyStmt E Bool:   [ .. | .... E ]
                    E :: Bool
              Translation: guard E >> ...

    Array comprehensions are handled like list comprehensions.

    Note [How RecStmt works]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Example:
       HsDo [ BindStmt x ex

            , RecStmt { recS_rec_ids   = [a, c]
                      , recS_stmts     = [ BindStmt b (return (a,c))
                                         , LetStmt a = ...b...
                                         , BindStmt c ec ]
                      , recS_later_ids = [a, b]

            , return (a b) ]

    Here, the RecStmt binds a,b,c; but
      - Only a,b are used in the stmts *following* the RecStmt,
      - Only a,c are used in the stmts *inside* the RecStmt
            *before* their bindings

    Why do we need *both* rec_ids and later_ids?  For monads they could be
    combined into a single set of variables, but not for arrows.  That
    follows from the types of the respective feedback operators:

            mfix :: MonadFix m => (a -> m a) -> m a
            loop :: ArrowLoop a => a (b,d) (c,d) -> a b c

    * For mfix, the 'a' covers the union of the later_ids and the rec_ids
    * For 'loop', 'c' is the later_ids and 'd' is the rec_ids

    Note [Typing a RecStmt]
    ~~~~~~~~~~~~~~~~~~~~~~~
    A (RecStmt stmts) types as if you had written

      (v1,..,vn, _, ..., _) <- mfix (\~(_, ..., _, r1, ..., rm) ->
                                     do { stmts
                                        ; return (v1,..vn, r1, ..., rm) })

    where v1..vn are the later_ids
          r1..rm are the rec_ids

    Note [Monad Comprehensions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Monad comprehensions require separate functions like 'return' and
    '>>=' for desugaring. These functions are stored in the statements
    used in monad comprehensions. For example, the 'return' of the 'LastStmt'
    expression is used to lift the body of the monad comprehension:

      [ body | stmts ]
       =>
      stmts >>= \bndrs -> return body

    In transform and grouping statements ('then ..' and 'then group ..') the
    'return' function is required for nested monad comprehensions, for example:

      [ body | stmts, then f, rest ]
       =>
      f [ env | stmts ] >>= \bndrs -> [ body | rest ]

    BodyStmts require the 'Control.Monad.guard' function for boolean
    expressions:

      [ body | exp, stmts ]
       =>
      guard exp >> [ body | stmts ]

    Parallel statements require the 'Control.Monad.Zip.mzip' function:

      [ body | stmts1 | stmts2 | .. ]
       =>
      mzip stmts1 (mzip stmts2 (..)) >>= \(bndrs1, (bndrs2, ..)) -> return body

    In any other context than 'MonadComp', the fields for most of these
    'SyntaxExpr's stay bottom.

    Note [Applicative BodyStmt]

    (#12143) For the purposes of ApplicativeDo, we treat any BodyStmt
    as if it was a BindStmt with a wildcard pattern.  For example,

      do
        x <- A
        B
        return x

    is transformed as if it were

      do
        x <- A
        _ <- B
        return x

    so it transforms to

      (\(x,_) -> x) <$> A <*> B

    But we have to remember when we treat a BodyStmt like a BindStmt,
    because in error messages we want to emit the original syntax the user
    wrote, not our internal representation.  So ApplicativeArgOne has a
    Bool flag that is True when the original statement was a BodyStmt, so
    that we can pretty-print it correctly.
    -}

    instance (Outputable (StmtLR idL idL (LHsExpr idL)),
              Outputable (XXParStmtBlock idL idR))
            => Outputable (ParStmtBlock idL idR) where
      ppr (ParStmtBlock _ stmts _ _) = interpp'SP stmts
      ppr (XParStmtBlock x)          = ppr x

    instance (OutputableBndrId pl, OutputableBndrId pr,
              Outputable body)
             => Outputable (StmtLR (GhcPass pl) (GhcPass pr) body) where
        ppr stmt = pprStmt stmt

    pprStmt :: forall idL idR body . (OutputableBndrId idL,
                                      OutputableBndrId idR,
                                      Outputable body)
            => (StmtLR (GhcPass idL) (GhcPass idR) body) -> SDoc
    pprStmt (LastStmt _ expr ret_stripped _)
      = whenPprDebug (text "[last]") <+>
           (if ret_stripped then text "return" else empty) <+>
           ppr expr
    pprStmt (BindStmt _ pat expr _ _) = hsep [ppr pat, larrow, ppr expr]
    pprStmt (LetStmt _ (L _ binds))   = hsep [text "let", pprBinds binds]
    pprStmt (BodyStmt _ expr _ _)     = ppr expr
    pprStmt (ParStmt _ stmtss _ _)   = sep (punctuate (text " | ") (map ppr stmtss))

    pprStmt (TransStmt { trS_stmts = stmts, trS_by = by
                       , trS_using = using, trS_form = form })
      = sep $ punctuate comma (map ppr stmts ++ [pprTransStmt by using form])

    pprStmt (RecStmt { recS_stmts = segment, recS_rec_ids = rec_ids
                     , recS_later_ids = later_ids })
      = text "rec" <+>
        vcat [ ppr_do_stmts segment
             , whenPprDebug (vcat [ text "rec_ids=" <> ppr rec_ids
                                , text "later_ids=" <> ppr later_ids])]

    pprStmt (ApplicativeStmt _ args mb_join)
      = getPprStyle $ \style ->
          if userStyle style
             then pp_for_user
             else pp_debug
      where
      -- make all the Applicative stuff invisible in error messages by
      -- flattening the whole ApplicativeStmt nest back to a sequence
      -- of statements.
       pp_for_user = vcat $ concatMap flattenArg args

       -- ppr directly rather than transforming here, because we need to
       -- inject a "return" which is hard when we're polymorphic in the id
       -- type.
       flattenStmt :: ExprLStmt (GhcPass idL) -> [SDoc]
       flattenStmt (L _ (ApplicativeStmt _ args _)) = concatMap flattenArg args
       flattenStmt stmt = [ppr stmt]

       flattenArg :: forall a . (a, ApplicativeArg (GhcPass idL)) -> [SDoc]
       flattenArg (_, ApplicativeArgOne _ pat expr isBody _)
         | isBody =  -- See Note [Applicative BodyStmt]
         [ppr (BodyStmt (panic "pprStmt") expr noSyntaxExpr noSyntaxExpr
                 :: ExprStmt (GhcPass idL))]
         | otherwise =
         [ppr (BindStmt (panic "pprStmt") pat expr noSyntaxExpr noSyntaxExpr
                 :: ExprStmt (GhcPass idL))]
       flattenArg (_, ApplicativeArgMany _ stmts _ _) =
         concatMap flattenStmt stmts
       flattenArg (_, XApplicativeArg nec) = noExtCon nec

       pp_debug =
         let
             ap_expr = sep (punctuate (text " |") (map pp_arg args))
         in
           if isNothing mb_join
              then ap_expr
              else text "join" <+> parens ap_expr

       pp_arg :: (a, ApplicativeArg (GhcPass idL)) -> SDoc
       pp_arg (_, ApplicativeArgOne _ pat expr isBody _)
         | isBody =  -- See Note [Applicative BodyStmt]
         ppr (BodyStmt (panic "pprStmt") expr noSyntaxExpr noSyntaxExpr
                :: ExprStmt (GhcPass idL))
         | otherwise =
         ppr (BindStmt (panic "pprStmt") pat expr noSyntaxExpr noSyntaxExpr
                :: ExprStmt (GhcPass idL))
       pp_arg (_, ApplicativeArgMany _ stmts return pat) =
         ppr pat <+>
         text "<-" <+>
         ppr (HsDo (panic "pprStmt") DoExpr (noLoc
                   (stmts ++
                       [noLoc (LastStmt noExtField (noLoc return) False noSyntaxExpr)])))
       pp_arg (_, XApplicativeArg x) = ppr x

    pprStmt (XStmtLR x) = ppr x

    pprTransformStmt :: (OutputableBndrId p)
                     => [IdP (GhcPass p)] -> LHsExpr (GhcPass p)
                     -> Maybe (LHsExpr (GhcPass p)) -> SDoc
    pprTransformStmt bndrs using by
      = sep [ text "then" <+> whenPprDebug (braces (ppr bndrs))
            , nest 2 (ppr using)
            , nest 2 (pprBy by)]

    pprTransStmt :: Outputable body => Maybe body -> body -> TransForm -> SDoc
    pprTransStmt by using ThenForm
      = sep [ text "then", nest 2 (ppr using), nest 2 (pprBy by)]
    pprTransStmt by using GroupForm
      = sep [ text "then group", nest 2 (pprBy by), nest 2 (ptext (sLit "using") <+> ppr using)]

    pprBy :: Outputable body => Maybe body -> SDoc
    pprBy Nothing  = empty
    pprBy (Just e) = text "by" <+> ppr e

    pprDo :: (OutputableBndrId p, Outputable body)
          => HsStmtContext any -> [LStmt (GhcPass p) body] -> SDoc
    pprDo DoExpr        stmts = text "do"  <+> ppr_do_stmts stmts
    pprDo GhciStmtCtxt  stmts = text "do"  <+> ppr_do_stmts stmts
    pprDo ArrowExpr     stmts = text "do"  <+> ppr_do_stmts stmts
    pprDo MDoExpr       stmts = text "mdo" <+> ppr_do_stmts stmts
    pprDo ListComp      stmts = brackets    $ pprComp stmts
    pprDo MonadComp     stmts = brackets    $ pprComp stmts
    pprDo _             _     = panic "pprDo" -- PatGuard, ParStmtCxt

    ppr_do_stmts :: (OutputableBndrId idL, OutputableBndrId idR,
                     Outputable body)
                 => [LStmtLR (GhcPass idL) (GhcPass idR) body] -> SDoc
    -- Print a bunch of do stmts
    ppr_do_stmts stmts = pprDeeperList vcat (map ppr stmts)

    pprComp :: (OutputableBndrId p, Outputable body)
            => [LStmt (GhcPass p) body] -> SDoc
    pprComp quals     -- Prints:  body | qual1, ..., qualn
      | Just (initStmts, L _ (LastStmt _ body _ _)) <- snocView quals
      = if null initStmts
           -- If there are no statements in a list comprehension besides the last
           -- one, we simply treat it like a normal list. This does arise
           -- occasionally in code that GHC generates, e.g., in implementations of
           -- 'range' for derived 'Ix' instances for product datatypes with exactly
           -- one constructor (e.g., see #12583).
           then ppr body
           else hang (ppr body <+> vbar) 2 (pprQuals initStmts)
      | otherwise
      = pprPanic "pprComp" (pprQuals quals)

    pprQuals :: (OutputableBndrId p, Outputable body)
             => [LStmt (GhcPass p) body] -> SDoc
    -- Show list comprehension qualifiers separated by commas
    pprQuals quals = interpp'SP quals

    {-
    ************************************************************************
    *                                                                      *
                    Template Haskell quotation brackets
    *                                                                      *
    ************************************************************************
    -}

    -- | Haskell Splice
    data HsSplice id
       = HsTypedSplice       --  $$z  or $$(f 4)
            (XTypedSplice id)
            SpliceDecoration -- Whether $$( ) variant found, for pretty printing
            (IdP id)         -- A unique name to identify this splice point
            (LHsExpr id)     -- See Note [Pending Splices]

       | HsUntypedSplice     --  $z  or $(f 4)
            (XUntypedSplice id)
            SpliceDecoration -- Whether $( ) variant found, for pretty printing
            (IdP id)         -- A unique name to identify this splice point
            (LHsExpr id)     -- See Note [Pending Splices]

       | HsQuasiQuote        -- See Note [Quasi-quote overview] in TcSplice
            (XQuasiQuote id)
            (IdP id)         -- Splice point
            (IdP id)         -- Quoter
            SrcSpan          -- The span of the enclosed string
            FastString       -- The enclosed string

       -- AZ:TODO: use XSplice instead of HsSpliced
       | HsSpliced  -- See Note [Delaying modFinalizers in untyped splices] in
                    -- RnSplice.
                    -- This is the result of splicing a splice. It is produced by
                    -- the renamer and consumed by the typechecker. It lives only
                    -- between the two.
            (XSpliced id)
            ThModFinalizers     -- TH finalizers produced by the splice.
            (HsSplicedThing id) -- The result of splicing
       | HsSplicedT
          DelayedSplice
       | XSplice (XXSplice id)  -- Note [Trees that Grow] extension point

    type instance XTypedSplice   (GhcPass _) = NoExtField
    type instance XUntypedSplice (GhcPass _) = NoExtField
    type instance XQuasiQuote    (GhcPass _) = NoExtField
    type instance XSpliced       (GhcPass _) = NoExtField
    type instance XXSplice       (GhcPass _) = NoExtCon

    -- | A splice can appear with various decorations wrapped around it. This data
    -- type captures explicitly how it was originally written, for use in the pretty
    -- printer.
    data SpliceDecoration
      = HasParens -- ^ $( splice ) or $$( splice )
      | HasDollar -- ^ $splice or $$splice
      | NoParens  -- ^ bare splice
      deriving (Data, Eq, Show)

    instance Outputable SpliceDecoration where
      ppr x = text $ show x

    isTypedSplice :: HsSplice id -> Bool
    isTypedSplice (HsTypedSplice {}) = True
    isTypedSplice _                  = False   -- Quasi-quotes are untyped splices

    -- | Finalizers produced by a splice with
    -- 'Language.Haskell.TH.Syntax.addModFinalizer'
    --
    -- See Note [Delaying modFinalizers in untyped splices] in RnSplice. For how
    -- this is used.
    --
    newtype ThModFinalizers = ThModFinalizers [ForeignRef (TH.Q ())]

    -- A Data instance which ignores the argument of 'ThModFinalizers'.
    instance Data ThModFinalizers where
      gunfold _ z _ = z $ ThModFinalizers []
      toConstr  a   = mkConstr (dataTypeOf a) "ThModFinalizers" [] Data.Prefix
      dataTypeOf a  = mkDataType "HsExpr.ThModFinalizers" [toConstr a]

    -- See Note [Running typed splices in the zonker]
    -- These are the arguments that are passed to `TcSplice.runTopSplice`
    data DelayedSplice =
      DelayedSplice
        TcLclEnv          -- The local environment to run the splice in
        (LHsExpr GhcRn)   -- The original renamed expression
        TcType            -- The result type of running the splice, unzonked
        (LHsExpr GhcTcId) -- The typechecked expression to run and splice in the result

    -- A Data instance which ignores the argument of 'DelayedSplice'.
    instance Data DelayedSplice where
      gunfold _ _ _ = panic "DelayedSplice"
      toConstr  a   = mkConstr (dataTypeOf a) "DelayedSplice" [] Data.Prefix
      dataTypeOf a  = mkDataType "HsExpr.DelayedSplice" [toConstr a]

    -- | Haskell Spliced Thing
    --
    -- Values that can result from running a splice.
    data HsSplicedThing id
        = HsSplicedExpr (HsExpr id) -- ^ Haskell Spliced Expression
        | HsSplicedTy   (HsType id) -- ^ Haskell Spliced Type
        | HsSplicedPat  (Pat id)    -- ^ Haskell Spliced Pattern

    -- See Note [Pending Splices]
    type SplicePointName = Name

    -- | Pending Renamer Splice
    data PendingRnSplice
      = PendingRnSplice UntypedSpliceFlavour SplicePointName (LHsExpr GhcRn)

    data UntypedSpliceFlavour
      = UntypedExpSplice
      | UntypedPatSplice
      | UntypedTypeSplice
      | UntypedDeclSplice
      deriving Data

    -- | Pending Type-checker Splice
    data PendingTcSplice
      = PendingTcSplice SplicePointName (LHsExpr GhcTc)

    {-
    Note [Pending Splices]
    ~~~~~~~~~~~~~~~~~~~~~~
    When we rename an untyped bracket, we name and lift out all the nested
    splices, so that when the typechecker hits the bracket, it can
    typecheck those nested splices without having to walk over the untyped
    bracket code.  So for example
        [| f $(g x) |]
    looks like

        HsBracket (HsApp (HsVar "f") (HsSpliceE _ (g x)))

    which the renamer rewrites to

        HsRnBracketOut (HsApp (HsVar f) (HsSpliceE sn (g x)))
                       [PendingRnSplice UntypedExpSplice sn (g x)]

    * The 'sn' is the Name of the splice point, the SplicePointName

    * The PendingRnExpSplice gives the splice that splice-point name maps to;
      and the typechecker can now conveniently find these sub-expressions

    * The other copy of the splice, in the second argument of HsSpliceE
                                    in the renamed first arg of HsRnBracketOut
      is used only for pretty printing

    There are four varieties of pending splices generated by the renamer,
    distinguished by their UntypedSpliceFlavour

     * Pending expression splices (UntypedExpSplice), e.g.,
           [|$(f x) + 2|]

       UntypedExpSplice is also used for
         * quasi-quotes, where the pending expression expands to
              $(quoter "...blah...")
           (see RnSplice.makePending, HsQuasiQuote case)

         * cross-stage lifting, where the pending expression expands to
              $(lift x)
           (see RnSplice.checkCrossStageLifting)

     * Pending pattern splices (UntypedPatSplice), e.g.,
           [| \$(f x) -> x |]

     * Pending type splices (UntypedTypeSplice), e.g.,
           [| f :: $(g x) |]

     * Pending declaration (UntypedDeclSplice), e.g.,
           [| let $(f x) in ... |]

    There is a fifth variety of pending splice, which is generated by the type
    checker:

      * Pending *typed* expression splices, (PendingTcSplice), e.g.,
            [||1 + $$(f 2)||]

    It would be possible to eliminate HsRnBracketOut and use HsBracketOut for the
    output of the renamer. However, when pretty printing the output of the renamer,
    e.g., in a type error message, we *do not* want to print out the pending
    splices. In contrast, when pretty printing the output of the type checker, we
    *do* want to print the pending splices. So splitting them up seems to make
    sense, although I hate to add another constructor to HsExpr.
    -}

    instance OutputableBndrId p
           => Outputable (HsSplicedThing (GhcPass p)) where
      ppr (HsSplicedExpr e) = ppr_expr e
      ppr (HsSplicedTy   t) = ppr t
      ppr (HsSplicedPat  p) = ppr p

    instance (OutputableBndrId p) => Outputable (HsSplice (GhcPass p)) where
      ppr s = pprSplice s

    pprPendingSplice :: (OutputableBndrId p)
                     => SplicePointName -> LHsExpr (GhcPass p) -> SDoc
    pprPendingSplice n e = angleBrackets (ppr n <> comma <+> ppr e)

    pprSpliceDecl ::  (OutputableBndrId p)
              => HsSplice (GhcPass p) -> SpliceExplicitFlag -> SDoc
    pprSpliceDecl e@HsQuasiQuote{} _ = pprSplice e
    pprSpliceDecl e ExplicitSplice   = text "$(" <> ppr_splice_decl e <> text ")"
    pprSpliceDecl e ImplicitSplice   = ppr_splice_decl e

    ppr_splice_decl :: (OutputableBndrId p)
                    => HsSplice (GhcPass p) -> SDoc
    ppr_splice_decl (HsUntypedSplice _ _ n e) = ppr_splice empty n e empty
    ppr_splice_decl e = pprSplice e

    pprSplice :: (OutputableBndrId p) => HsSplice (GhcPass p) -> SDoc
    pprSplice (HsTypedSplice _ HasParens  n e)
      = ppr_splice (text "$$(") n e (text ")")
    pprSplice (HsTypedSplice _ HasDollar n e)
      = ppr_splice (text "$$") n e empty
    pprSplice (HsTypedSplice _ NoParens n e)
      = ppr_splice empty n e empty
    pprSplice (HsUntypedSplice _ HasParens  n e)
      = ppr_splice (text "$(") n e (text ")")
    pprSplice (HsUntypedSplice _ HasDollar n e)
      = ppr_splice (text "$")  n e empty
    pprSplice (HsUntypedSplice _ NoParens n e)
      = ppr_splice empty  n e empty
    pprSplice (HsQuasiQuote _ n q _ s)      = ppr_quasi n q s
    pprSplice (HsSpliced _ _ thing)         = ppr thing
    pprSplice (HsSplicedT {})               = text "Unevaluated typed splice"
    pprSplice (XSplice x)                   = ppr x

    ppr_quasi :: OutputableBndr p => p -> p -> FastString -> SDoc
    ppr_quasi n quoter quote = whenPprDebug (brackets (ppr n)) <>
                               char '[' <> ppr quoter <> vbar <>
                               ppr quote <> text "|]"

    ppr_splice :: (OutputableBndrId p)
               => SDoc -> (IdP (GhcPass p)) -> LHsExpr (GhcPass p) -> SDoc -> SDoc
    ppr_splice herald n e trail
        = herald <> whenPprDebug (brackets (ppr n)) <> ppr e <> trail

    -- | Haskell Bracket
    data HsBracket p
      = ExpBr  (XExpBr p)   (LHsExpr p)    -- [|  expr  |]
      | PatBr  (XPatBr p)   (LPat p)      -- [p| pat   |]
      | DecBrL (XDecBrL p)  [LHsDecl p]   -- [d| decls |]; result of parser
      | DecBrG (XDecBrG p)  (HsGroup p)   -- [d| decls |]; result of renamer
      | TypBr  (XTypBr p)   (LHsType p)   -- [t| type  |]
      | VarBr  (XVarBr p)   Bool (IdP p)  -- True: 'x, False: ''T
                                    -- (The Bool flag is used only in pprHsBracket)
      | TExpBr (XTExpBr p) (LHsExpr p)    -- [||  expr  ||]
      | XBracket (XXBracket p)            -- Note [Trees that Grow] extension point

    type instance XExpBr      (GhcPass _) = NoExtField
    type instance XPatBr      (GhcPass _) = NoExtField
    type instance XDecBrL     (GhcPass _) = NoExtField
    type instance XDecBrG     (GhcPass _) = NoExtField
    type instance XTypBr      (GhcPass _) = NoExtField
    type instance XVarBr      (GhcPass _) = NoExtField
    type instance XTExpBr     (GhcPass _) = NoExtField
    type instance XXBracket   (GhcPass _) = NoExtCon

    isTypedBracket :: HsBracket id -> Bool
    isTypedBracket (TExpBr {}) = True
    isTypedBracket _           = False

    instance OutputableBndrId p
              => Outputable (HsBracket (GhcPass p)) where
      ppr = pprHsBracket

    pprHsBracket :: (OutputableBndrId p) => HsBracket (GhcPass p) -> SDoc
    pprHsBracket (ExpBr _ e)   = thBrackets empty (ppr e)
    pprHsBracket (PatBr _ p)   = thBrackets (char 'p') (ppr p)
    pprHsBracket (DecBrG _ gp) = thBrackets (char 'd') (ppr gp)
    pprHsBracket (DecBrL _ ds) = thBrackets (char 'd') (vcat (map ppr ds))
    pprHsBracket (TypBr _ t)   = thBrackets (char 't') (ppr t)
    pprHsBracket (VarBr _ True n)
      = char '\'' <> pprPrefixOcc n
    pprHsBracket (VarBr _ False n)
      = text "''" <> pprPrefixOcc n
    pprHsBracket (TExpBr _ e)  = thTyBrackets (ppr e)
    pprHsBracket (XBracket e)  = ppr e

    thBrackets :: SDoc -> SDoc -> SDoc
    thBrackets pp_kind pp_body = char '[' <> pp_kind <> vbar <+>
                                 pp_body <+> text "|]"

    thTyBrackets :: SDoc -> SDoc
    thTyBrackets pp_body = text "[||" <+> pp_body <+> ptext (sLit "||]")

    instance Outputable PendingRnSplice where
      ppr (PendingRnSplice _ n e) = pprPendingSplice n e

    instance Outputable PendingTcSplice where
      ppr (PendingTcSplice n e) = pprPendingSplice n e

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Enumerations and list comprehensions}
    *                                                                      *
    ************************************************************************
    -}

    -- | Arithmetic Sequence Information
    data ArithSeqInfo id
      = From            (LHsExpr id)
      | FromThen        (LHsExpr id)
                        (LHsExpr id)
      | FromTo          (LHsExpr id)
                        (LHsExpr id)
      | FromThenTo      (LHsExpr id)
                        (LHsExpr id)
                        (LHsExpr id)
    -- AZ: Sould ArithSeqInfo have a TTG extension?

    instance OutputableBndrId p
             => Outputable (ArithSeqInfo (GhcPass p)) where
        ppr (From e1)             = hcat [ppr e1, pp_dotdot]
        ppr (FromThen e1 e2)      = hcat [ppr e1, comma, space, ppr e2, pp_dotdot]
        ppr (FromTo e1 e3)        = hcat [ppr e1, pp_dotdot, ppr e3]
        ppr (FromThenTo e1 e2 e3)
          = hcat [ppr e1, comma, space, ppr e2, pp_dotdot, ppr e3]

    pp_dotdot :: SDoc
    pp_dotdot = text " .. "

    {-
    ************************************************************************
    *                                                                      *
    \subsection{HsMatchCtxt}
    *                                                                      *
    ************************************************************************
    -}

    -- | Haskell Match Context
    --
    -- Context of a pattern match. This is more subtle than it would seem. See Note
    -- [Varieties of pattern matches].
    data HsMatchContext id -- Not an extensible tag
      = FunRhs { mc_fun        :: Located id    -- ^ function binder of @f@
               , mc_fixity     :: LexicalFixity -- ^ fixing of @f@
               , mc_strictness :: SrcStrictness -- ^ was @f@ banged?
                                                -- See Note [FunBind vs PatBind]
               }
                                    -- ^A pattern matching on an argument of a
                                    -- function binding
      | LambdaExpr                  -- ^Patterns of a lambda
      | CaseAlt                     -- ^Patterns and guards on a case alternative
      | IfAlt                       -- ^Guards of a multi-way if alternative
      | ProcExpr                    -- ^Patterns of a proc
      | PatBindRhs                  -- ^A pattern binding  eg [y] <- e = e
      | PatBindGuards               -- ^Guards of pattern bindings, e.g.,
                                    --    (Just b) | Just _ <- x = e
                                    --             | otherwise   = e'

      | RecUpd                      -- ^Record update [used only in DsExpr to
                                    --    tell matchWrapper what sort of
                                    --    runtime error message to generate]

      | StmtCtxt (HsStmtContext id) -- ^Pattern of a do-stmt, list comprehension,
                                    -- pattern guard, etc

      | ThPatSplice            -- ^A Template Haskell pattern splice
      | ThPatQuote             -- ^A Template Haskell pattern quotation [p| (a,b) |]
      | PatSyn                 -- ^A pattern synonym declaration
      deriving Functor
    deriving instance (Data id) => Data (HsMatchContext id)

    instance OutputableBndr id => Outputable (HsMatchContext id) where
      ppr m@(FunRhs{})          = text "FunRhs" <+> ppr (mc_fun m) <+> ppr (mc_fixity m)
      ppr LambdaExpr            = text "LambdaExpr"
      ppr CaseAlt               = text "CaseAlt"
      ppr IfAlt                 = text "IfAlt"
      ppr ProcExpr              = text "ProcExpr"
      ppr PatBindRhs            = text "PatBindRhs"
      ppr PatBindGuards         = text "PatBindGuards"
      ppr RecUpd                = text "RecUpd"
      ppr (StmtCtxt _)          = text "StmtCtxt _"
      ppr ThPatSplice           = text "ThPatSplice"
      ppr ThPatQuote            = text "ThPatQuote"
      ppr PatSyn                = text "PatSyn"

    isPatSynCtxt :: HsMatchContext id -> Bool
    isPatSynCtxt ctxt =
      case ctxt of
        PatSyn -> True
        _      -> False

    -- | Haskell Statement Context. It expects to be parameterised with one of
    -- 'RdrName', 'Name' or 'Id'
    data HsStmtContext id
      = ListComp
      | MonadComp

      | DoExpr                           -- ^do { ... }
      | MDoExpr                          -- ^mdo { ... }  ie recursive do-expression
      | ArrowExpr                        -- ^do-notation in an arrow-command context

      | GhciStmtCtxt                     -- ^A command-line Stmt in GHCi pat <- rhs
      | PatGuard (HsMatchContext id)     -- ^Pattern guard for specified thing
      | ParStmtCtxt (HsStmtContext id)   -- ^A branch of a parallel stmt
      | TransStmtCtxt (HsStmtContext id) -- ^A branch of a transform stmt
      deriving Functor
    deriving instance (Data id) => Data (HsStmtContext id)

    isComprehensionContext :: HsStmtContext id -> Bool
    -- Uses comprehension syntax [ e | quals ]
    isComprehensionContext ListComp          = True
    isComprehensionContext MonadComp         = True
    isComprehensionContext (ParStmtCtxt c)   = isComprehensionContext c
    isComprehensionContext (TransStmtCtxt c) = isComprehensionContext c
    isComprehensionContext _ = False

    -- | Should pattern match failure in a 'HsStmtContext' be desugared using
    -- 'MonadFail'?
    isMonadFailStmtContext :: HsStmtContext id -> Bool
    isMonadFailStmtContext MonadComp            = True
    isMonadFailStmtContext DoExpr               = True
    isMonadFailStmtContext MDoExpr              = True
    isMonadFailStmtContext GhciStmtCtxt         = True
    isMonadFailStmtContext (ParStmtCtxt ctxt)   = isMonadFailStmtContext ctxt
    isMonadFailStmtContext (TransStmtCtxt ctxt) = isMonadFailStmtContext ctxt
    isMonadFailStmtContext _ = False -- ListComp, PatGuard, ArrowExpr

    isMonadCompContext :: HsStmtContext id -> Bool
    isMonadCompContext MonadComp = True
    isMonadCompContext _         = False

    matchSeparator :: HsMatchContext id -> SDoc
    matchSeparator (FunRhs {})   = text "="
    matchSeparator CaseAlt       = text "->"
    matchSeparator IfAlt         = text "->"
    matchSeparator LambdaExpr    = text "->"
    matchSeparator ProcExpr      = text "->"
    matchSeparator PatBindRhs    = text "="
    matchSeparator PatBindGuards = text "="
    matchSeparator (StmtCtxt _)  = text "<-"
    matchSeparator RecUpd        = text "=" -- This can be printed by the pattern
                                           -- match checker trace
    matchSeparator ThPatSplice  = panic "unused"
    matchSeparator ThPatQuote   = panic "unused"
    matchSeparator PatSyn       = panic "unused"

    pprMatchContext :: (Outputable (NameOrRdrName id),Outputable id)
                    => HsMatchContext id -> SDoc
    pprMatchContext ctxt
      | want_an ctxt = text "an" <+> pprMatchContextNoun ctxt
      | otherwise    = text "a"  <+> pprMatchContextNoun ctxt
      where
        want_an (FunRhs {}) = True  -- Use "an" in front
        want_an ProcExpr    = True
        want_an _           = False

    pprMatchContextNoun :: (Outputable (NameOrRdrName id),Outputable id)
                        => HsMatchContext id -> SDoc
    pprMatchContextNoun (FunRhs {mc_fun=L _ fun})
                                        = text "equation for"
                                          <+> quotes (ppr fun)
    pprMatchContextNoun CaseAlt         = text "case alternative"
    pprMatchContextNoun IfAlt           = text "multi-way if alternative"
    pprMatchContextNoun RecUpd          = text "record-update construct"
    pprMatchContextNoun ThPatSplice     = text "Template Haskell pattern splice"
    pprMatchContextNoun ThPatQuote      = text "Template Haskell pattern quotation"
    pprMatchContextNoun PatBindRhs      = text "pattern binding"
    pprMatchContextNoun PatBindGuards   = text "pattern binding guards"
    pprMatchContextNoun LambdaExpr      = text "lambda abstraction"
    pprMatchContextNoun ProcExpr        = text "arrow abstraction"
    pprMatchContextNoun (StmtCtxt ctxt) = text "pattern binding in"
                                          $$ pprAStmtContext ctxt
    pprMatchContextNoun PatSyn          = text "pattern synonym declaration"

    -----------------
    pprAStmtContext, pprStmtContext :: (Outputable id,
                                        Outputable (NameOrRdrName id))
                                    => HsStmtContext id -> SDoc
    pprAStmtContext ctxt = article <+> pprStmtContext ctxt
      where
        pp_an = text "an"
        pp_a  = text "a"
        article = case ctxt of
                      MDoExpr       -> pp_an
                      GhciStmtCtxt  -> pp_an
                      _             -> pp_a

    -----------------
    pprStmtContext GhciStmtCtxt    = text "interactive GHCi command"
    pprStmtContext DoExpr          = text "'do' block"
    pprStmtContext MDoExpr         = text "'mdo' block"
    pprStmtContext ArrowExpr       = text "'do' block in an arrow command"
    pprStmtContext ListComp        = text "list comprehension"
    pprStmtContext MonadComp       = text "monad comprehension"
    pprStmtContext (PatGuard ctxt) = text "pattern guard for" $$ pprMatchContext ctxt

    -- Drop the inner contexts when reporting errors, else we get
    --     Unexpected transform statement
    --     in a transformed branch of
    --          transformed branch of
    --          transformed branch of monad comprehension
    pprStmtContext (ParStmtCtxt c) =
      ifPprDebug (sep [text "parallel branch of", pprAStmtContext c])
                 (pprStmtContext c)
    pprStmtContext (TransStmtCtxt c) =
      ifPprDebug (sep [text "transformed branch of", pprAStmtContext c])
                 (pprStmtContext c)

    instance (Outputable (GhcPass p), Outputable (NameOrRdrName (GhcPass p)))
          => Outputable (HsStmtContext (GhcPass p)) where
        ppr = pprStmtContext

    -- Used to generate the string for a *runtime* error message
    matchContextErrString :: Outputable id
                          => HsMatchContext id -> SDoc
    matchContextErrString (FunRhs{mc_fun=L _ fun})   = text "function" <+> ppr fun
    matchContextErrString CaseAlt                    = text "case"
    matchContextErrString IfAlt                      = text "multi-way if"
    matchContextErrString PatBindRhs                 = text "pattern binding"
    matchContextErrString PatBindGuards              = text "pattern binding guards"
    matchContextErrString RecUpd                     = text "record update"
    matchContextErrString LambdaExpr                 = text "lambda"
    matchContextErrString ProcExpr                   = text "proc"
    matchContextErrString ThPatSplice                = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString ThPatQuote                 = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString PatSyn                     = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString (StmtCtxt (ParStmtCtxt c))   = matchContextErrString (StmtCtxt c)
    matchContextErrString (StmtCtxt (TransStmtCtxt c)) = matchContextErrString (StmtCtxt c)
    matchContextErrString (StmtCtxt (PatGuard _))      = text "pattern guard"
    matchContextErrString (StmtCtxt GhciStmtCtxt)      = text "interactive GHCi command"
    matchContextErrString (StmtCtxt DoExpr)            = text "'do' block"
    matchContextErrString (StmtCtxt ArrowExpr)         = text "'do' block"
    matchContextErrString (StmtCtxt MDoExpr)           = text "'mdo' block"
    matchContextErrString (StmtCtxt ListComp)          = text "list comprehension"
    matchContextErrString (StmtCtxt MonadComp)         = text "monad comprehension"

    pprMatchInCtxt :: (OutputableBndrId idR,
                       -- TODO:AZ these constraints do not make sense
                     Outputable (NameOrRdrName (NameOrRdrName (IdP (GhcPass idR)))),
                     Outputable body)
                   => Match (GhcPass idR) body -> SDoc
    pprMatchInCtxt match  = hang (text "In" <+> pprMatchContext (m_ctxt match)
                                            <> colon)
                                 4 (pprMatch match)

    pprStmtInCtxt :: (OutputableBndrId idL,
                      OutputableBndrId idR,
                      Outputable body)
                  => HsStmtContext (IdP (GhcPass idL))
                  -> StmtLR (GhcPass idL) (GhcPass idR) body
                  -> SDoc
    pprStmtInCtxt ctxt (LastStmt _ e _ _)
      | isComprehensionContext ctxt      -- For [ e | .. ], do not mutter about "stmts"
      = hang (text "In the expression:") 2 (ppr e)

    pprStmtInCtxt ctxt stmt
      = hang (text "In a stmt of" <+> pprAStmtContext ctxt <> colon)
           2 (ppr_stmt stmt)
      where
        -- For Group and Transform Stmts, don't print the nested stmts!
        ppr_stmt (TransStmt { trS_by = by, trS_using = using
                            , trS_form = form }) = pprTransStmt by using form
        ppr_stmt stmt = pprStmt stmt