GHC/Hs/Pat.hs

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

    \section[PatSyntax]{Abstract Haskell syntax---patterns}
    -}

    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE DeriveFunctor #-}
    {-# LANGUAGE DeriveFoldable #-}
    {-# LANGUAGE DeriveTraversable #-}
    {-# LANGUAGE CPP #-}
    {-# LANGUAGE StandaloneDeriving #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE UndecidableInstances #-} -- Wrinkle in Note [Trees That Grow]
                                          -- in module GHC.Hs.Extension
    {-# LANGUAGE ConstraintKinds #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE ViewPatterns      #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TypeApplications #-}
    {-# LANGUAGE LambdaCase #-}

    module GHC.Hs.Pat (
            Pat(..), LPat,
            ConPatTc (..),
            CoPat (..),
            ListPatTc(..),
            ConLikeP,

            HsConPatDetails, hsConPatArgs,
            HsRecFields(..), HsRecField'(..), LHsRecField',
            HsRecField, LHsRecField,
            HsRecUpdField, LHsRecUpdField,
            hsRecFields, hsRecFieldSel, hsRecFieldId, hsRecFieldsArgs,
            hsRecUpdFieldId, hsRecUpdFieldOcc, hsRecUpdFieldRdr,

            mkPrefixConPat, mkCharLitPat, mkNilPat,

            isSimplePat,
            looksLazyPatBind,
            isBangedLPat,
            patNeedsParens, parenthesizePat,
            isIrrefutableHsPat,

            collectEvVarsPat, collectEvVarsPats,

            pprParendLPat, pprConArgs
        ) where

    import GHC.Prelude

    import {-# SOURCE #-} GHC.Hs.Expr (SyntaxExpr, LHsExpr, HsSplice, pprLExpr, pprSplice)

    -- friends:
    import GHC.Hs.Binds
    import GHC.Hs.Lit
    import GHC.Hs.Extension
    import GHC.Hs.Type
    import GHC.Tc.Types.Evidence
    import GHC.Types.Basic
    -- others:
    import GHC.Core.Ppr ( {- instance OutputableBndr TyVar -} )
    import GHC.Builtin.Types
    import GHC.Types.Var
    import GHC.Types.Name.Reader ( RdrName )
    import GHC.Core.ConLike
    import GHC.Core.DataCon
    import GHC.Core.TyCon
    import GHC.Utils.Outputable
    import GHC.Core.Type
    import GHC.Types.SrcLoc
    import GHC.Data.Bag -- collect ev vars from pats
    import GHC.Data.Maybe
    import GHC.Types.Name (Name)
    -- libraries:
    import Data.Data hiding (TyCon,Fixity)

    type LPat p = XRec p Pat

    -- | Pattern
    --
    -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnBang'

    -- For details on above see note [Api annotations] in GHC.Parser.Annotation
    data Pat p
      =     ------------ Simple patterns ---------------
        WildPat     (XWildPat p)        -- ^ Wildcard Pattern
            -- The sole reason for a type on a WildPat is to
            -- support hsPatType :: Pat Id -> Type

           -- AZ:TODO above comment needs to be updated
      | VarPat      (XVarPat p)
                    (Located (IdP p))  -- ^ Variable Pattern

                                 -- See Note [Located RdrNames] in GHC.Hs.Expr
      | LazyPat     (XLazyPat p)
                    (LPat p)                -- ^ Lazy Pattern
        -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnTilde'

        -- For details on above see note [Api annotations] in GHC.Parser.Annotation

      | AsPat       (XAsPat p)
                    (Located (IdP p)) (LPat p)    -- ^ As pattern
        -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnAt'

        -- For details on above see note [Api annotations] in GHC.Parser.Annotation

      | ParPat      (XParPat p)
                    (LPat p)                -- ^ Parenthesised pattern
                                            -- See Note [Parens in HsSyn] in GHC.Hs.Expr
        -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'('@,
        --                                    'GHC.Parser.Annotation.AnnClose' @')'@

        -- For details on above see note [Api annotations] in GHC.Parser.Annotation
      | BangPat     (XBangPat p)
                    (LPat p)                -- ^ Bang pattern
        -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnBang'

        -- For details on above see note [Api annotations] in GHC.Parser.Annotation

            ------------ Lists, tuples, arrays ---------------
      | ListPat     (XListPat p)
                    [LPat p]
                       -- For OverloadedLists a Just (ty,fn) gives
                       -- overall type of the pattern, and the toList
    -- function to convert the scrutinee to a list value

        -- ^ Syntactic List
        --
        -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'['@,
        --                                    'GHC.Parser.Annotation.AnnClose' @']'@

        -- For details on above see note [Api annotations] in GHC.Parser.Annotation

      | TuplePat    (XTuplePat p)
                      -- after typechecking, holds the types of the tuple components
                    [LPat p]         -- Tuple sub-patterns
                    Boxity           -- UnitPat is TuplePat []
            -- You might think that the post typechecking Type was redundant,
            -- because we can get the pattern type by getting the types of the
            -- sub-patterns.
            -- But it's essential
            --      data T a where
            --        T1 :: Int -> T Int
            --      f :: (T a, a) -> Int
            --      f (T1 x, z) = z
            -- When desugaring, we must generate
            --      f = /\a. \v::a.  case v of (t::T a, w::a) ->
            --                       case t of (T1 (x::Int)) ->
            -- Note the (w::a), NOT (w::Int), because we have not yet
            -- refined 'a' to Int.  So we must know that the second component
            -- of the tuple is of type 'a' not Int.  See selectMatchVar
            -- (June 14: I'm not sure this comment is right; the sub-patterns
            --           will be wrapped in CoPats, no?)
        -- ^ Tuple sub-patterns
        --
        -- - 'GHC.Parser.Annotation.AnnKeywordId' :
        --            'GHC.Parser.Annotation.AnnOpen' @'('@ or @'(#'@,
        --            'GHC.Parser.Annotation.AnnClose' @')'@ or  @'#)'@

      | SumPat      (XSumPat p)        -- after typechecker, types of the alternative
                    (LPat p)           -- Sum sub-pattern
                    ConTag             -- Alternative (one-based)
                    Arity              -- Arity (INVARIANT: ≥ 2)
        -- ^ Anonymous sum pattern
        --
        -- - 'GHC.Parser.Annotation.AnnKeywordId' :
        --            'GHC.Parser.Annotation.AnnOpen' @'(#'@,
        --            'GHC.Parser.Annotation.AnnClose' @'#)'@

        -- For details on above see note [Api annotations] in GHC.Parser.Annotation

            ------------ Constructor patterns ---------------
      | ConPat {
            pat_con_ext :: XConPat p,
            pat_con     :: Located (ConLikeP p),
            pat_args    :: HsConPatDetails p
        }
        -- ^ Constructor Pattern

            ------------ View patterns ---------------
      -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnRarrow'

      -- For details on above see note [Api annotations] in GHC.Parser.Annotation
      | ViewPat       (XViewPat p)     -- The overall type of the pattern
                                       -- (= the argument type of the view function)
                                       -- for hsPatType.
                      (LHsExpr p)
                      (LPat p)
        -- ^ View Pattern

            ------------ Pattern splices ---------------
      -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'$('@
      --        'GHC.Parser.Annotation.AnnClose' @')'@

      -- For details on above see note [Api annotations] in GHC.Parser.Annotation
      | SplicePat       (XSplicePat p)
                        (HsSplice p)    -- ^ Splice Pattern (Includes quasi-quotes)

            ------------ Literal and n+k patterns ---------------
      | LitPat          (XLitPat p)
                        (HsLit p)           -- ^ Literal Pattern
                                            -- Used for *non-overloaded* literal patterns:
                                            -- Int#, Char#, Int, Char, String, etc.

      | NPat                -- Natural Pattern
                            -- Used for all overloaded literals,
                            -- including overloaded strings with -XOverloadedStrings
                        (XNPat p)            -- Overall type of pattern. Might be
                                             -- different than the literal's type
                                             -- if (==) or negate changes the type
                        (Located (HsOverLit p))     -- ALWAYS positive
                        (Maybe (SyntaxExpr p)) -- Just (Name of 'negate') for
                                               -- negative patterns, Nothing
                                               -- otherwise
                        (SyntaxExpr p)       -- Equality checker, of type t->t->Bool

      -- ^ Natural Pattern
      --
      -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnVal' @'+'@

      -- For details on above see note [Api annotations] in GHC.Parser.Annotation
      | NPlusKPat       (XNPlusKPat p)           -- Type of overall pattern
                        (Located (IdP p))        -- n+k pattern
                        (Located (HsOverLit p))  -- It'll always be an HsIntegral
                        (HsOverLit p)            -- See Note [NPlusK patterns] in GHC.Tc.Gen.Pat
                         -- NB: This could be (PostTc ...), but that induced a
                         -- a new hs-boot file. Not worth it.

                        (SyntaxExpr p)   -- (>=) function, of type t1->t2->Bool
                        (SyntaxExpr p)   -- Name of '-' (see GHC.Rename.Env.lookupSyntax)
      -- ^ n+k pattern

            ------------ Pattern type signatures ---------------
      -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnDcolon'

      -- For details on above see note [Api annotations] in GHC.Parser.Annotation
      | SigPat          (XSigPat p)             -- After typechecker: Type
                        (LPat p)                -- Pattern with a type signature
                        (HsPatSigType (NoGhcTc p)) --  Signature can bind both
                                                   --  kind and type vars

        -- ^ Pattern with a type signature

      -- | Trees that Grow extension point for new constructors
      | XPat
          !(XXPat p)

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

    data ListPatTc
      = ListPatTc
          Type                             -- The type of the elements
          (Maybe (Type, SyntaxExpr GhcTc)) -- For rebindable syntax

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

    type instance XVarPat  (GhcPass _) = NoExtField
    type instance XLazyPat (GhcPass _) = NoExtField
    type instance XAsPat   (GhcPass _) = NoExtField
    type instance XParPat  (GhcPass _) = NoExtField
    type instance XBangPat (GhcPass _) = NoExtField

    -- Note: XListPat cannot be extended when using GHC 8.0.2 as the bootstrap
    -- compiler, as it triggers https://gitlab.haskell.org/ghc/ghc/issues/14396 for
    -- `SyntaxExpr`
    type instance XListPat GhcPs = NoExtField
    type instance XListPat GhcRn = Maybe (SyntaxExpr GhcRn)
    type instance XListPat GhcTc = ListPatTc

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

    type instance XConPat GhcPs = NoExtField
    type instance XConPat GhcRn = NoExtField
    type instance XConPat GhcTc = ConPatTc

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

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

    type instance XSplicePat (GhcPass _) = NoExtField
    type instance XLitPat    (GhcPass _) = NoExtField

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

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

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

    type instance XXPat GhcPs = NoExtCon
    type instance XXPat GhcRn = NoExtCon
    type instance XXPat GhcTc = CoPat
      -- After typechecking, we add one extra constructor: CoPat

    type family ConLikeP x

    type instance ConLikeP GhcPs = RdrName -- IdP GhcPs
    type instance ConLikeP GhcRn = Name -- IdP GhcRn
    type instance ConLikeP GhcTc = ConLike

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

    -- | Haskell Constructor Pattern Details
    type HsConPatDetails p = HsConDetails (LPat p) (HsRecFields p (LPat p))

    hsConPatArgs :: HsConPatDetails p -> [LPat p]
    hsConPatArgs (PrefixCon ps)   = ps
    hsConPatArgs (RecCon fs)      = map (hsRecFieldArg . unLoc) (rec_flds fs)
    hsConPatArgs (InfixCon p1 p2) = [p1,p2]

    -- | This is the extension field for ConPat, added after typechecking
    -- It adds quite a few extra fields, to support elaboration of pattern matching.
    data ConPatTc
      = ConPatTc
        { -- | The universal arg types  1-1 with the universal
          -- tyvars of the constructor/pattern synonym
          -- Use (conLikeResTy pat_con cpt_arg_tys) to get
          -- the type of the pattern
          cpt_arg_tys :: [Type]

        , -- | Existentially bound type variables
          -- in correctly-scoped order e.g. [k:*  x:k]
          cpt_tvs   :: [TyVar]

        , -- | Ditto *coercion variables* and *dictionaries*
          -- One reason for putting coercion variable here  I think
          --      is to ensure their kinds are zonked
          cpt_dicts :: [EvVar]

        , -- | Bindings involving those dictionaries
          cpt_binds :: TcEvBinds

        , -- ^ Extra wrapper to pass to the matcher
          -- Only relevant for pattern-synonyms;
          --   ignored for data cons
          cpt_wrap  :: HsWrapper
        }

    -- | Coercion Pattern (translation only)
    --
    -- During desugaring a (CoPat co pat) turns into a cast with 'co' on the
    -- scrutinee, followed by a match on 'pat'.
    data CoPat
      = CoPat
        { -- | Coercion Pattern
          -- If co :: t1 ~ t2, p :: t2,
          -- then (CoPat co p) :: t1
          co_cpt_wrap :: HsWrapper

        , -- | Why not LPat?  Ans: existing locn will do
          co_pat_inner :: Pat GhcTc

        , -- | Type of whole pattern, t1
          co_pat_ty :: Type
        }

    -- | Haskell Record Fields
    --
    -- HsRecFields is used only for patterns and expressions (not data type
    -- declarations)
    data HsRecFields p arg         -- A bunch of record fields
                                    --      { x = 3, y = True }
            -- Used for both expressions and patterns
      = HsRecFields { rec_flds   :: [LHsRecField p arg],
                      rec_dotdot :: Maybe (Located Int) }  -- Note [DotDot fields]
      deriving (Functor, Foldable, Traversable)

    -- Note [DotDot fields]
    -- ~~~~~~~~~~~~~~~~~~~~
    -- The rec_dotdot field means this:
    --   Nothing => the normal case
    --   Just n  => the group uses ".." notation,
    --
    -- In the latter case:
    --
    --   *before* renamer: rec_flds are exactly the n user-written fields
    --
    --   *after* renamer:  rec_flds includes *all* fields, with
    --                     the first 'n' being the user-written ones
    --                     and the remainder being 'filled in' implicitly

    -- | Located Haskell Record Field
    type LHsRecField' p arg = Located (HsRecField' p arg)

    -- | Located Haskell Record Field
    type LHsRecField  p arg = Located (HsRecField  p arg)

    -- | Located Haskell Record Update Field
    type LHsRecUpdField p   = Located (HsRecUpdField p)

    -- | Haskell Record Field
    type HsRecField    p arg = HsRecField' (FieldOcc p) arg

    -- | Haskell Record Update Field
    type HsRecUpdField p     = HsRecField' (AmbiguousFieldOcc p) (LHsExpr p)

    -- | Haskell Record Field
    --
    -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnEqual',
    --
    -- For details on above see note [Api annotations] in GHC.Parser.Annotation
    data HsRecField' id arg = HsRecField {
            hsRecFieldLbl :: Located id,
            hsRecFieldArg :: arg,           -- ^ Filled in by renamer when punning
            hsRecPun      :: Bool           -- ^ Note [Punning]
      } deriving (Data, Functor, Foldable, Traversable)

    -- Note [Punning]
    -- ~~~~~~~~~~~~~~
    -- If you write T { x, y = v+1 }, the HsRecFields will be
    --      HsRecField x x True ...
    --      HsRecField y (v+1) False ...
    -- That is, for "punned" field x is expanded (in the renamer)
    -- to x=x; but with a punning flag so we can detect it later
    -- (e.g. when pretty printing)
    --
    -- If the original field was qualified, we un-qualify it, thus
    --    T { A.x } means T { A.x = x }

    -- Note [HsRecField and HsRecUpdField]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    -- A HsRecField (used for record construction and pattern matching)
    -- contains an unambiguous occurrence of a field (i.e. a FieldOcc).
    -- We can't just store the Name, because thanks to
    -- DuplicateRecordFields this may not correspond to the label the user
    -- wrote.
    --
    -- A HsRecUpdField (used for record update) contains a potentially
    -- ambiguous occurrence of a field (an AmbiguousFieldOcc).  The
    -- renamer will fill in the selector function if it can, but if the
    -- selector is ambiguous the renamer will defer to the typechecker.
    -- After the typechecker, a unique selector will have been determined.
    --
    -- The renamer produces an Unambiguous result if it can, rather than
    -- just doing the lookup in the typechecker, so that completely
    -- unambiguous updates can be represented by 'GHC.HsToCore.Quote.repUpdFields'.
    --
    -- For example, suppose we have:
    --
    --     data S = MkS { x :: Int }
    --     data T = MkT { x :: Int }
    --
    --     f z = (z { x = 3 }) :: S
    --
    -- The parsed HsRecUpdField corresponding to the record update will have:
    --
    --     hsRecFieldLbl = Unambiguous "x" noExtField :: AmbiguousFieldOcc RdrName
    --
    -- After the renamer, this will become:
    --
    --     hsRecFieldLbl = Ambiguous   "x" noExtField :: AmbiguousFieldOcc Name
    --
    -- (note that the Unambiguous constructor is not type-correct here).
    -- The typechecker will determine the particular selector:
    --
    --     hsRecFieldLbl = Unambiguous "x" $sel:x:MkS  :: AmbiguousFieldOcc Id
    --
    -- See also Note [Disambiguating record fields] in GHC.Tc.Gen.Expr.

    hsRecFields :: HsRecFields p arg -> [XCFieldOcc p]
    hsRecFields rbinds = map (unLoc . hsRecFieldSel . unLoc) (rec_flds rbinds)

    -- Probably won't typecheck at once, things have changed :/
    hsRecFieldsArgs :: HsRecFields p arg -> [arg]
    hsRecFieldsArgs rbinds = map (hsRecFieldArg . unLoc) (rec_flds rbinds)

    hsRecFieldSel :: HsRecField pass arg -> Located (XCFieldOcc pass)
    hsRecFieldSel = fmap extFieldOcc . hsRecFieldLbl

    hsRecFieldId :: HsRecField GhcTc arg -> Located Id
    hsRecFieldId = hsRecFieldSel

    hsRecUpdFieldRdr :: HsRecUpdField (GhcPass p) -> Located RdrName
    hsRecUpdFieldRdr = fmap rdrNameAmbiguousFieldOcc . hsRecFieldLbl

    hsRecUpdFieldId :: HsRecField' (AmbiguousFieldOcc GhcTc) arg -> Located Id
    hsRecUpdFieldId = fmap extFieldOcc . hsRecUpdFieldOcc

    hsRecUpdFieldOcc :: HsRecField' (AmbiguousFieldOcc GhcTc) arg -> LFieldOcc GhcTc
    hsRecUpdFieldOcc = fmap unambiguousFieldOcc . hsRecFieldLbl

    {-
    ************************************************************************
    *                                                                      *
    *              Printing patterns
    *                                                                      *
    ************************************************************************
    -}

    instance OutputableBndrId p => Outputable (Pat (GhcPass p)) where
        ppr = pprPat

    -- | Print with type info if -dppr-debug is on
    pprPatBndr :: OutputableBndr name => name -> SDoc
    pprPatBndr var
      = getPprDebug $ \case
          True -> parens (pprBndr LambdaBind var) -- Could pass the site to pprPat
                                                  -- but is it worth it?
          False -> pprPrefixOcc var

    pprParendLPat :: (OutputableBndrId p)
                  => PprPrec -> LPat (GhcPass p) -> SDoc
    pprParendLPat p = pprParendPat p . unLoc

    pprParendPat :: forall p. OutputableBndrId p
                 => PprPrec
                 -> Pat (GhcPass p)
                 -> SDoc
    pprParendPat p pat = sdocOption sdocPrintTypecheckerElaboration $ \ print_tc_elab ->
        if need_parens print_tc_elab pat
        then parens (pprPat pat)
        else pprPat pat
      where
        need_parens print_tc_elab pat
          | GhcTc <- ghcPass @p
          , XPat ext <- pat
          , CoPat {} <- ext
          = print_tc_elab

          | otherwise
          = patNeedsParens p pat
          -- For a CoPat we need parens if we are going to show it, which
          -- we do if -fprint-typechecker-elaboration is on (c.f. pprHsWrapper)
          -- But otherwise the CoPat is discarded, so it
          -- is the pattern inside that matters.  Sigh.

    pprPat :: forall p. (OutputableBndrId p) => Pat (GhcPass p) -> SDoc
    pprPat (VarPat _ lvar)          = pprPatBndr (unLoc lvar)
    pprPat (WildPat _)              = char '_'
    pprPat (LazyPat _ pat)          = char '~' <> pprParendLPat appPrec pat
    pprPat (BangPat _ pat)          = char '!' <> pprParendLPat appPrec pat
    pprPat (AsPat _ name pat)       = hcat [pprPrefixOcc (unLoc name), char '@',
                                            pprParendLPat appPrec pat]
    pprPat (ViewPat _ expr pat)     = hcat [pprLExpr expr, text " -> ", ppr pat]
    pprPat (ParPat _ pat)           = parens (ppr pat)
    pprPat (LitPat _ s)             = ppr s
    pprPat (NPat _ l Nothing  _)    = ppr l
    pprPat (NPat _ l (Just _) _)    = char '-' <> ppr l
    pprPat (NPlusKPat _ n k _ _ _)  = hcat [ppr n, char '+', ppr k]
    pprPat (SplicePat _ splice)     = pprSplice splice
    pprPat (SigPat _ pat ty)        = ppr pat <+> dcolon <+> ppr_ty
      where ppr_ty = case ghcPass @p of
                       GhcPs -> ppr ty
                       GhcRn -> ppr ty
                       GhcTc -> ppr ty
    pprPat (ListPat _ pats)         = brackets (interpp'SP pats)
    pprPat (TuplePat _ pats bx)
        -- Special-case unary boxed tuples so that they are pretty-printed as
        -- `Solo x`, not `(x)`
      | [pat] <- pats
      , Boxed <- bx
      = hcat [text (mkTupleStr Boxed 1), pprParendLPat appPrec pat]
      | otherwise
      = tupleParens (boxityTupleSort bx) (pprWithCommas ppr pats)
    pprPat (SumPat _ pat alt arity) = sumParens (pprAlternative ppr pat alt arity)
    pprPat (ConPat { pat_con = con
                   , pat_args = details
                   , pat_con_ext = ext
                   }
           )
      = case ghcPass @p of
          GhcPs -> pprUserCon (unLoc con) details
          GhcRn -> pprUserCon (unLoc con) details
          GhcTc -> sdocOption sdocPrintTypecheckerElaboration $ \case
            False -> pprUserCon (unLoc con) details
            True  ->
              -- Tiresome; in 'GHC.Tc.Gen.Bind.tcRhs' we print out a typechecked Pat in an
              -- error message, and we want to make sure it prints nicely
              ppr con
                <> braces (sep [ hsep (map pprPatBndr (tvs ++ dicts))
                               , ppr binds ])
                <+> pprConArgs details
            where ConPatTc { cpt_tvs = tvs
                           , cpt_dicts = dicts
                           , cpt_binds = binds
                           } = ext
    pprPat (XPat ext) = case ghcPass @p of
    #if __GLASGOW_HASKELL__ < 811
      GhcPs -> noExtCon ext
      GhcRn -> noExtCon ext
    #endif
      GhcTc -> pprHsWrapper co $ \parens ->
          if parens
          then pprParendPat appPrec pat
          else pprPat pat
        where CoPat co pat _ = ext

    pprUserCon :: (OutputableBndr con, OutputableBndrId p)
               => con -> HsConPatDetails (GhcPass p) -> SDoc
    pprUserCon c (InfixCon p1 p2) = ppr p1 <+> pprInfixOcc c <+> ppr p2
    pprUserCon c details          = pprPrefixOcc c <+> pprConArgs details

    pprConArgs :: (OutputableBndrId p)
               => HsConPatDetails (GhcPass p) -> SDoc
    pprConArgs (PrefixCon pats) = fsep (map (pprParendLPat appPrec) pats)
    pprConArgs (InfixCon p1 p2) = sep [ pprParendLPat appPrec p1
                                      , pprParendLPat appPrec p2 ]
    pprConArgs (RecCon rpats)   = ppr rpats

    instance (Outputable arg)
          => Outputable (HsRecFields p arg) where
      ppr (HsRecFields { rec_flds = flds, rec_dotdot = Nothing })
            = braces (fsep (punctuate comma (map ppr flds)))
      ppr (HsRecFields { rec_flds = flds, rec_dotdot = Just (unLoc -> n) })
            = braces (fsep (punctuate comma (map ppr (take n flds) ++ [dotdot])))
            where
              dotdot = text ".." <+> whenPprDebug (ppr (drop n flds))

    instance (Outputable p, Outputable arg)
          => Outputable (HsRecField' p arg) where
      ppr (HsRecField { hsRecFieldLbl = f, hsRecFieldArg = arg,
                        hsRecPun = pun })
        = ppr f <+> (ppUnless pun $ equals <+> ppr arg)

    {-
    ************************************************************************
    *                                                                      *
    *              Building patterns
    *                                                                      *
    ************************************************************************
    -}

    mkPrefixConPat :: DataCon ->
                      [LPat GhcTc] -> [Type] -> LPat GhcTc
    -- Make a vanilla Prefix constructor pattern
    mkPrefixConPat dc pats tys
      = noLoc $ ConPat { pat_con = noLoc (RealDataCon dc)
                       , pat_args = PrefixCon pats
                       , pat_con_ext = ConPatTc
                         { cpt_tvs = []
                         , cpt_dicts = []
                         , cpt_binds = emptyTcEvBinds
                         , cpt_arg_tys = tys
                         , cpt_wrap = idHsWrapper
                         }
                       }

    mkNilPat :: Type -> LPat GhcTc
    mkNilPat ty = mkPrefixConPat nilDataCon [] [ty]

    mkCharLitPat :: SourceText -> Char -> LPat GhcTc
    mkCharLitPat src c = mkPrefixConPat charDataCon
                              [noLoc $ LitPat noExtField (HsCharPrim src c)] []

    {-
    ************************************************************************
    *                                                                      *
    * Predicates for checking things about pattern-lists in EquationInfo   *
    *                                                                      *
    ************************************************************************

    \subsection[Pat-list-predicates]{Look for interesting things in patterns}

    Unlike in the Wadler chapter, where patterns are either ``variables''
    or ``constructors,'' here we distinguish between:
    \begin{description}
    \item[unfailable:]
    Patterns that cannot fail to match: variables, wildcards, and lazy
    patterns.

    These are the irrefutable patterns; the two other categories
    are refutable patterns.

    \item[constructor:]
    A non-literal constructor pattern (see next category).

    \item[literal patterns:]
    At least the numeric ones may be overloaded.
    \end{description}

    A pattern is in {\em exactly one} of the above three categories; `as'
    patterns are treated specially, of course.

    The 1.3 report defines what ``irrefutable'' and ``failure-free'' patterns are.
    -}

    isBangedLPat :: LPat (GhcPass p) -> Bool
    isBangedLPat = isBangedPat . unLoc

    isBangedPat :: Pat (GhcPass p) -> Bool
    isBangedPat (ParPat _ p) = isBangedLPat p
    isBangedPat (BangPat {}) = True
    isBangedPat _            = False

    looksLazyPatBind :: HsBind (GhcPass p) -> Bool
    -- Returns True of anything *except*
    --     a StrictHsBind (as above) or
    --     a VarPat
    -- In particular, returns True of a pattern binding with a compound pattern, like (I# x)
    -- Looks through AbsBinds
    looksLazyPatBind (PatBind { pat_lhs = p })
      = looksLazyLPat p
    looksLazyPatBind (AbsBinds { abs_binds = binds })
      = anyBag (looksLazyPatBind . unLoc) binds
    looksLazyPatBind _
      = False

    looksLazyLPat :: LPat (GhcPass p) -> Bool
    looksLazyLPat = looksLazyPat . unLoc

    looksLazyPat :: Pat (GhcPass p) -> Bool
    looksLazyPat (ParPat _ p)  = looksLazyLPat p
    looksLazyPat (AsPat _ _ p) = looksLazyLPat p
    looksLazyPat (BangPat {})  = False
    looksLazyPat (VarPat {})   = False
    looksLazyPat (WildPat {})  = False
    looksLazyPat _             = True

    isIrrefutableHsPat :: forall p. (OutputableBndrId p) => LPat (GhcPass p) -> Bool
    -- (isIrrefutableHsPat p) is true if matching against p cannot fail,
    -- in the sense of falling through to the next pattern.
    --      (NB: this is not quite the same as the (silly) defn
    --      in 3.17.2 of the Haskell 98 report.)
    --
    -- WARNING: isIrrefutableHsPat returns False if it's in doubt.
    -- Specifically on a ConPatIn, which is what it sees for a
    -- (LPat Name) in the renamer, it doesn't know the size of the
    -- constructor family, so it returns False.  Result: only
    -- tuple patterns are considered irrefutable at the renamer stage.
    --
    -- But if it returns True, the pattern is definitely irrefutable
    isIrrefutableHsPat
      = goL
      where
        goL :: LPat (GhcPass p) -> Bool
        goL = go . unLoc

        go :: Pat (GhcPass p) -> Bool
        go (WildPat {})        = True
        go (VarPat {})         = True
        go (LazyPat {})        = True
        go (BangPat _ pat)     = goL pat
        go (ParPat _ pat)      = goL pat
        go (AsPat _ _ pat)     = goL pat
        go (ViewPat _ _ pat)   = goL pat
        go (SigPat _ pat _)    = goL pat
        go (TuplePat _ pats _) = all goL pats
        go (SumPat {})         = False
                        -- See Note [Unboxed sum patterns aren't irrefutable]
        go (ListPat {})        = False

        go (ConPat
            { pat_con  = con
            , pat_args = details })
                               = case ghcPass @p of
           GhcPs -> False -- Conservative
           GhcRn -> False -- Conservative
           GhcTc -> case con of
             L _ (PatSynCon _pat)  -> False -- Conservative
             L _ (RealDataCon con) ->
               isJust (tyConSingleDataCon_maybe (dataConTyCon con))
               -- NB: tyConSingleDataCon_maybe, *not* isProductTyCon, because
               -- the latter is false of existentials. See #4439
               && all goL (hsConPatArgs details)
        go (LitPat {})         = False
        go (NPat {})           = False
        go (NPlusKPat {})      = False

        -- We conservatively assume that no TH splices are irrefutable
        -- since we cannot know until the splice is evaluated.
        go (SplicePat {})      = False

        go (XPat ext)          = case ghcPass @p of
    #if __GLASGOW_HASKELL__ < 811
          GhcPs -> noExtCon ext
          GhcRn -> noExtCon ext
    #endif
          GhcTc -> go pat
            where CoPat _ pat _ = ext

    -- | Is the pattern any of combination of:
    --
    -- - (pat)
    -- - pat :: Type
    -- - ~pat
    -- - !pat
    -- - x (variable)
    isSimplePat :: LPat (GhcPass x) -> Maybe (IdP (GhcPass x))
    isSimplePat p = case unLoc p of
      ParPat _ x -> isSimplePat x
      SigPat _ x _ -> isSimplePat x
      LazyPat _ x -> isSimplePat x
      BangPat _ x -> isSimplePat x
      VarPat _ x -> Just (unLoc x)
      _ -> Nothing

    {- Note [Unboxed sum patterns aren't irrefutable]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Unlike unboxed tuples, unboxed sums are *not* irrefutable when used as
    patterns. A simple example that demonstrates this is from #14228:

      pattern Just' x = (# x | #)
      pattern Nothing' = (# | () #)

      foo x = case x of
        Nothing' -> putStrLn "nothing"
        Just'    -> putStrLn "just"

    In foo, the pattern Nothing' (that is, (# x | #)) is certainly not irrefutable,
    as does not match an unboxed sum value of the same arity—namely, (# | y #)
    (covered by Just'). In fact, no unboxed sum pattern is irrefutable, since the
    minimum unboxed sum arity is 2.

    Failing to mark unboxed sum patterns as non-irrefutable would cause the Just'
    case in foo to be unreachable, as GHC would mistakenly believe that Nothing'
    is the only thing that could possibly be matched!
    -}

    -- | @'patNeedsParens' p pat@ returns 'True' if the pattern @pat@ needs
    -- parentheses under precedence @p@.
    patNeedsParens :: forall p. IsPass p => PprPrec -> Pat (GhcPass p) -> Bool
    patNeedsParens p = go
      where
        go :: Pat (GhcPass p) -> Bool
        go (NPlusKPat {})    = p > opPrec
        go (SplicePat {})    = False
        go (ConPat { pat_args = ds})
                             = conPatNeedsParens p ds
        go (SigPat {})       = p >= sigPrec
        go (ViewPat {})      = True
        go (XPat ext)        = case ghcPass @p of
          GhcPs -> noExtCon ext
          GhcRn -> noExtCon ext
          GhcTc -> go inner
            where CoPat _ inner _ = ext
        go (WildPat {})      = False
        go (VarPat {})       = False
        go (LazyPat {})      = False
        go (BangPat {})      = False
        go (ParPat {})       = False
        go (AsPat {})        = False
        go (TuplePat {})     = False
        go (SumPat {})       = False
        go (ListPat {})      = False
        go (LitPat _ l)      = hsLitNeedsParens p l
        go (NPat _ lol _ _)  = hsOverLitNeedsParens p (unLoc lol)

    -- | @'conPatNeedsParens' p cp@ returns 'True' if the constructor patterns @cp@
    -- needs parentheses under precedence @p@.
    conPatNeedsParens :: PprPrec -> HsConDetails a b -> Bool
    conPatNeedsParens p = go
      where
        go (PrefixCon args) = p >= appPrec && not (null args)
        go (InfixCon {})    = p >= opPrec
        go (RecCon {})      = False

    -- | @'parenthesizePat' p pat@ checks if @'patNeedsParens' p pat@ is true, and
    -- if so, surrounds @pat@ with a 'ParPat'. Otherwise, it simply returns @pat@.
    parenthesizePat :: IsPass p
                    => PprPrec
                    -> LPat (GhcPass p)
                    -> LPat (GhcPass p)
    parenthesizePat p lpat@(L loc pat)
      | patNeedsParens p pat = L loc (ParPat noExtField lpat)
      | otherwise            = lpat

    {-
    % Collect all EvVars from all constructor patterns
    -}

    -- May need to add more cases
    collectEvVarsPats :: [Pat GhcTc] -> Bag EvVar
    collectEvVarsPats = unionManyBags . map collectEvVarsPat

    collectEvVarsLPat :: LPat GhcTc -> Bag EvVar
    collectEvVarsLPat = collectEvVarsPat . unLoc

    collectEvVarsPat :: Pat GhcTc -> Bag EvVar
    collectEvVarsPat pat =
      case pat of
        LazyPat _ p      -> collectEvVarsLPat p
        AsPat _ _ p      -> collectEvVarsLPat p
        ParPat  _ p      -> collectEvVarsLPat p
        BangPat _ p      -> collectEvVarsLPat p
        ListPat _ ps     -> unionManyBags $ map collectEvVarsLPat ps
        TuplePat _ ps _  -> unionManyBags $ map collectEvVarsLPat ps
        SumPat _ p _ _   -> collectEvVarsLPat p
        ConPat
          { pat_args  = args
          , pat_con_ext = ConPatTc
            { cpt_dicts = dicts
            }
          }
                         -> unionBags (listToBag dicts)
                                       $ unionManyBags
                                       $ map collectEvVarsLPat
                                       $ hsConPatArgs args
        SigPat  _ p _    -> collectEvVarsLPat p
        XPat (CoPat _ p _) -> collectEvVarsPat  p
        _other_pat       -> emptyBag