GHC/Hs/Types.hs

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

    GHC.Hs.Types: Abstract syntax: user-defined types
    -}

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

    module GHC.Hs.Types (
            HsType(..), NewHsTypeX(..), LHsType, HsKind, LHsKind,
            HsTyVarBndr(..), LHsTyVarBndr, ForallVisFlag(..),
            LHsQTyVars(..),
            HsImplicitBndrs(..),
            HsWildCardBndrs(..),
            LHsSigType, LHsSigWcType, LHsWcType,
            HsTupleSort(..),
            HsContext, LHsContext, noLHsContext,
            HsTyLit(..),
            HsIPName(..), hsIPNameFS,
            HsArg(..), numVisibleArgs,
            LHsTypeArg,

            LBangType, BangType,
            HsSrcBang(..), HsImplBang(..),
            SrcStrictness(..), SrcUnpackedness(..),
            getBangType, getBangStrictness,

            ConDeclField(..), LConDeclField, pprConDeclFields,

            HsConDetails(..),

            FieldOcc(..), LFieldOcc, mkFieldOcc,
            AmbiguousFieldOcc(..), mkAmbiguousFieldOcc,
            rdrNameAmbiguousFieldOcc, selectorAmbiguousFieldOcc,
            unambiguousFieldOcc, ambiguousFieldOcc,

            mkAnonWildCardTy, pprAnonWildCard,

            mkHsImplicitBndrs, mkHsWildCardBndrs, hsImplicitBody,
            mkEmptyImplicitBndrs, mkEmptyWildCardBndrs,
            mkHsQTvs, hsQTvExplicit, emptyLHsQTvs, isEmptyLHsQTvs,
            isHsKindedTyVar, hsTvbAllKinded, isLHsForAllTy,
            hsScopedTvs, hsWcScopedTvs, dropWildCards,
            hsTyVarName, hsAllLTyVarNames, hsLTyVarLocNames,
            hsLTyVarName, hsLTyVarNames, hsLTyVarLocName, hsExplicitLTyVarNames,
            splitLHsInstDeclTy, getLHsInstDeclHead, getLHsInstDeclClass_maybe,
            splitLHsPatSynTy,
            splitLHsForAllTy, splitLHsForAllTyInvis,
            splitLHsQualTy, splitLHsSigmaTy, splitLHsSigmaTyInvis,
            splitHsFunType, hsTyGetAppHead_maybe,
            mkHsOpTy, mkHsAppTy, mkHsAppTys, mkHsAppKindTy,
            ignoreParens, hsSigType, hsSigWcType,
            hsLTyVarBndrToType, hsLTyVarBndrsToTypes,
            hsTyKindSig,
            hsConDetailsArgs,

            -- Printing
            pprHsType, pprHsForAll, pprHsForAllExtra, pprHsExplicitForAll,
            pprLHsContext,
            hsTypeNeedsParens, parenthesizeHsType, parenthesizeHsContext
        ) where

    #include "HsVersions.h"

    import GhcPrelude

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

    import GHC.Hs.Extension

    import Id ( Id )
    import Name( Name, NamedThing(getName) )
    import RdrName ( RdrName )
    import DataCon( HsSrcBang(..), HsImplBang(..),
                    SrcStrictness(..), SrcUnpackedness(..) )
    import TysPrim( funTyConName )
    import TysWiredIn( mkTupleStr )
    import Type
    import GHC.Hs.Doc
    import BasicTypes
    import SrcLoc
    import Outputable
    import FastString
    import Maybes( isJust )
    import Util ( count, debugIsOn )

    import Data.Data hiding ( Fixity, Prefix, Infix )

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Bang annotations}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located Bang Type
    type LBangType pass = Located (BangType pass)

    -- | Bang Type
    --
    -- In the parser, strictness and packedness annotations bind more tightly
    -- than docstrings. This means that when consuming a 'BangType' (and looking
    -- for 'HsBangTy') we must be ready to peer behind a potential layer of
    -- 'HsDocTy'. See #15206 for motivation and 'getBangType' for an example.
    type BangType pass  = HsType pass       -- Bangs are in the HsType data type

    getBangType :: LHsType a -> LHsType a
    getBangType                 (L _ (HsBangTy _ _ lty))       = lty
    getBangType (L _ (HsDocTy x (L _ (HsBangTy _ _ lty)) lds)) =
      addCLoc lty lds (HsDocTy x lty lds)
    getBangType lty                                            = lty

    getBangStrictness :: LHsType a -> HsSrcBang
    getBangStrictness                 (L _ (HsBangTy _ s _))     = s
    getBangStrictness (L _ (HsDocTy _ (L _ (HsBangTy _ s _)) _)) = s
    getBangStrictness _ = (HsSrcBang NoSourceText NoSrcUnpack NoSrcStrict)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Data types}
    *                                                                      *
    ************************************************************************

    This is the syntax for types as seen in type signatures.

    Note [HsBSig binder lists]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider a binder (or pattern) decorated with a type or kind,
       \ (x :: a -> a). blah
       forall (a :: k -> *) (b :: k). blah
    Then we use a LHsBndrSig on the binder, so that the
    renamer can decorate it with the variables bound
    by the pattern ('a' in the first example, 'k' in the second),
    assuming that neither of them is in scope already
    See also Note [Kind and type-variable binders] in RnTypes

    Note [HsType binders]
    ~~~~~~~~~~~~~~~~~~~~~
    The system for recording type and kind-variable binders in HsTypes
    is a bit complicated.  Here's how it works.

    * In a HsType,
         HsForAllTy   represents an /explicit, user-written/ 'forall'
                       e.g.   forall a b.   {...} or
                              forall a b -> {...}
         HsQualTy     represents an /explicit, user-written/ context
                       e.g.   (Eq a, Show a) => ...
                      The context can be empty if that's what the user wrote
      These constructors represent what the user wrote, no more
      and no less.

    * The ForallVisFlag field of HsForAllTy represents whether a forall is
      invisible (e.g., forall a b. {...}, with a dot) or visible
      (e.g., forall a b -> {...}, with an arrow).

    * HsTyVarBndr describes a quantified type variable written by the
      user.  For example
         f :: forall a (b :: *).  blah
      here 'a' and '(b::*)' are each a HsTyVarBndr.  A HsForAllTy has
      a list of LHsTyVarBndrs.

    * HsImplicitBndrs is a wrapper that gives the implicitly-quantified
      kind and type variables of the wrapped thing.  It is filled in by
      the renamer. For example, if the user writes
         f :: a -> a
      the HsImplicitBinders binds the 'a' (not a HsForAllTy!).
      NB: this implicit quantification is purely lexical: we bind any
          type or kind variables that are not in scope. The type checker
          may subsequently quantify over further kind variables.

    * HsWildCardBndrs is a wrapper that binds the wildcard variables
      of the wrapped thing.  It is filled in by the renamer
         f :: _a -> _
      The enclosing HsWildCardBndrs binds the wildcards _a and _.

    * The explicit presence of these wrappers specifies, in the HsSyn,
      exactly where implicit quantification is allowed, and where
      wildcards are allowed.

    * LHsQTyVars is used in data/class declarations, where the user gives
      explicit *type* variable bindings, but we need to implicitly bind
      *kind* variables.  For example
          class C (a :: k -> *) where ...
      The 'k' is implicitly bound in the hsq_tvs field of LHsQTyVars

    Note [The wildcard story for types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Types can have wildcards in them, to support partial type signatures,
    like       f :: Int -> (_ , _a) -> _a

    A wildcard in a type can be

      * An anonymous wildcard,
            written '_'
        In HsType this is represented by HsWildCardTy.
        The renamer leaves it untouched, and it is later given fresh meta tyvars in
        the typechecker.

      * A named wildcard,
            written '_a', '_foo', etc
        In HsType this is represented by (HsTyVar "_a")
        i.e. a perfectly ordinary type variable that happens
             to start with an underscore

    Note carefully:

    * When NamedWildCards is off, type variables that start with an
      underscore really /are/ ordinary type variables.  And indeed, even
      when NamedWildCards is on you can bind _a explicitly as an ordinary
      type variable:
            data T _a _b = MkT _b _a
      Or even:
            f :: forall _a. _a -> _b
      Here _a is an ordinary forall'd binder, but (With NamedWildCards)
      _b is a named wildcard.  (See the comments in #10982)

    * Named wildcards are bound by the HsWildCardBndrs construct, which wraps
      types that are allowed to have wildcards. Unnamed wildcards however are left
      unchanged until typechecking, where we give them fresh wild tyavrs and
      determine whether or not to emit hole constraints on each wildcard
      (we don't if it's a visible type/kind argument or a type family pattern).
      See related notes Note [Wildcards in visible kind application]
      and Note [Wildcards in visible type application] in TcHsType.hs

    * After type checking is done, we report what types the wildcards
      got unified with.

    Note [Ordering of implicit variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Since the advent of -XTypeApplications, GHC makes promises about the ordering
    of implicit variable quantification. Specifically, we offer that implicitly
    quantified variables (such as those in const :: a -> b -> a, without a `forall`)
    will occur in left-to-right order of first occurrence. Here are a few examples:

      const :: a -> b -> a       -- forall a b. ...
      f :: Eq a => b -> a -> a   -- forall a b. ...  contexts are included

      type a <-< b = b -> a
      g :: a <-< b               -- forall a b. ...  type synonyms matter

      class Functor f where
        fmap :: (a -> b) -> f a -> f b   -- forall f a b. ...
        -- The f is quantified by the class, so only a and b are considered in fmap

    This simple story is complicated by the possibility of dependency: all variables
    must come after any variables mentioned in their kinds.

      typeRep :: Typeable a => TypeRep (a :: k)   -- forall k a. ...

    The k comes first because a depends on k, even though the k appears later than
    the a in the code. Thus, GHC does a *stable topological sort* on the variables.
    By "stable", we mean that any two variables who do not depend on each other
    preserve their existing left-to-right ordering.

    Implicitly bound variables are collected by the extract- family of functions
    (extractHsTysRdrTyVars, extractHsTyVarBndrsKVs, etc.) in RnTypes.
    These functions thus promise to keep left-to-right ordering.
    Look for pointers to this note to see the places where the action happens.

    Note that we also maintain this ordering in kind signatures. Even though
    there's no visible kind application (yet), having implicit variables be
    quantified in left-to-right order in kind signatures is nice since:

    * It's consistent with the treatment for type signatures.
    * It can affect how types are displayed with -fprint-explicit-kinds (see
      #15568 for an example), which is a situation where knowing the order in
      which implicit variables are quantified can be useful.
    * In the event that visible kind application is implemented, the order in
      which we would expect implicit variables to be ordered in kinds will have
      already been established.
    -}

    -- | Located Haskell Context
    type LHsContext pass = Located (HsContext pass)
          -- ^ 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnUnit'
          -- For details on above see note [Api annotations] in ApiAnnotation

    noLHsContext :: LHsContext pass
    -- Use this when there is no context in the original program
    -- It would really be more kosher to use a Maybe, to distinguish
    --     class () => C a where ...
    -- from
    --     class C a where ...
    noLHsContext = noLoc []

    -- | Haskell Context
    type HsContext pass = [LHsType pass]

    -- | Located Haskell Type
    type LHsType pass = Located (HsType pass)
          -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnComma' when
          --   in a list

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

    -- | Haskell Kind
    type HsKind pass = HsType pass

    -- | Located Haskell Kind
    type LHsKind pass = Located (HsKind pass)
          -- ^ 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDcolon'

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

    --------------------------------------------------
    --             LHsQTyVars
    --  The explicitly-quantified binders in a data/type declaration

    -- | Located Haskell Type Variable Binder
    type LHsTyVarBndr pass = Located (HsTyVarBndr pass)
                             -- See Note [HsType binders]

    -- | Located Haskell Quantified Type Variables
    data LHsQTyVars pass   -- See Note [HsType binders]
      = HsQTvs { hsq_ext :: XHsQTvs pass

               , hsq_explicit :: [LHsTyVarBndr pass]
                    -- Explicit variables, written by the user
                    -- See Note [HsForAllTy tyvar binders]
        }
      | XLHsQTyVars (XXLHsQTyVars pass)

    type HsQTvsRn = [Name]  -- Implicit variables
      -- For example, in   data T (a :: k1 -> k2) = ...
      -- the 'a' is explicit while 'k1', 'k2' are implicit

    type instance XHsQTvs GhcPs = NoExtField
    type instance XHsQTvs GhcRn = HsQTvsRn
    type instance XHsQTvs GhcTc = HsQTvsRn

    type instance XXLHsQTyVars  (GhcPass _) = NoExtCon

    mkHsQTvs :: [LHsTyVarBndr GhcPs] -> LHsQTyVars GhcPs
    mkHsQTvs tvs = HsQTvs { hsq_ext = noExtField, hsq_explicit = tvs }

    hsQTvExplicit :: LHsQTyVars pass -> [LHsTyVarBndr pass]
    hsQTvExplicit = hsq_explicit

    emptyLHsQTvs :: LHsQTyVars GhcRn
    emptyLHsQTvs = HsQTvs { hsq_ext = [], hsq_explicit = [] }

    isEmptyLHsQTvs :: LHsQTyVars GhcRn -> Bool
    isEmptyLHsQTvs (HsQTvs { hsq_ext = imp, hsq_explicit = exp })
      = null imp && null exp
    isEmptyLHsQTvs _ = False

    ------------------------------------------------
    --            HsImplicitBndrs
    -- Used to quantify the implicit binders of a type
    --    * Implicit binders of a type signature (LHsSigType/LHsSigWcType)
    --    * Patterns in a type/data family instance (HsTyPats)

    -- | Haskell Implicit Binders
    data HsImplicitBndrs pass thing   -- See Note [HsType binders]
      = HsIB { hsib_ext  :: XHsIB pass thing -- after renamer: [Name]
                                             -- Implicitly-bound kind & type vars
                                             -- Order is important; see
                                             -- Note [Ordering of implicit variables]
                                             -- in RnTypes

             , hsib_body :: thing            -- Main payload (type or list of types)
        }
      | XHsImplicitBndrs (XXHsImplicitBndrs pass thing)

    type instance XHsIB              GhcPs _ = NoExtField
    type instance XHsIB              GhcRn _ = [Name]
    type instance XHsIB              GhcTc _ = [Name]

    type instance XXHsImplicitBndrs  (GhcPass _) _ = NoExtCon

    -- | Haskell Wildcard Binders
    data HsWildCardBndrs pass thing
        -- See Note [HsType binders]
        -- See Note [The wildcard story for types]
      = HsWC { hswc_ext :: XHsWC pass thing
                    -- after the renamer
                    -- Wild cards, only named
                    -- See Note [Wildcards in visible kind application]

             , hswc_body :: thing
                    -- Main payload (type or list of types)
                    -- If there is an extra-constraints wildcard,
                    -- it's still there in the hsc_body.
        }
      | XHsWildCardBndrs (XXHsWildCardBndrs pass thing)

    type instance XHsWC              GhcPs b = NoExtField
    type instance XHsWC              GhcRn b = [Name]
    type instance XHsWC              GhcTc b = [Name]

    type instance XXHsWildCardBndrs  (GhcPass _) b = NoExtCon

    -- | Located Haskell Signature Type
    type LHsSigType   pass = HsImplicitBndrs pass (LHsType pass)    -- Implicit only

    -- | Located Haskell Wildcard Type
    type LHsWcType    pass = HsWildCardBndrs pass (LHsType pass)    -- Wildcard only

    -- | Located Haskell Signature Wildcard Type
    type LHsSigWcType pass = HsWildCardBndrs pass (LHsSigType pass) -- Both

    -- See Note [Representing type signatures]

    hsImplicitBody :: HsImplicitBndrs (GhcPass p) thing -> thing
    hsImplicitBody (HsIB { hsib_body = body }) = body
    hsImplicitBody (XHsImplicitBndrs nec) = noExtCon nec

    hsSigType :: LHsSigType (GhcPass p) -> LHsType (GhcPass p)
    hsSigType = hsImplicitBody

    hsSigWcType :: LHsSigWcType pass -> LHsType pass
    hsSigWcType sig_ty = hsib_body (hswc_body sig_ty)

    dropWildCards :: LHsSigWcType pass -> LHsSigType pass
    -- Drop the wildcard part of a LHsSigWcType
    dropWildCards sig_ty = hswc_body sig_ty

    {- Note [Representing type signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    HsSigType is used to represent an explicit user type signature
    such as   f :: a -> a
         or   g (x :: a -> a) = x

    A HsSigType is just a HsImplicitBndrs wrapping a LHsType.
     * The HsImplicitBndrs binds the /implicitly/ quantified tyvars
     * The LHsType binds the /explicitly/ quantified tyvars

    E.g. For a signature like
       f :: forall (a::k). blah
    we get
       HsIB { hsib_vars = [k]
            , hsib_body = HsForAllTy { hst_bndrs = [(a::*)]
                                     , hst_body = blah }
    The implicit kind variable 'k' is bound by the HsIB;
    the explicitly forall'd tyvar 'a' is bound by the HsForAllTy
    -}

    mkHsImplicitBndrs :: thing -> HsImplicitBndrs GhcPs thing
    mkHsImplicitBndrs x = HsIB { hsib_ext  = noExtField
                               , hsib_body = x }

    mkHsWildCardBndrs :: thing -> HsWildCardBndrs GhcPs thing
    mkHsWildCardBndrs x = HsWC { hswc_body = x
                               , hswc_ext  = noExtField }

    -- Add empty binders.  This is a bit suspicious; what if
    -- the wrapped thing had free type variables?
    mkEmptyImplicitBndrs :: thing -> HsImplicitBndrs GhcRn thing
    mkEmptyImplicitBndrs x = HsIB { hsib_ext = []
                                  , hsib_body = x }

    mkEmptyWildCardBndrs :: thing -> HsWildCardBndrs GhcRn thing
    mkEmptyWildCardBndrs x = HsWC { hswc_body = x
                                  , hswc_ext  = [] }

    --------------------------------------------------
    -- | These names are used early on to store the names of implicit
    -- parameters.  They completely disappear after type-checking.
    newtype HsIPName = HsIPName FastString
      deriving( Eq, Data )

    hsIPNameFS :: HsIPName -> FastString
    hsIPNameFS (HsIPName n) = n

    instance Outputable HsIPName where
        ppr (HsIPName n) = char '?' <> ftext n -- Ordinary implicit parameters

    instance OutputableBndr HsIPName where
        pprBndr _ n   = ppr n         -- Simple for now
        pprInfixOcc  n = ppr n
        pprPrefixOcc n = ppr n

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

    -- | Haskell Type Variable Binder
    data HsTyVarBndr pass
      = UserTyVar        -- no explicit kinding
             (XUserTyVar pass)
             (Located (IdP pass))
            -- See Note [Located RdrNames] in GHC.Hs.Expr
      | KindedTyVar
             (XKindedTyVar pass)
             (Located (IdP pass))
             (LHsKind pass)  -- The user-supplied kind signature
            -- ^
            --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
            --          'ApiAnnotation.AnnDcolon', 'ApiAnnotation.AnnClose'

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

      | XTyVarBndr
          (XXTyVarBndr pass)

    type instance XUserTyVar    (GhcPass _) = NoExtField
    type instance XKindedTyVar  (GhcPass _) = NoExtField

    type instance XXTyVarBndr   (GhcPass _) = NoExtCon

    -- | Does this 'HsTyVarBndr' come with an explicit kind annotation?
    isHsKindedTyVar :: HsTyVarBndr pass -> Bool
    isHsKindedTyVar (UserTyVar {})   = False
    isHsKindedTyVar (KindedTyVar {}) = True
    isHsKindedTyVar (XTyVarBndr {})  = False

    -- | Do all type variables in this 'LHsQTyVars' come with kind annotations?
    hsTvbAllKinded :: LHsQTyVars pass -> Bool
    hsTvbAllKinded = all (isHsKindedTyVar . unLoc) . hsQTvExplicit

    instance NamedThing (HsTyVarBndr GhcRn) where
      getName (UserTyVar _ v) = unLoc v
      getName (KindedTyVar _ v _) = unLoc v
      getName (XTyVarBndr nec) = noExtCon nec

    -- | Haskell Type
    data HsType pass
      = HsForAllTy   -- See Note [HsType binders]
          { hst_xforall :: XForAllTy pass
          , hst_fvf     :: ForallVisFlag -- Is this `forall a -> {...}` or
                                         --         `forall a. {...}`?
          , hst_bndrs   :: [LHsTyVarBndr pass]
                                           -- Explicit, user-supplied 'forall a b c'
          , hst_body    :: LHsType pass      -- body type
          }
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnForall',
          --         'ApiAnnotation.AnnDot','ApiAnnotation.AnnDarrow'
          -- For details on above see note [Api annotations] in ApiAnnotation

      | HsQualTy   -- See Note [HsType binders]
          { hst_xqual :: XQualTy pass
          , hst_ctxt  :: LHsContext pass       -- Context C => blah
          , hst_body  :: LHsType pass }

      | HsTyVar  (XTyVar pass)
                  PromotionFlag    -- Whether explicitly promoted,
                                   -- for the pretty printer
                 (Located (IdP pass))
                      -- Type variable, type constructor, or data constructor
                      -- see Note [Promotions (HsTyVar)]
                      -- See Note [Located RdrNames] in GHC.Hs.Expr
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      | HsAppTy             (XAppTy pass)
                            (LHsType pass)
                            (LHsType pass)
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      | HsAppKindTy         (XAppKindTy pass) -- type level type app
                            (LHsType pass)
                            (LHsKind pass)

      | HsFunTy             (XFunTy pass)
                            (LHsType pass)   -- function type
                            (LHsType pass)
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnRarrow',

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

      | HsListTy            (XListTy pass)
                            (LHsType pass)  -- Element type
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'['@,
          --         'ApiAnnotation.AnnClose' @']'@

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

      | HsTupleTy           (XTupleTy pass)
                            HsTupleSort
                            [LHsType pass]  -- Element types (length gives arity)
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'(' or '(#'@,
        --         'ApiAnnotation.AnnClose' @')' or '#)'@

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

      | HsSumTy             (XSumTy pass)
                            [LHsType pass]  -- Element types (length gives arity)
        -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'(#'@,
        --         'ApiAnnotation.AnnClose' '#)'@

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

      | HsOpTy              (XOpTy pass)
                            (LHsType pass) (Located (IdP pass)) (LHsType pass)
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      | HsParTy             (XParTy pass)
                            (LHsType pass)   -- See Note [Parens in HsSyn] in GHC.Hs.Expr
            -- Parenthesis preserved for the precedence re-arrangement in RnTypes
            -- It's important that a * (b + c) doesn't get rearranged to (a*b) + c!
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'('@,
          --         'ApiAnnotation.AnnClose' @')'@

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

      | HsIParamTy          (XIParamTy pass)
                            (Located HsIPName) -- (?x :: ty)
                            (LHsType pass)   -- Implicit parameters as they occur in
                                             -- contexts
          -- ^
          -- > (?x :: ty)
          --
          -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDcolon'

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

      | HsStarTy            (XStarTy pass)
                            Bool             -- Is this the Unicode variant?
                                             -- Note [HsStarTy]
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

      | HsKindSig           (XKindSig pass)
                            (LHsType pass)  -- (ty :: kind)
                            (LHsKind pass)  -- A type with a kind signature
          -- ^
          -- > (ty :: kind)
          --
          -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'('@,
          --         'ApiAnnotation.AnnDcolon','ApiAnnotation.AnnClose' @')'@

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

      | HsSpliceTy          (XSpliceTy pass)
                            (HsSplice pass)   -- Includes quasi-quotes
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'$('@,
          --         'ApiAnnotation.AnnClose' @')'@

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

      | HsDocTy             (XDocTy pass)
                            (LHsType pass) LHsDocString -- A documented type
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      | HsBangTy    (XBangTy pass)
                    HsSrcBang (LHsType pass)   -- Bang-style type annotations
          -- ^ - 'ApiAnnotation.AnnKeywordId' :
          --         'ApiAnnotation.AnnOpen' @'{-\# UNPACK' or '{-\# NOUNPACK'@,
          --         'ApiAnnotation.AnnClose' @'#-}'@
          --         'ApiAnnotation.AnnBang' @\'!\'@

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

      | HsRecTy     (XRecTy pass)
                    [LConDeclField pass]    -- Only in data type declarations
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @'{'@,
          --         'ApiAnnotation.AnnClose' @'}'@

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

      -- | HsCoreTy (XCoreTy pass) Type -- An escape hatch for tunnelling a *closed*
      --                                -- Core Type through HsSyn.
      --     -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      | HsExplicitListTy       -- A promoted explicit list
            (XExplicitListTy pass)
            PromotionFlag      -- whether explcitly promoted, for pretty printer
            [LHsType pass]
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @"'["@,
          --         'ApiAnnotation.AnnClose' @']'@

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

      | HsExplicitTupleTy      -- A promoted explicit tuple
            (XExplicitTupleTy pass)
            [LHsType pass]
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen' @"'("@,
          --         'ApiAnnotation.AnnClose' @')'@

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

      | HsTyLit (XTyLit pass) HsTyLit      -- A promoted numeric literal.
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      | HsWildCardTy (XWildCardTy pass)  -- A type wildcard
          -- See Note [The wildcard story for types]
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

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

      -- For adding new constructors via Trees that Grow
      | XHsType
          (XXType pass)

    data NewHsTypeX
      = NHsCoreTy Type -- An escape hatch for tunnelling a *closed*
                       -- Core Type through HsSyn.
        deriving Data
          -- ^ - 'ApiAnnotation.AnnKeywordId' : None

    instance Outputable NewHsTypeX where
      ppr (NHsCoreTy ty) = ppr ty

    type instance XForAllTy        (GhcPass _) = NoExtField
    type instance XQualTy          (GhcPass _) = NoExtField
    type instance XTyVar           (GhcPass _) = NoExtField
    type instance XAppTy           (GhcPass _) = NoExtField
    type instance XFunTy           (GhcPass _) = NoExtField
    type instance XListTy          (GhcPass _) = NoExtField
    type instance XTupleTy         (GhcPass _) = NoExtField
    type instance XSumTy           (GhcPass _) = NoExtField
    type instance XOpTy            (GhcPass _) = NoExtField
    type instance XParTy           (GhcPass _) = NoExtField
    type instance XIParamTy        (GhcPass _) = NoExtField
    type instance XStarTy          (GhcPass _) = NoExtField
    type instance XKindSig         (GhcPass _) = NoExtField

    type instance XAppKindTy       (GhcPass _) = SrcSpan -- Where the `@` lives

    type instance XSpliceTy        GhcPs = NoExtField
    type instance XSpliceTy        GhcRn = NoExtField
    type instance XSpliceTy        GhcTc = Kind

    type instance XDocTy           (GhcPass _) = NoExtField
    type instance XBangTy          (GhcPass _) = NoExtField
    type instance XRecTy           (GhcPass _) = NoExtField

    type instance XExplicitListTy  GhcPs = NoExtField
    type instance XExplicitListTy  GhcRn = NoExtField
    type instance XExplicitListTy  GhcTc = Kind

    type instance XExplicitTupleTy GhcPs = NoExtField
    type instance XExplicitTupleTy GhcRn = NoExtField
    type instance XExplicitTupleTy GhcTc = [Kind]

    type instance XTyLit           (GhcPass _) = NoExtField

    type instance XWildCardTy      (GhcPass _) = NoExtField

    type instance XXType         (GhcPass _) = NewHsTypeX

    -- Note [Literal source text] in BasicTypes for SourceText fields in
    -- the following
    -- | Haskell Type Literal
    data HsTyLit
      = HsNumTy SourceText Integer
      | HsStrTy SourceText FastString
        deriving Data

    {-
    Note [HsForAllTy tyvar binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    After parsing:
      * Implicit => empty
        Explicit => the variables the user wrote

    After renaming
      * Implicit => the *type* variables free in the type
        Explicit => the variables the user wrote (renamed)

    Qualified currently behaves exactly as Implicit,
    but it is deprecated to use it for implicit quantification.
    In this case, GHC 7.10 gives a warning; see
    Note [Context quantification] in RnTypes, and #4426.
    In GHC 8.0, Qualified will no longer bind variables
    and this will become an error.

    The kind variables bound in the hsq_implicit field come both
      a) from the kind signatures on the kind vars (eg k1)
      b) from the scope of the forall (eg k2)
    Example:   f :: forall (a::k1) b. T a (b::k2)

    Note [Unit tuples]
    ~~~~~~~~~~~~~~~~~~
    Consider the type
        type instance F Int = ()
    We want to parse that "()"
        as HsTupleTy HsBoxedOrConstraintTuple [],
    NOT as HsTyVar unitTyCon

    Why? Because F might have kind (* -> Constraint), so we when parsing we
    don't know if that tuple is going to be a constraint tuple or an ordinary
    unit tuple.  The HsTupleSort flag is specifically designed to deal with
    that, but it has to work for unit tuples too.

    Note [Promotions (HsTyVar)]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    HsTyVar: A name in a type or kind.
      Here are the allowed namespaces for the name.
        In a type:
          Var: not allowed
          Data: promoted data constructor
          Tv: type variable
          TcCls before renamer: type constructor, class constructor, or promoted data constructor
          TcCls after renamer: type constructor or class constructor
        In a kind:
          Var, Data: not allowed
          Tv: kind variable
          TcCls: kind constructor or promoted type constructor

      The 'Promoted' field in an HsTyVar captures whether the type was promoted in
      the source code by prefixing an apostrophe.

    Note [HsStarTy]
    ~~~~~~~~~~~~~~~
    When the StarIsType extension is enabled, we want to treat '*' and its Unicode
    variant identically to 'Data.Kind.Type'. Unfortunately, doing so in the parser
    would mean that when we pretty-print it back, we don't know whether the user
    wrote '*' or 'Type', and lose the parse/ppr roundtrip property.

    As a workaround, we parse '*' as HsStarTy (if it stands for 'Data.Kind.Type')
    and then desugar it to 'Data.Kind.Type' in the typechecker (see tc_hs_type).
    When '*' is a regular type operator (StarIsType is disabled), HsStarTy is not
    involved.

    Note [Promoted lists and tuples]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Notice the difference between
       HsListTy    HsExplicitListTy
       HsTupleTy   HsExplicitListTupleTy

    E.g.    f :: [Int]                      HsListTy

            g3  :: T '[]                   All these use
            g2  :: T '[True]                  HsExplicitListTy
            g1  :: T '[True,False]
            g1a :: T [True,False]             (can omit ' where unambiguous)

      kind of T :: [Bool] -> *        This kind uses HsListTy!

    E.g.    h :: (Int,Bool)                 HsTupleTy; f is a pair
            k :: S '(True,False)            HsExplicitTypleTy; S is indexed by
                                               a type-level pair of booleans
            kind of S :: (Bool,Bool) -> *   This kind uses HsExplicitTupleTy

    Note [Distinguishing tuple kinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Apart from promotion, tuples can have one of three different kinds:

            x :: (Int, Bool)                -- Regular boxed tuples
            f :: Int# -> (# Int#, Int# #)   -- Unboxed tuples
            g :: (Eq a, Ord a) => a         -- Constraint tuples

    For convenience, internally we use a single constructor for all of these,
    namely HsTupleTy, but keep track of the tuple kind (in the first argument to
    HsTupleTy, a HsTupleSort). We can tell if a tuple is unboxed while parsing,
    because of the #. However, with -XConstraintKinds we can only distinguish
    between constraint and boxed tuples during type checking, in general. Hence the
    four constructors of HsTupleSort:

            HsUnboxedTuple                  -> Produced by the parser
            HsBoxedTuple                    -> Certainly a boxed tuple
            HsConstraintTuple               -> Certainly a constraint tuple
            HsBoxedOrConstraintTuple        -> Could be a boxed or a constraint
                                            tuple. Produced by the parser only,
                                            disappears after type checking
    -}

    -- | Haskell Tuple Sort
    data HsTupleSort = HsUnboxedTuple
                     | HsBoxedTuple
                     | HsConstraintTuple
                     | HsBoxedOrConstraintTuple
                     deriving Data

    -- | Located Constructor Declaration Field
    type LConDeclField pass = Located (ConDeclField pass)
          -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnComma' when
          --   in a list

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

    -- | Constructor Declaration Field
    data ConDeclField pass  -- Record fields have Haddoc docs on them
      = ConDeclField { cd_fld_ext  :: XConDeclField pass,
                       cd_fld_names :: [LFieldOcc pass],
                                       -- ^ See Note [ConDeclField passs]
                       cd_fld_type :: LBangType pass,
                       cd_fld_doc  :: Maybe LHsDocString }
          -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDcolon'

          -- For details on above see note [Api annotations] in ApiAnnotation
      | XConDeclField (XXConDeclField pass)

    type instance XConDeclField  (GhcPass _) = NoExtField
    type instance XXConDeclField (GhcPass _) = NoExtCon

    instance OutputableBndrId p
           => Outputable (ConDeclField (GhcPass p)) where
      ppr (ConDeclField _ fld_n fld_ty _) = ppr fld_n <+> dcolon <+> ppr fld_ty
      ppr (XConDeclField x) = ppr x

    -- HsConDetails is used for patterns/expressions *and* for data type
    -- declarations
    -- | Haskell Constructor Details
    data HsConDetails arg rec
      = PrefixCon [arg]             -- C p1 p2 p3
      | RecCon    rec               -- C { x = p1, y = p2 }
      | InfixCon  arg arg           -- p1 `C` p2
      deriving Data

    instance (Outputable arg, Outputable rec)
             => Outputable (HsConDetails arg rec) where
      ppr (PrefixCon args) = text "PrefixCon" <+> ppr args
      ppr (RecCon rec)     = text "RecCon:" <+> ppr rec
      ppr (InfixCon l r)   = text "InfixCon:" <+> ppr [l, r]

    hsConDetailsArgs ::
         HsConDetails (LHsType a) (Located [LConDeclField a])
      -> [LHsType a]
    hsConDetailsArgs details = case details of
      InfixCon a b -> [a,b]
      PrefixCon xs -> xs
      RecCon r -> map (cd_fld_type . unLoc) (unLoc r)

    {-
    Note [ConDeclField passs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~

    A ConDeclField contains a list of field occurrences: these always
    include the field label as the user wrote it.  After the renamer, it
    will additionally contain the identity of the selector function in the
    second component.

    Due to DuplicateRecordFields, the OccName of the selector function
    may have been mangled, which is why we keep the original field label
    separately.  For example, when DuplicateRecordFields is enabled

        data T = MkT { x :: Int }

    gives

        ConDeclField { cd_fld_names = [L _ (FieldOcc "x" $sel:x:MkT)], ... }.
    -}

    -----------------------
    -- A valid type must have a for-all at the top of the type, or of the fn arg
    -- types

    ---------------------
    hsWcScopedTvs :: LHsSigWcType GhcRn -> [Name]
    -- Get the lexically-scoped type variables of a HsSigType
    --  - the explicitly-given forall'd type variables
    --  - the named wildcars; see Note [Scoping of named wildcards]
    -- because they scope in the same way
    hsWcScopedTvs sig_ty
      | HsWC { hswc_ext = nwcs, hswc_body = sig_ty1 }  <- sig_ty
      , HsIB { hsib_ext = vars
             , hsib_body = sig_ty2 } <- sig_ty1
      = case sig_ty2 of
          L _ (HsForAllTy { hst_fvf = vis_flag
                          , hst_bndrs = tvs }) ->
            ASSERT( vis_flag == ForallInvis ) -- See Note [hsScopedTvs vis_flag]
            vars ++ nwcs ++ hsLTyVarNames tvs
          _                                    -> nwcs
    hsWcScopedTvs (HsWC _ (XHsImplicitBndrs nec)) = noExtCon nec
    hsWcScopedTvs (XHsWildCardBndrs nec) = noExtCon nec

    hsScopedTvs :: LHsSigType GhcRn -> [Name]
    -- Same as hsWcScopedTvs, but for a LHsSigType
    hsScopedTvs sig_ty
      | HsIB { hsib_ext = vars
             , hsib_body = sig_ty2 } <- sig_ty
      , L _ (HsForAllTy { hst_fvf = vis_flag
                        , hst_bndrs = tvs }) <- sig_ty2
      = ASSERT( vis_flag == ForallInvis ) -- See Note [hsScopedTvs vis_flag]
        vars ++ hsLTyVarNames tvs
      | otherwise
      = []

    {- Note [Scoping of named wildcards]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      f :: _a -> _a
      f x = let g :: _a -> _a
                g = ...
            in ...

    Currently, for better or worse, the "_a" variables are all the same. So
    although there is no explicit forall, the "_a" scopes over the definition.
    I don't know if this is a good idea, but there it is.
    -}

    {- Note [hsScopedTvs vis_flag]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -XScopedTypeVariables can be defined in terms of a desugaring to
    -XTypeAbstractions (GHC Proposal #50):

        fn :: forall a b c. tau(a,b,c)            fn :: forall a b c. tau(a,b,c)
        fn = defn(a,b,c)                   ==>    fn @x @y @z = defn(x,y,z)

    That is, for every type variable of the leading 'forall' in the type signature,
    we add an invisible binder at term level.

    This model does not extend to visible forall, as discussed here:

    * https://gitlab.haskell.org/ghc/ghc/issues/16734#note_203412
    * https://github.com/ghc-proposals/ghc-proposals/pull/238

    The conclusion of these discussions can be summarized as follows:

      > Assuming support for visible 'forall' in terms, consider this example:
      >
      >     vfn :: forall x y -> tau(x,y)
      >     vfn = \a b -> ...
      >
      > The user has written their own binders 'a' and 'b' to stand for 'x' and
      > 'y', and we definitely should not desugar this into:
      >
      >     vfn :: forall x y -> tau(x,y)
      >     vfn x y = \a b -> ...         -- bad!

    At the moment, GHC does not support visible 'forall' in terms, so we simply cement
    our assumptions with an assert:

        hsScopedTvs (HsForAllTy { hst_fvf = vis_flag, ... }) =
          ASSERT( vis_flag == ForallInvis )
          ...

    In the future, this assert can be safely turned into a pattern match to support
    visible forall in terms:

        hsScopedTvs (HsForAllTy { hst_fvf = ForallInvis, ... }) = ...
    -}

    ---------------------
    hsTyVarName :: HsTyVarBndr (GhcPass p) -> IdP (GhcPass p)
    hsTyVarName (UserTyVar _ (L _ n))     = n
    hsTyVarName (KindedTyVar _ (L _ n) _) = n
    hsTyVarName (XTyVarBndr nec) = noExtCon nec

    hsLTyVarName :: LHsTyVarBndr (GhcPass p) -> IdP (GhcPass p)
    hsLTyVarName = hsTyVarName . unLoc

    hsLTyVarNames :: [LHsTyVarBndr (GhcPass p)] -> [IdP (GhcPass p)]
    hsLTyVarNames = map hsLTyVarName

    hsExplicitLTyVarNames :: LHsQTyVars (GhcPass p) -> [IdP (GhcPass p)]
    -- Explicit variables only
    hsExplicitLTyVarNames qtvs = map hsLTyVarName (hsQTvExplicit qtvs)

    hsAllLTyVarNames :: LHsQTyVars GhcRn -> [Name]
    -- All variables
    hsAllLTyVarNames (HsQTvs { hsq_ext = kvs
                             , hsq_explicit = tvs })
      = kvs ++ hsLTyVarNames tvs
    hsAllLTyVarNames (XLHsQTyVars nec) = noExtCon nec

    hsLTyVarLocName :: LHsTyVarBndr (GhcPass p) -> Located (IdP (GhcPass p))
    hsLTyVarLocName = onHasSrcSpan hsTyVarName

    hsLTyVarLocNames :: LHsQTyVars (GhcPass p) -> [Located (IdP (GhcPass p))]
    hsLTyVarLocNames qtvs = map hsLTyVarLocName (hsQTvExplicit qtvs)

    -- | Convert a LHsTyVarBndr to an equivalent LHsType.
    hsLTyVarBndrToType :: LHsTyVarBndr (GhcPass p) -> LHsType (GhcPass p)
    hsLTyVarBndrToType = onHasSrcSpan cvt
      where cvt (UserTyVar _ n) = HsTyVar noExtField NotPromoted n
            cvt (KindedTyVar _ (L name_loc n) kind)
              = HsKindSig noExtField
                       (L name_loc (HsTyVar noExtField NotPromoted (L name_loc n))) kind
            cvt (XTyVarBndr nec) = noExtCon nec

    -- | Convert a LHsTyVarBndrs to a list of types.
    -- Works on *type* variable only, no kind vars.
    hsLTyVarBndrsToTypes :: LHsQTyVars (GhcPass p) -> [LHsType (GhcPass p)]
    hsLTyVarBndrsToTypes (HsQTvs { hsq_explicit = tvbs }) = map hsLTyVarBndrToType tvbs
    hsLTyVarBndrsToTypes (XLHsQTyVars nec) = noExtCon nec

    -- | Get the kind signature of a type, ignoring parentheses:
    --
    --   hsTyKindSig   `Maybe                    `   =   Nothing
    --   hsTyKindSig   `Maybe ::   Type -> Type  `   =   Just  `Type -> Type`
    --   hsTyKindSig   `Maybe :: ((Type -> Type))`   =   Just  `Type -> Type`
    --
    -- This is used to extract the result kind of type synonyms with a CUSK:
    --
    --  type S = (F :: res_kind)
    --                 ^^^^^^^^
    --
    hsTyKindSig :: LHsType pass -> Maybe (LHsKind pass)
    hsTyKindSig lty =
      case unLoc lty of
        HsParTy _ lty'    -> hsTyKindSig lty'
        HsKindSig _ _ k   -> Just k
        _                 -> Nothing

    ---------------------
    ignoreParens :: LHsType pass -> LHsType pass
    ignoreParens (L _ (HsParTy _ ty)) = ignoreParens ty
    ignoreParens ty                   = ty

    isLHsForAllTy :: LHsType p -> Bool
    isLHsForAllTy (L _ (HsForAllTy {})) = True
    isLHsForAllTy _                     = False

    {-
    ************************************************************************
    *                                                                      *
                    Building types
    *                                                                      *
    ************************************************************************
    -}

    mkAnonWildCardTy :: HsType GhcPs
    mkAnonWildCardTy = HsWildCardTy noExtField

    mkHsOpTy :: LHsType (GhcPass p) -> Located (IdP (GhcPass p))
             -> LHsType (GhcPass p) -> HsType (GhcPass p)
    mkHsOpTy ty1 op ty2 = HsOpTy noExtField ty1 op ty2

    mkHsAppTy :: LHsType (GhcPass p) -> LHsType (GhcPass p) -> LHsType (GhcPass p)
    mkHsAppTy t1 t2
      = addCLoc t1 t2 (HsAppTy noExtField t1 (parenthesizeHsType appPrec t2))

    mkHsAppTys :: LHsType (GhcPass p) -> [LHsType (GhcPass p)]
               -> LHsType (GhcPass p)
    mkHsAppTys = foldl' mkHsAppTy

    mkHsAppKindTy :: XAppKindTy (GhcPass p) -> LHsType (GhcPass p) -> LHsType (GhcPass p)
                  -> LHsType (GhcPass p)
    mkHsAppKindTy ext ty k
      = addCLoc ty k (HsAppKindTy ext ty k)

    {-
    ************************************************************************
    *                                                                      *
                    Decomposing HsTypes
    *                                                                      *
    ************************************************************************
    -}

    ---------------------------------
    -- splitHsFunType decomposes a type (t1 -> t2 ... -> tn)
    -- Breaks up any parens in the result type:
    --      splitHsFunType (a -> (b -> c)) = ([a,b], c)
    -- Also deals with (->) t1 t2; that is why it only works on LHsType Name
    --   (see #9096)
    splitHsFunType :: LHsType GhcRn -> ([LHsType GhcRn], LHsType GhcRn)
    splitHsFunType (L _ (HsParTy _ ty))
      = splitHsFunType ty

    splitHsFunType (L _ (HsFunTy _ x y))
      | (args, res) <- splitHsFunType y
      = (x:args, res)
    {- This is not so correct, because it won't work with visible kind app, in case
      someone wants to write '(->) @k1 @k2 t1 t2'. Fixing this would require changing
      ConDeclGADT abstract syntax -}
    splitHsFunType orig_ty@(L _ (HsAppTy _ t1 t2))
      = go t1 [t2]
      where  -- Look for (->) t1 t2, possibly with parenthesisation
        go (L _ (HsTyVar _ _ (L _ fn))) tys | fn == funTyConName
                                     , [t1,t2] <- tys
                                     , (args, res) <- splitHsFunType t2
                                     = (t1:args, res)
        go (L _ (HsAppTy _ t1 t2)) tys = go t1 (t2:tys)
        go (L _ (HsParTy _ ty))    tys = go ty tys
        go _                       _   = ([], orig_ty)  -- Failure to match

    splitHsFunType other = ([], other)

    -- retrieve the name of the "head" of a nested type application
    -- somewhat like splitHsAppTys, but a little more thorough
    -- used to examine the result of a GADT-like datacon, so it doesn't handle
    -- *all* cases (like lists, tuples, (~), etc.)
    hsTyGetAppHead_maybe :: LHsType (GhcPass p)
                         -> Maybe (Located (IdP (GhcPass p)))
    hsTyGetAppHead_maybe = go
      where
        go (L _ (HsTyVar _ _ ln))          = Just ln
        go (L _ (HsAppTy _ l _))           = go l
        go (L _ (HsAppKindTy _ t _))       = go t
        go (L _ (HsOpTy _ _ (L loc n) _))  = Just (L loc n)
        go (L _ (HsParTy _ t))             = go t
        go (L _ (HsKindSig _ t _))         = go t
        go _                               = Nothing

    ------------------------------------------------------------
    -- Arguments in an expression/type after splitting
    data HsArg tm ty
      = HsValArg tm   -- Argument is an ordinary expression     (f arg)
      | HsTypeArg SrcSpan ty -- Argument is a visible type application (f @ty)
                             -- SrcSpan is location of the `@`
      | HsArgPar SrcSpan -- See Note [HsArgPar]

    numVisibleArgs :: [HsArg tm ty] -> Arity
    numVisibleArgs = count is_vis
      where is_vis (HsValArg _) = True
            is_vis _            = False

    -- type level equivalent
    type LHsTypeArg p = HsArg (LHsType p) (LHsKind p)

    instance (Outputable tm, Outputable ty) => Outputable (HsArg tm ty) where
      ppr (HsValArg tm)    = ppr tm
      ppr (HsTypeArg _ ty) = char '@' <> ppr ty
      ppr (HsArgPar sp)    = text "HsArgPar"  <+> ppr sp
    {-
    Note [HsArgPar]
    A HsArgPar indicates that everything to the left of this in the argument list is
    enclosed in parentheses together with the function itself. It is necessary so
    that we can recreate the parenthesis structure in the original source after
    typechecking the arguments.

    The SrcSpan is the span of the original HsPar

    ((f arg1) arg2 arg3) results in an input argument list of
    [HsValArg arg1, HsArgPar span1, HsValArg arg2, HsValArg arg3, HsArgPar span2]

    -}

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

    -- | Decompose a pattern synonym type signature into its constituent parts.
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(forall a. <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsPatSynTy :: LHsType pass
                     -> ( [LHsTyVarBndr pass]    -- universals
                        , LHsContext pass        -- required constraints
                        , [LHsTyVarBndr pass]    -- existentials
                        , LHsContext pass        -- provided constraints
                        , LHsType pass)          -- body type
    splitLHsPatSynTy ty = (univs, reqs, exis, provs, ty4)
      where
        (univs, ty1) = splitLHsForAllTyInvis ty
        (reqs,  ty2) = splitLHsQualTy ty1
        (exis,  ty3) = splitLHsForAllTyInvis ty2
        (provs, ty4) = splitLHsQualTy ty3

    -- | Decompose a sigma type (of the form @forall <tvs>. context => body@)
    -- into its constituent parts.
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(forall a. <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsSigmaTy :: LHsType pass
                    -> ([LHsTyVarBndr pass], LHsContext pass, LHsType pass)
    splitLHsSigmaTy ty
      | (tvs, ty1)  <- splitLHsForAllTy ty
      , (ctxt, ty2) <- splitLHsQualTy ty1
      = (tvs, ctxt, ty2)

    -- | Like 'splitLHsSigmaTy', but only splits type variable binders that were
    -- quantified invisibly (e.g., @forall a.@, with a dot).
    --
    -- This function is used to split apart certain types, such as instance
    -- declaration types, which disallow visible @forall@s. For instance, if GHC
    -- split apart the @forall@ in @instance forall a -> Show (Blah a)@, then that
    -- declaration would mistakenly be accepted!
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(forall a. <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsSigmaTyInvis :: LHsType pass
                         -> ([LHsTyVarBndr pass], LHsContext pass, LHsType pass)
    splitLHsSigmaTyInvis ty
      | (tvs,  ty1) <- splitLHsForAllTyInvis ty
      , (ctxt, ty2) <- splitLHsQualTy ty1
      = (tvs, ctxt, ty2)

    -- | Decompose a type of the form @forall <tvs>. body@) into its constituent
    -- parts.
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(forall a. <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsForAllTy :: LHsType pass -> ([LHsTyVarBndr pass], LHsType pass)
    splitLHsForAllTy (L _ (HsParTy _ ty)) = splitLHsForAllTy ty
    splitLHsForAllTy (L _ (HsForAllTy { hst_bndrs = tvs, hst_body = body })) = (tvs, body)
    splitLHsForAllTy body              = ([], body)

    -- | Like 'splitLHsForAllTy', but only splits type variable binders that
    -- were quantified invisibly (e.g., @forall a.@, with a dot).
    --
    -- This function is used to split apart certain types, such as instance
    -- declaration types, which disallow visible @forall@s. For instance, if GHC
    -- split apart the @forall@ in @instance forall a -> Show (Blah a)@, then that
    -- declaration would mistakenly be accepted!
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(forall a. <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsForAllTyInvis :: LHsType pass -> ([LHsTyVarBndr pass], LHsType pass)
    splitLHsForAllTyInvis lty@(L _ ty) =
      case ty of
        HsParTy _ ty' -> splitLHsForAllTyInvis ty'
        HsForAllTy { hst_fvf = fvf', hst_bndrs = tvs', hst_body = body' }
          |  fvf' == ForallInvis
          -> (tvs', body')
        _ -> ([], lty)

    -- | Decompose a type of the form @context => body@ into its constituent parts.
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(context => <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsQualTy :: LHsType pass -> (LHsContext pass, LHsType pass)
    splitLHsQualTy (L _ (HsParTy _ ty)) = splitLHsQualTy ty
    splitLHsQualTy (L _ (HsQualTy { hst_ctxt = ctxt, hst_body = body })) = (ctxt,     body)
    splitLHsQualTy body              = (noLHsContext, body)

    -- | Decompose a type class instance type (of the form
    -- @forall <tvs>. context => instance_head@) into its constituent parts.
    --
    -- Note that this function looks through parentheses, so it will work on types
    -- such as @(forall <tvs>. <...>)@. The downside to this is that it is not
    -- generally possible to take the returned types and reconstruct the original
    -- type (parentheses and all) from them.
    splitLHsInstDeclTy :: LHsSigType GhcRn
                       -> ([Name], LHsContext GhcRn, LHsType GhcRn)
    -- Split up an instance decl type, returning the pieces
    splitLHsInstDeclTy (HsIB { hsib_ext = itkvs
                             , hsib_body = inst_ty })
      | (tvs, cxt, body_ty) <- splitLHsSigmaTyInvis inst_ty
      = (itkvs ++ hsLTyVarNames tvs, cxt, body_ty)
             -- Return implicitly bound type and kind vars
             -- For an instance decl, all of them are in scope
    splitLHsInstDeclTy (XHsImplicitBndrs nec) = noExtCon nec

    getLHsInstDeclHead :: LHsSigType (GhcPass p) -> LHsType (GhcPass p)
    getLHsInstDeclHead inst_ty
      | (_tvs, _cxt, body_ty) <- splitLHsSigmaTyInvis (hsSigType inst_ty)
      = body_ty

    getLHsInstDeclClass_maybe :: LHsSigType (GhcPass p)
                              -> Maybe (Located (IdP (GhcPass p)))
    -- Works on (HsSigType RdrName)
    getLHsInstDeclClass_maybe inst_ty
      = do { let head_ty = getLHsInstDeclHead inst_ty
           ; cls <- hsTyGetAppHead_maybe head_ty
           ; return cls }

    {-
    ************************************************************************
    *                                                                      *
                    FieldOcc
    *                                                                      *
    ************************************************************************
    -}

    -- | Located Field Occurrence
    type LFieldOcc pass = Located (FieldOcc pass)

    -- | Field Occurrence
    --
    -- Represents an *occurrence* of an unambiguous field.  We store
    -- both the 'RdrName' the user originally wrote, and after the
    -- renamer, the selector function.
    data FieldOcc pass = FieldOcc { extFieldOcc     :: XCFieldOcc pass
                                  , rdrNameFieldOcc :: Located RdrName
                                     -- ^ See Note [Located RdrNames] in GHC.Hs.Expr
                                  }

      | XFieldOcc
          (XXFieldOcc pass)
    deriving instance Eq  (XCFieldOcc (GhcPass p)) => Eq  (FieldOcc (GhcPass p))
    deriving instance Ord (XCFieldOcc (GhcPass p)) => Ord (FieldOcc (GhcPass p))

    type instance XCFieldOcc GhcPs = NoExtField
    type instance XCFieldOcc GhcRn = Name
    type instance XCFieldOcc GhcTc = Id

    type instance XXFieldOcc (GhcPass _) = NoExtCon

    instance Outputable (FieldOcc pass) where
      ppr = ppr . rdrNameFieldOcc

    mkFieldOcc :: Located RdrName -> FieldOcc GhcPs
    mkFieldOcc rdr = FieldOcc noExtField rdr

    -- | Ambiguous Field Occurrence
    --
    -- Represents an *occurrence* of a field that is potentially
    -- ambiguous after the renamer, with the ambiguity resolved by the
    -- typechecker.  We always store the 'RdrName' that the user
    -- originally wrote, and store the selector function after the renamer
    -- (for unambiguous occurrences) or the typechecker (for ambiguous
    -- occurrences).
    --
    -- See Note [HsRecField and HsRecUpdField] in GHC.Hs.Pat and
    -- Note [Disambiguating record fields] in TcExpr.
    -- See Note [Located RdrNames] in GHC.Hs.Expr
    data AmbiguousFieldOcc pass
      = Unambiguous (XUnambiguous pass) (Located RdrName)
      | Ambiguous   (XAmbiguous pass)   (Located RdrName)
      | XAmbiguousFieldOcc (XXAmbiguousFieldOcc pass)

    type instance XUnambiguous GhcPs = NoExtField
    type instance XUnambiguous GhcRn = Name
    type instance XUnambiguous GhcTc = Id

    type instance XAmbiguous GhcPs = NoExtField
    type instance XAmbiguous GhcRn = NoExtField
    type instance XAmbiguous GhcTc = Id

    type instance XXAmbiguousFieldOcc (GhcPass _) = NoExtCon

    instance Outputable (AmbiguousFieldOcc (GhcPass p)) where
      ppr = ppr . rdrNameAmbiguousFieldOcc

    instance OutputableBndr (AmbiguousFieldOcc (GhcPass p)) where
      pprInfixOcc  = pprInfixOcc . rdrNameAmbiguousFieldOcc
      pprPrefixOcc = pprPrefixOcc . rdrNameAmbiguousFieldOcc

    mkAmbiguousFieldOcc :: Located RdrName -> AmbiguousFieldOcc GhcPs
    mkAmbiguousFieldOcc rdr = Unambiguous noExtField rdr

    rdrNameAmbiguousFieldOcc :: AmbiguousFieldOcc (GhcPass p) -> RdrName
    rdrNameAmbiguousFieldOcc (Unambiguous _ (L _ rdr)) = rdr
    rdrNameAmbiguousFieldOcc (Ambiguous   _ (L _ rdr)) = rdr
    rdrNameAmbiguousFieldOcc (XAmbiguousFieldOcc nec)
      = noExtCon nec

    selectorAmbiguousFieldOcc :: AmbiguousFieldOcc GhcTc -> Id
    selectorAmbiguousFieldOcc (Unambiguous sel _) = sel
    selectorAmbiguousFieldOcc (Ambiguous   sel _) = sel
    selectorAmbiguousFieldOcc (XAmbiguousFieldOcc nec)
      = noExtCon nec

    unambiguousFieldOcc :: AmbiguousFieldOcc GhcTc -> FieldOcc GhcTc
    unambiguousFieldOcc (Unambiguous rdr sel) = FieldOcc rdr sel
    unambiguousFieldOcc (Ambiguous   rdr sel) = FieldOcc rdr sel
    unambiguousFieldOcc (XAmbiguousFieldOcc nec) = noExtCon nec

    ambiguousFieldOcc :: FieldOcc GhcTc -> AmbiguousFieldOcc GhcTc
    ambiguousFieldOcc (FieldOcc sel rdr) = Unambiguous sel rdr
    ambiguousFieldOcc (XFieldOcc nec) = noExtCon nec

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Pretty printing}
    *                                                                      *
    ************************************************************************
    -}

    instance OutputableBndrId p => Outputable (HsType (GhcPass p)) where
        ppr ty = pprHsType ty

    instance Outputable HsTyLit where
        ppr = ppr_tylit

    instance OutputableBndrId p
           => Outputable (LHsQTyVars (GhcPass p)) where
        ppr (HsQTvs { hsq_explicit = tvs }) = interppSP tvs
        ppr (XLHsQTyVars x) = ppr x

    instance OutputableBndrId p
           => Outputable (HsTyVarBndr (GhcPass p)) where
        ppr (UserTyVar _ n)     = ppr n
        ppr (KindedTyVar _ n k) = parens $ hsep [ppr n, dcolon, ppr k]
        ppr (XTyVarBndr nec)    = noExtCon nec

    instance Outputable thing
           => Outputable (HsImplicitBndrs (GhcPass p) thing) where
        ppr (HsIB { hsib_body = ty }) = ppr ty
        ppr (XHsImplicitBndrs x) = ppr x

    instance Outputable thing
           => Outputable (HsWildCardBndrs (GhcPass p) thing) where
        ppr (HsWC { hswc_body = ty }) = ppr ty
        ppr (XHsWildCardBndrs x) = ppr x

    pprAnonWildCard :: SDoc
    pprAnonWildCard = char '_'

    -- | Prints a forall; When passed an empty list, prints @forall .@/@forall ->@
    -- only when @-dppr-debug@ is enabled.
    pprHsForAll :: (OutputableBndrId p)
                => ForallVisFlag -> [LHsTyVarBndr (GhcPass p)]
                -> LHsContext (GhcPass p) -> SDoc
    pprHsForAll = pprHsForAllExtra Nothing

    -- | Version of 'pprHsForAll' that can also print an extra-constraints
    -- wildcard, e.g. @_ => a -> Bool@ or @(Show a, _) => a -> String@. This
    -- underscore will be printed when the 'Maybe SrcSpan' argument is a 'Just'
    -- containing the location of the extra-constraints wildcard. A special
    -- function for this is needed, as the extra-constraints wildcard is removed
    -- from the actual context and type, and stored in a separate field, thus just
    -- printing the type will not print the extra-constraints wildcard.
    pprHsForAllExtra :: (OutputableBndrId p)
                     => Maybe SrcSpan -> ForallVisFlag
                     -> [LHsTyVarBndr (GhcPass p)]
                     -> LHsContext (GhcPass p) -> SDoc
    pprHsForAllExtra extra fvf qtvs cxt
      = pp_forall <+> pprLHsContextExtra (isJust extra) cxt
      where
        pp_forall | null qtvs = whenPprDebug (forAllLit <> separator)
                  | otherwise = forAllLit <+> interppSP qtvs <> separator

        separator = ppr_forall_separator fvf

    -- | Version of 'pprHsForAll' or 'pprHsForAllExtra' that will always print
    -- @forall.@ when passed @Just []@. Prints nothing if passed 'Nothing'
    pprHsExplicitForAll :: (OutputableBndrId p)
                        => ForallVisFlag
                        -> Maybe [LHsTyVarBndr (GhcPass p)] -> SDoc
    pprHsExplicitForAll fvf (Just qtvs) = forAllLit <+> interppSP qtvs
                                                     <> ppr_forall_separator fvf
    pprHsExplicitForAll _   Nothing     = empty

    -- | Prints an arrow for visible @forall@s (e.g., @forall a ->@) and a dot for
    -- invisible @forall@s (e.g., @forall a.@).
    ppr_forall_separator :: ForallVisFlag -> SDoc
    ppr_forall_separator ForallVis   = space <> arrow
    ppr_forall_separator ForallInvis = dot

    pprLHsContext :: (OutputableBndrId p)
                  => LHsContext (GhcPass p) -> SDoc
    pprLHsContext lctxt
      | null (unLoc lctxt) = empty
      | otherwise          = pprLHsContextAlways lctxt

    -- For use in a HsQualTy, which always gets printed if it exists.
    pprLHsContextAlways :: (OutputableBndrId p)
                        => LHsContext (GhcPass p) -> SDoc
    pprLHsContextAlways (L _ ctxt)
      = case ctxt of
          []       -> parens empty             <+> darrow
          [L _ ty] -> ppr_mono_ty ty           <+> darrow
          _        -> parens (interpp'SP ctxt) <+> darrow

    -- True <=> print an extra-constraints wildcard, e.g. @(Show a, _) =>@
    pprLHsContextExtra :: (OutputableBndrId p)
                       => Bool -> LHsContext (GhcPass p) -> SDoc
    pprLHsContextExtra show_extra lctxt@(L _ ctxt)
      | not show_extra = pprLHsContext lctxt
      | null ctxt      = char '_' <+> darrow
      | otherwise      = parens (sep (punctuate comma ctxt')) <+> darrow
      where
        ctxt' = map ppr ctxt ++ [char '_']

    pprConDeclFields :: (OutputableBndrId p)
                     => [LConDeclField (GhcPass p)] -> SDoc
    pprConDeclFields fields = braces (sep (punctuate comma (map ppr_fld fields)))
      where
        ppr_fld (L _ (ConDeclField { cd_fld_names = ns, cd_fld_type = ty,
                                     cd_fld_doc = doc }))
            = ppr_names ns <+> dcolon <+> ppr ty <+> ppr_mbDoc doc
        ppr_fld (L _ (XConDeclField x)) = ppr x
        ppr_names [n] = ppr n
        ppr_names ns = sep (punctuate comma (map ppr ns))

    {-
    Note [Printing KindedTyVars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    #3830 reminded me that we should really only print the kind
    signature on a KindedTyVar if the kind signature was put there by the
    programmer.  During kind inference GHC now adds a PostTcKind to UserTyVars,
    rather than converting to KindedTyVars as before.

    (As it happens, the message in #3830 comes out a different way now,
    and the problem doesn't show up; but having the flag on a KindedTyVar
    seems like the Right Thing anyway.)
    -}

    -- Printing works more-or-less as for Types

    pprHsType :: (OutputableBndrId p) => HsType (GhcPass p) -> SDoc
    pprHsType ty = ppr_mono_ty ty

    ppr_mono_lty :: (OutputableBndrId p) => LHsType (GhcPass p) -> SDoc
    ppr_mono_lty ty = ppr_mono_ty (unLoc ty)

    ppr_mono_ty :: (OutputableBndrId p) => HsType (GhcPass p) -> SDoc
    ppr_mono_ty (HsForAllTy { hst_fvf = fvf, hst_bndrs = tvs, hst_body = ty })
      = sep [pprHsForAll fvf tvs noLHsContext, ppr_mono_lty ty]

    ppr_mono_ty (HsQualTy { hst_ctxt = ctxt, hst_body = ty })
      = sep [pprLHsContextAlways ctxt, ppr_mono_lty ty]

    ppr_mono_ty (HsBangTy _ b ty)   = ppr b <> ppr_mono_lty ty
    ppr_mono_ty (HsRecTy _ flds)      = pprConDeclFields flds
    ppr_mono_ty (HsTyVar _ prom (L _ name))
      | isPromoted prom = quote (pprPrefixOcc name)
      | otherwise       = pprPrefixOcc name
    ppr_mono_ty (HsFunTy _ ty1 ty2)   = ppr_fun_ty ty1 ty2
    ppr_mono_ty (HsTupleTy _ con tys)
        -- Special-case unary boxed tuples so that they are pretty-printed as
        -- `Unit x`, not `(x)`
      | [ty] <- tys
      , BoxedTuple <- std_con
      = sep [text (mkTupleStr Boxed 1), ppr_mono_lty ty]
      | otherwise
      = tupleParens std_con (pprWithCommas ppr tys)
      where std_con = case con of
                        HsUnboxedTuple -> UnboxedTuple
                        _              -> BoxedTuple
    ppr_mono_ty (HsSumTy _ tys)
      = tupleParens UnboxedTuple (pprWithBars ppr tys)
    ppr_mono_ty (HsKindSig _ ty kind)
      = ppr_mono_lty ty <+> dcolon <+> ppr kind
    ppr_mono_ty (HsListTy _ ty)       = brackets (ppr_mono_lty ty)
    ppr_mono_ty (HsIParamTy _ n ty)   = (ppr n <+> dcolon <+> ppr_mono_lty ty)
    ppr_mono_ty (HsSpliceTy _ s)      = pprSplice s
    ppr_mono_ty (HsExplicitListTy _ prom tys)
      | isPromoted prom = quote $ brackets (maybeAddSpace tys $ interpp'SP tys)
      | otherwise       = brackets (interpp'SP tys)
    ppr_mono_ty (HsExplicitTupleTy _ tys)
        -- Special-case unary boxed tuples so that they are pretty-printed as
        -- `'Unit x`, not `'(x)`
      | [ty] <- tys
      = quote $ sep [text (mkTupleStr Boxed 1), ppr_mono_lty ty]
      | otherwise
      = quote $ parens (maybeAddSpace tys $ interpp'SP tys)
    ppr_mono_ty (HsTyLit _ t)       = ppr_tylit t
    ppr_mono_ty (HsWildCardTy {})   = char '_'

    ppr_mono_ty (HsStarTy _ isUni)  = char (if isUni then '★' else '*')

    ppr_mono_ty (HsAppTy _ fun_ty arg_ty)
      = hsep [ppr_mono_lty fun_ty, ppr_mono_lty arg_ty]
    ppr_mono_ty (HsAppKindTy _ ty k)
      = ppr_mono_lty ty <+> char '@' <> ppr_mono_lty k
    ppr_mono_ty (HsOpTy _ ty1 (L _ op) ty2)
      = sep [ ppr_mono_lty ty1
            , sep [pprInfixOcc op, ppr_mono_lty ty2 ] ]

    ppr_mono_ty (HsParTy _ ty)
      = parens (ppr_mono_lty ty)
      -- Put the parens in where the user did
      -- But we still use the precedence stuff to add parens because
      --    toHsType doesn't put in any HsParTys, so we may still need them

    ppr_mono_ty (HsDocTy _ ty doc)
      -- AZ: Should we add parens?  Should we introduce "-- ^"?
      = ppr_mono_lty ty <+> ppr (unLoc doc)
      -- we pretty print Haddock comments on types as if they were
      -- postfix operators

    ppr_mono_ty (XHsType t) = ppr t

    --------------------------
    ppr_fun_ty :: (OutputableBndrId p)
               => LHsType (GhcPass p) -> LHsType (GhcPass p) -> SDoc
    ppr_fun_ty ty1 ty2
      = let p1 = ppr_mono_lty ty1
            p2 = ppr_mono_lty ty2
        in
        sep [p1, arrow <+> p2]

    --------------------------
    ppr_tylit :: HsTyLit -> SDoc
    ppr_tylit (HsNumTy _ i) = integer i
    ppr_tylit (HsStrTy _ s) = text (show s)

    -- | @'hsTypeNeedsParens' p t@ returns 'True' if the type @t@ needs parentheses
    -- under precedence @p@.
    hsTypeNeedsParens :: PprPrec -> HsType pass -> Bool
    hsTypeNeedsParens p = go
      where
        go (HsForAllTy{})        = p >= funPrec
        go (HsQualTy{})          = p >= funPrec
        go (HsBangTy{})          = p > topPrec
        go (HsRecTy{})           = False
        go (HsTyVar{})           = False
        go (HsFunTy{})           = p >= funPrec
        go (HsTupleTy{})         = False
        go (HsSumTy{})           = False
        go (HsKindSig{})         = p >= sigPrec
        go (HsListTy{})          = False
        go (HsIParamTy{})        = p > topPrec
        go (HsSpliceTy{})        = False
        go (HsExplicitListTy{})  = False
        go (HsExplicitTupleTy{}) = False
        go (HsTyLit{})           = False
        go (HsWildCardTy{})      = False
        go (HsStarTy{})          = False
        go (HsAppTy{})           = p >= appPrec
        go (HsAppKindTy{})       = p >= appPrec
        go (HsOpTy{})            = p >= opPrec
        go (HsParTy{})           = False
        go (HsDocTy _ (L _ t) _) = go t
        go (XHsType{})           = False

    maybeAddSpace :: [LHsType pass] -> SDoc -> SDoc
    -- See Note [Printing promoted type constructors]
    -- in IfaceType.  This code implements the same
    -- logic for printing HsType
    maybeAddSpace tys doc
      | (ty : _) <- tys
      , lhsTypeHasLeadingPromotionQuote ty = space <> doc
      | otherwise                          = doc

    lhsTypeHasLeadingPromotionQuote :: LHsType pass -> Bool
    lhsTypeHasLeadingPromotionQuote ty
      = goL ty
      where
        goL (L _ ty) = go ty

        go (HsForAllTy{})        = False
        go (HsQualTy{ hst_ctxt = ctxt, hst_body = body})
          | L _ (c:_) <- ctxt    = goL c
          | otherwise            = goL body
        go (HsBangTy{})          = False
        go (HsRecTy{})           = False
        go (HsTyVar _ p _)       = isPromoted p
        go (HsFunTy _ arg _)     = goL arg
        go (HsListTy{})          = False
        go (HsTupleTy{})         = False
        go (HsSumTy{})           = False
        go (HsOpTy _ t1 _ _)     = goL t1
        go (HsKindSig _ t _)     = goL t
        go (HsIParamTy{})        = False
        go (HsSpliceTy{})        = False
        go (HsExplicitListTy _ p _) = isPromoted p
        go (HsExplicitTupleTy{}) = True
        go (HsTyLit{})           = False
        go (HsWildCardTy{})      = False
        go (HsStarTy{})          = False
        go (HsAppTy _ t _)       = goL t
        go (HsAppKindTy _ t _)   = goL t
        go (HsParTy{})           = False
        go (HsDocTy _ t _)       = goL t
        go (XHsType{})           = False

    -- | @'parenthesizeHsType' p ty@ checks if @'hsTypeNeedsParens' p ty@ is
    -- true, and if so, surrounds @ty@ with an 'HsParTy'. Otherwise, it simply
    -- returns @ty@.
    parenthesizeHsType :: PprPrec -> LHsType (GhcPass p) -> LHsType (GhcPass p)
    parenthesizeHsType p lty@(L loc ty)
      | hsTypeNeedsParens p ty = L loc (HsParTy noExtField lty)
      | otherwise              = lty

    -- | @'parenthesizeHsContext' p ctxt@ checks if @ctxt@ is a single constraint
    -- @c@ such that @'hsTypeNeedsParens' p c@ is true, and if so, surrounds @c@
    -- with an 'HsParTy' to form a parenthesized @ctxt@. Otherwise, it simply
    -- returns @ctxt@ unchanged.
    parenthesizeHsContext :: PprPrec
                          -> LHsContext (GhcPass p) -> LHsContext (GhcPass p)
    parenthesizeHsContext p lctxt@(L loc ctxt) =
      case ctxt of
        [c] -> L loc [parenthesizeHsType p c]
        _   -> lctxt -- Other contexts are already "parenthesized" by virtue of
                     -- being tuples.