content: {-# LANGUAGE AllowAmbiguousTypes     #-} -- for unXRec, etc.
  {-# LANGUAGE ConstraintKinds         #-}
  {-# LANGUAGE DataKinds               #-}
  {-# LANGUAGE DeriveDataTypeable      #-}
  {-# LANGUAGE EmptyCase               #-}
  {-# LANGUAGE EmptyDataDeriving       #-}
  {-# LANGUAGE FlexibleContexts        #-}
  {-# LANGUAGE FlexibleInstances       #-}
  {-# LANGUAGE GADTs                   #-}
  {-# LANGUAGE MultiParamTypeClasses   #-}
  {-# LANGUAGE RankNTypes              #-}
  {-# LANGUAGE ScopedTypeVariables     #-}
  {-# LANGUAGE TypeFamilyDependencies  #-}
  {-# LANGUAGE UndecidableInstances    #-} -- Wrinkle in Note [Trees That Grow]
                                           -- in module Language.Haskell.Syntax.Extension
  
  -- See Note [Language.Haskell.Syntax.* Hierarchy] for why not GHC.Hs.*
  module Language.Haskell.Syntax.Extension where
  
  -- This module captures the type families to precisely identify the extension
  -- points for GHC.Hs syntax
  
  import Data.Type.Equality (type (~))
  
  import Data.Data hiding ( Fixity )
  import Data.Kind (Type)
  
  import Data.Eq
  import Data.Ord
  
  {-
  Note [Trees That Grow]
  ~~~~~~~~~~~~~~~~~~~~~~
  
  See https://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow
  
  The hsSyn AST is reused across multiple compiler passes. We also have the
  Template Haskell AST, and the haskell-src-exts one (outside of GHC)
  
  Supporting multiple passes means the AST has various warts on it to cope with
  the specifics for the phases, such as the 'ValBindsOut', 'ConPatOut',
  'SigPatOut' etc.
  
  The growable AST will allow each of these variants to be captured explicitly,
  such that they only exist in the given compiler pass AST, as selected by the
  type parameter to the AST.
  
  In addition it will allow tool writers to define their own extensions to capture
  additional information for the tool, in a natural way.
  
  A further goal is to provide a means to harmonise the Template Haskell and
  haskell-src-exts ASTs as well.
  
  Wrinkle: In order to print out the AST, we need to know it is Outputable.
  We also sometimes need to branch on the particular pass that we're in
  (e.g. to print out type information once we know it). In order to allow
  both of these actions, we define OutputableBndrId, which gathers the necessary
  OutputableBndr and IsPass constraints. The use of this constraint in instances
  generally requires UndecidableInstances.
  
  See also Note [IsPass] and Note [NoGhcTc] in GHC.Hs.Extension.
  
  -}
  
  -- | A placeholder type for TTG extension points that are not currently
  -- used to represent any particular value.
  --
  -- This should not be confused with 'DataConCantHappen', which are found in unused
  -- extension /constructors/ and therefore should never be inhabited. In
  -- contrast, 'NoExtField' is used in extension /points/ (e.g., as the field of
  -- some constructor), so it must have an inhabitant to construct AST passes
  -- that manipulate fields with that extension point as their type.
  data NoExtField = NoExtField
    deriving (Data,Eq,Ord)
  
  -- | Used when constructing a term with an unused extension point.
  noExtField :: NoExtField
  noExtField = NoExtField
  
  {-
  Note [Constructor cannot occur]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Some data constructors can't occur in certain phases; e.g. the output
  of the type checker never has OverLabel. We signal this by
  * setting the extension field to DataConCantHappen
  * using dataConCantHappen in the cases that can't happen
  
  For example:
  
     type instance XOverLabel GhcTc = DataConCantHappen
  
     dsExpr :: HsExpr GhcTc -> blah
     dsExpr (HsOverLabel x _) = dataConCantHappen x
  
  The function dataConCantHappen is defined thus:
     dataConCantHappen :: DataConCantHappen -> a
     dataConCantHappen x = case x of {}
  (i.e. identically to Data.Void.absurd, but more helpfully named).
  Remember DataConCantHappen is a type whose only element is bottom.
  
  This should not be confused with 'NoExtField', which are found in unused
  extension /points/ (not /constructors/) and therefore can be inhabited.
  
  It would be better to omit the pattern match altogether, but we
  can only do that if the extension field was strict (#18764).
  See also [DataConCantHappen and strict fields].
  -}
  data DataConCantHappen
    deriving (Data,Eq,Ord)
  
  -- | Eliminate a 'DataConCantHappen'. See Note [Constructor cannot occur].
  dataConCantHappen :: DataConCantHappen -> a
  dataConCantHappen x = case x of {}
  
  -- | GHC's L prefixed variants wrap their vanilla variant in this type family,
  -- to add 'SrcLoc' info via 'Located'. Other passes than 'GhcPass' not
  -- interested in location information can define this as
  -- @type instance XRec NoLocated a = a@.
  -- See Note [XRec and SrcSpans in the AST]
  type family XRec p a = r | r -> a
  
  type family Anno a = b -- See Note [XRec and Anno in the AST] in GHC.Parser.Annotation
  
  {-
  Note [XRec and SrcSpans in the AST]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  XRec is meant to replace most of the uses of `Located` in the AST. It is another
  extension point meant to make it easier for non-GHC applications to reuse the
  AST for their own purposes, and not have to deal the hassle of (perhaps) useless
  SrcSpans everywhere.
  
  instead of `Located (HsExpr p)` or similar types, we will now have `XRec p
  (HsExpr p)`
  
  XRec allows annotating certain points in the AST with extra
  information. This maybe be source spans (for GHC), nothing (for TH),
  types (for HIE files), exact print annotations (for exactprint) or
  anything else.
  
  This should hopefully bring us one step closer to sharing the AST between GHC
  and TH.
  
  We use the `UnXRec`, `MapXRec` and `WrapXRec` type classes to aid us in writing
  pass-polymorphic code that deals with `XRec`s
  -}
  
  -- | We can strip off the XRec to access the underlying data.
  -- See Note [XRec and SrcSpans in the AST]
  class UnXRec p where
    unXRec :: XRec p a -> a
  
  -- | We can map over the underlying type contained in an @XRec@ while preserving
  -- the annotation as is.
  class MapXRec p where
    mapXRec :: (Anno a ~ Anno b) => (a -> b) -> XRec p a -> XRec p b
  -- See Note [XRec and SrcSpans in the AST]
  -- See Note [XRec and Anno in the AST] in GHC.Parser.Annotation
  -- AZ: Is there a way to not have Anno in this file, but still have MapXRec?
  --     Perhaps define XRec with an additional b parameter, only used in Hs as (Anno b)?
  
  -- | The trivial wrapper that carries no additional information
  -- See Note [XRec and SrcSpans in the AST]
  class WrapXRec p a where
    wrapXRec :: a -> XRec p a
  
  -- | Maps the "normal" id type for a given pass
  type family IdP p
  
  type LIdP p = XRec p (IdP p)
  
  -- | Like 'IdP', except it keeps track of the user-written module qualification,
  -- if any.
  --
  -- See Note [IdOcc].
  type family IdOccP p
  type LIdOccP p = XRec p (IdOccP p)
  
  {- Note [IdOcc]
  ~~~~~~~~~~~~~~~
  When possible, in error messages we would like to report identifiers with the
  qualification that the user has written (provided this does not cause any
  ambiguity). To do this, we record the user-written qualification in the
  AST in the GhcRn stage. This is achieved with the 'WithUserRdr' data type.
  Thus:
  
    - instead of using 'IdP', we use 'IdOccP', which wraps an 'IdP' in
      'WithUserRdr' at GhcRn pass, in:
        - 'HsVar'
        - 'HsTyVar' (which is used for 'TyCon's, not just type variables)
        - 'HsOpTy'
    - we also use 'ConLikeP' which wraps a 'Name' in 'WithUserRdr' at 'GhcRn'
      pass, in:
        - 'RecordCon'
        - 'ConPat'
  
  This user-written module qualification is then consulted when pretty-printing
  expressions, e.g. we have:
  
    - ppr_expr (HsVar _ (L _ v)) = pprPrefixOcc v
  
  and 'pprPrefixOcc' uses the 'OutputableBndr' instance for 'WithUserRdr'.
  This happens in 'GHC.Types.Name.Ppr.mkQualName'.
  
  Test case: T25877.
  -}
  
  -- =====================================================================
  -- Type families for the HsBinds extension points
  
  -- HsLocalBindsLR type families
  type family XHsValBinds      x x'
  type family XHsIPBinds       x x'
  type family XEmptyLocalBinds x x'
  type family XXHsLocalBindsLR x x'
  
  -- HsValBindsLR type families
  type family XValBinds    x x'
  type family XXValBindsLR x x'
  
  -- HsBindLR type families
  type family XFunBind    x x'
  type family XPatBind    x x'
  type family XVarBind    x x'
  type family XPatSynBind x x'
  type family XXHsBindsLR x x'
  
  -- PatSynBind type families
  type family XPSB x x'
  type family XXPatSynBind x x'
  
  -- HsIPBinds type families
  type family XIPBinds    x
  type family XXHsIPBinds x
  
  -- IPBind type families
  type family XCIPBind x
  type family XXIPBind x
  
  -- Sig type families
  type family XTypeSig          x
  type family XPatSynSig        x
  type family XClassOpSig       x
  type family XIdSig            x
  type family XFixSig           x
  type family XInlineSig        x
  type family XSpecSig          x
  type family XSpecSigE         x
  type family XSpecInstSig      x
  type family XMinimalSig       x
  type family XSCCFunSig        x
  type family XCompleteMatchSig x
  type family XXSig             x
  
  -- FixitySig type families
  type family XFixitySig          x
  type family XXFixitySig         x
  
  -- StandaloneKindSig type families
  type family XStandaloneKindSig  x
  type family XXStandaloneKindSig x
  
  -- =====================================================================
  -- Type families for the HsDecls extension points
  
  -- HsDecl type families
  type family XTyClD       x
  type family XInstD       x
  type family XDerivD      x
  type family XValD        x
  type family XSigD        x
  type family XKindSigD    x
  type family XDefD        x
  type family XForD        x
  type family XWarningD    x
  type family XAnnD        x
  type family XRuleD       x
  type family XSpliceD     x
  type family XDocD        x
  type family XRoleAnnotD  x
  type family XXHsDecl     x
  
  -- -------------------------------------
  -- HsGroup type families
  type family XCHsGroup      x
  type family XXHsGroup      x
  
  -- -------------------------------------
  -- SpliceDecl type families
  type family XSpliceDecl       x
  type family XXSpliceDecl      x
  
  -- -------------------------------------
  -- TyClDecl type families
  type family XFamDecl       x
  type family XSynDecl       x
  type family XDataDecl      x
  type family XClassDecl     x
  type family XXTyClDecl     x
  
  -- -------------------------------------
  -- FunDep type families
  type family XCFunDep      x
  type family XXFunDep      x
  
  -- -------------------------------------
  -- TyClGroup type families
  type family XCTyClGroup      x
  type family XXTyClGroup      x
  
  -- -------------------------------------
  -- FamilyResultSig type families
  type family XNoSig            x
  type family XCKindSig         x -- Clashes with XKindSig above
  type family XTyVarSig         x
  type family XXFamilyResultSig x
  
  -- -------------------------------------
  -- FamilyDecl type families
  type family XCFamilyDecl      x
  type family XXFamilyDecl      x
  
  -- -------------------------------------
  -- HsDataDefn type families
  type family XCHsDataDefn      x
  type family XXHsDataDefn      x
  
  -- -------------------------------------
  -- HsDerivingClause type families
  type family XCHsDerivingClause      x
  type family XXHsDerivingClause      x
  
  -- -------------------------------------
  -- DerivClauseTys type families
  type family XDctSingle       x
  type family XDctMulti        x
  type family XXDerivClauseTys x
  
  -- -------------------------------------
  -- ConDecl type families
  type family XConDeclGADT   x
  type family XConDeclH98    x
  type family XXConDecl      x
  
  -- -------------------------------------
  -- FamEqn type families
  type family XCFamEqn      x r
  type family XXFamEqn      x r
  
  -- -------------------------------------
  -- TyFamInstDecl type families
  type family XCTyFamInstDecl x
  type family XXTyFamInstDecl x
  
  -- -------------------------------------
  -- ClsInstDecl type families
  type family XCClsInstDecl      x
  type family XXClsInstDecl      x
  
  -- -------------------------------------
  -- InstDecl type families
  type family XClsInstD      x
  type family XDataFamInstD  x
  type family XTyFamInstD    x
  type family XXInstDecl     x
  
  -- -------------------------------------
  -- DerivDecl type families
  type family XCDerivDecl      x
  type family XXDerivDecl      x
  
  -- -------------------------------------
  -- DerivStrategy type family
  type family XStockStrategy    x
  type family XAnyClassStrategy x
  type family XNewtypeStrategy  x
  type family XViaStrategy      x
  
  -- -------------------------------------
  -- DefaultDecl type families
  type family XCDefaultDecl      x
  type family XXDefaultDecl      x
  
  -- -------------------------------------
  -- ForeignDecl type families
  type family XForeignImport     x
  type family XForeignExport     x
  type family XXForeignDecl      x
  type family XCImport           x
  type family XXForeignImport    x
  type family XCExport           x
  type family XXForeignExport    x
  
  -- -------------------------------------
  -- RuleDecls type families
  type family XCRuleDecls      x
  type family XXRuleDecls      x
  
  -- -------------------------------------
  -- RuleDecl type families
  type family XHsRule          x
  type family XXRuleDecl       x
  
  -- -------------------------------------
  -- RuleBndrs type families
  type family XCRuleBndrs     x
  type family XXRuleBndrs     x
  
  -- -------------------------------------
  -- RuleBndr type families
  type family XCRuleBndr      x
  type family XRuleBndrSig    x
  type family XXRuleBndr      x
  
  -- -------------------------------------
  -- WarnDecls type families
  type family XWarnings        x
  type family XXWarnDecls      x
  
  -- -------------------------------------
  -- WarnDecl type families
  type family XWarning        x
  type family XXWarnDecl      x
  
  -- -------------------------------------
  -- AnnDecl type families
  type family XHsAnnotation  x
  type family XXAnnDecl      x
  
  -- -------------------------------------
  -- RoleAnnotDecl type families
  type family XCRoleAnnotDecl  x
  type family XXRoleAnnotDecl  x
  
  -- -------------------------------------
  -- InjectivityAnn type families
  type family XCInjectivityAnn  x
  type family XXInjectivityAnn  x
  
  -- =====================================================================
  -- Type families for the HsModule extension points
  
  type family XCModule x
  type family XXModule x
  
  -- =====================================================================
  -- Type families for the HsExpr extension points
  
  type family XVar            x
  type family XUnboundVar     x
  type family XRecSel         x
  type family XOverLabel      x
  type family XIPVar          x
  type family XOverLitE       x
  type family XLitE           x
  type family XLam            x
  type family XLamCase        x
  type family XApp            x
  type family XAppTypeE       x
  type family XOpApp          x
  type family XNegApp         x
  type family XPar            x
  type family XSectionL       x
  type family XSectionR       x
  type family XExplicitTuple  x
  type family XExplicitSum    x
  type family XCase           x
  type family XIf             x
  type family XMultiIf        x
  type family XLet            x
  type family XDo             x
  type family XExplicitList   x
  type family XRecordCon      x
  type family XRecordUpd      x
  type family XLHsRecUpdLabels x
  type family XLHsOLRecUpdLabels x
  type family XGetField       x
  type family XProjection     x
  type family XExprWithTySig  x
  type family XArithSeq       x
  type family XTypedBracket   x
  type family XUntypedBracket x
  type family XTypedSplice    x
  type family XUntypedSplice  x
  type family XProc           x
  type family XStatic         x
  type family XTick           x
  type family XBinTick        x
  type family XPragE          x
  type family XEmbTy          x
  type family XHole           x
  type family XForAll         x
  type family XQual           x
  type family XFunArr         x
  type family XXExpr          x
  
  -- -------------------------------------
  -- HsMatchContext type families
  type family XFunRhs
  
  -- -------------------------------------
  -- DotFieldOcc type families
  type family XCDotFieldOcc  x
  type family XXDotFieldOcc  x
  
  -- -------------------------------------
  -- HsPragE type families
  type family XSCC            x
  type family XXPragE         x
  
  -- -------------------------------------
  -- HsTupArg type families
  type family XPresent  x
  type family XMissing  x
  type family XXTupArg  x
  
  -- -------------------------------------
  -- HsUntypedSplice type families
  type family XUntypedSpliceExpr x
  type family XTypedSpliceExpr x
  type family XQuasiQuote        x
  type family XXUntypedSplice    x
  
  -- HsTypedSplice type families
  type family XXTypedSplice      x
  
  -- -------------------------------------
  -- HsQuoteBracket type families
  type family XExpBr  x
  type family XPatBr  x
  type family XDecBrL x
  type family XDecBrG x
  type family XTypBr  x
  type family XVarBr  x
  type family XXQuote x
  
  -- -------------------------------------
  -- HsCmdTop type families
  type family XCmdTop  x
  type family XXCmdTop x
  
  -- -------------------------------------
  -- MatchGroup type families
  type family XMG           x b
  type family XXMatchGroup  x b
  
  -- -------------------------------------
  -- Match type families
  type family XCMatch  x b
  type family XXMatch  x b
  
  -- -------------------------------------
  -- GRHSs type families
  type family XCGRHSs  x b
  type family XXGRHSs  x b
  
  -- -------------------------------------
  -- GRHS type families
  type family XCGRHS  x b
  type family XXGRHS  x b
  
  -- -------------------------------------
  -- StmtLR type families
  type family XLastStmt        x x' b
  type family XBindStmt        x x' b
  type family XBodyStmt        x x' b
  type family XLetStmt         x x' b
  type family XParStmt         x x' b
  type family XTransStmt       x x' b
  type family XRecStmt         x x' b
  type family XXStmtLR         x x' b
  
  -- -------------------------------------
  -- HsCmd type families
  type family XCmdArrApp   x
  type family XCmdArrForm  x
  type family XCmdApp      x
  type family XCmdLam      x
  type family XCmdPar      x
  type family XCmdCase     x
  type family XCmdLamCase  x
  type family XCmdIf       x
  type family XCmdLet      x
  type family XCmdDo       x
  type family XCmdWrap     x
  type family XXCmd        x
  
  -- -------------------------------------
  -- ParStmtBlock type families
  type family XParStmtBlock  x x'
  type family XXParStmtBlock x x'
  
  -- =====================================================================
  -- Type families for the HsLit extension points
  
  -- We define a type family for each extension point. This is based on prepending
  -- 'X' to the constructor name, for ease of reference.
  type family XHsChar x
  type family XHsCharPrim x
  type family XHsString x
  type family XHsMultilineString x
  type family XHsStringPrim x
  type family XHsInt x
  type family XHsIntPrim x
  type family XHsWordPrim x
  type family XHsInt8Prim x
  type family XHsInt16Prim x
  type family XHsInt32Prim x
  type family XHsInt64Prim x
  type family XHsWord8Prim x
  type family XHsWord16Prim x
  type family XHsWord32Prim x
  type family XHsWord64Prim x
  type family XHsFloatPrim x
  type family XHsDoublePrim x
  type family XXLit x
  
  -- -------------------------------------
  -- HsOverLit type families
  type family XOverLit  x
  type family XXOverLit x
  
  -- =====================================================================
  -- Type families for the HsPat extension points
  
  type family XWildPat     x
  type family XVarPat      x
  type family XLazyPat     x
  type family XAsPat       x
  type family XParPat      x
  type family XBangPat     x
  type family XListPat     x
  type family XTuplePat    x
  type family XSumPat      x
  type family XOrPat       x
  type family XConPat      x
  type family XViewPat     x
  type family XSplicePat   x
  type family XLitPat      x
  type family XNPat        x
  type family XNPlusKPat   x
  type family XSigPat      x
  type family XEmbTyPat    x
  type family XInvisPat    x
  type family XCoPat       x
  type family XXPat        x
  type family XHsFieldBind x
  
  -- =====================================================================
  -- Type families for the HsTypes type families
  
  
  -- -------------------------------------
  -- LHsQTyVars type families
  type family XHsQTvs       x
  type family XXLHsQTyVars  x
  
  -- -------------------------------------
  -- HsOuterTyVarBndrs type families
  type family XHsOuterImplicit    x
  type family XHsOuterExplicit    x flag
  type family XXHsOuterTyVarBndrs x
  
  -- -------------------------------------
  -- HsSigType type families
  type family XHsSig      x
  type family XXHsSigType x
  
  -- -------------------------------------
  -- HsWildCardBndrs type families
  type family XHsWC              x b
  type family XXHsWildCardBndrs  x b
  
  -- -------------------------------------
  -- HsPatSigType type families
  type family XHsPS x
  type family XXHsPatSigType x
  
  -- -------------------------------------
  -- HsTyPat type families
  type family XHsTP x
  type family XXHsTyPat x
  
  -- -------------------------------------
  -- HsType type families
  type family XForAllTy        x
  type family XQualTy          x
  type family XTyVar           x
  type family XAppTy           x
  type family XAppKindTy       x
  type family XFunTy           x
  type family XListTy          x
  type family XTupleTy         x
  type family XSumTy           x
  type family XOpTy            x
  type family XParTy           x
  type family XIParamTy        x
  type family XStarTy          x
  type family XKindSig         x
  type family XSpliceTy        x
  type family XDocTy           x
  type family XExplicitListTy  x
  type family XExplicitTupleTy x
  type family XTyLit           x
  type family XWildCardTy      x
  type family XXType           x
  
  -- ---------------------------------------------------------------------
  -- HsTyLit type families
  type family XNumTy           x
  type family XStrTy           x
  type family XCharTy          x
  type family XXTyLit          x
  
  -- ---------------------------------------------------------------------
  -- HsForAllTelescope type families
  type family XHsForAllVis        x
  type family XHsForAllInvis      x
  type family XXHsForAllTelescope x
  
  -- ---------------------------------------------------------------------
  -- HsTyVarBndr type families
  type family XTyVarBndr   x
  type family XXTyVarBndr  x
  
  -- ---------------------------------------------------------------------
  -- HsConDeclRecField type families
  type family XConDeclRecField  x
  type family XXConDeclRecField x
  
  -- ---------------------------------------------------------------------
  -- ConDeclField type families
  type family XConDeclField  x
  type family XXConDeclField x
  
  -- ---------------------------------------------------------------------
  -- FieldOcc type families
  type family XCFieldOcc x
  type family XXFieldOcc x
  
  -- =====================================================================
  -- Type families for the HsImpExp extension points
  
  -- -------------------------------------
  -- ImportDecl type families
  type family XCImportDecl       x
  type family XXImportDecl       x
  type family ImportDeclPkgQual  x -- stores the package qualifier in an import statement
  
  -- -------------------------------------
  -- IE type families
  type family XIEVar             x
  type family XIEThingAbs        x
  type family XIEThingAll        x
  type family XIEThingWith       x
  type family XIEModuleContents  x
  type family XIEGroup           x
  type family XIEDoc             x
  type family XIEDocNamed        x
  type family XXIE               x
  
  -- -------------------------------------
  -- IEWrappedName type families
  type family XIEName p
  type family XIEDefault p
  type family XIEPattern p
  type family XIEType p
  type family XIEData p
  type family XXIEWrappedName p
  
  
  
  -- =====================================================================
  -- Misc
  
  -- | See Note [NoGhcTc] in GHC.Hs.Extension. It has to be in this
  -- module because it is used like an extension point (in the data definitions
  -- of types that should be parameter-agnostic.
  type family NoGhcTc (p :: Type)
  
  -- =====================================================================
  -- End of Type family definitions
  -- =====================================================================
  