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

    {-# LANGUAGE DeriveDataTypeable #-}

    -- | Abstract Haskell syntax for expressions.
    module HsExpr where

    #include "HsVersions.h"

    -- friends:
    import HsDecls
    import HsPat
    import HsLit
    import HsTypes
    import HsBinds

    -- others:
    import Var
    import Name
    import BasicTypes
    import DataCon
    import SrcLoc
    import Outputable
    import FastString

    -- libraries:
    import Data.Data hiding (Fixity)

\end{code} %************************************************************************ %* * \subsection{Expressions proper} %* * %************************************************************************ \begin{code}

    -- * Expressions proper

    type LHsExpr id = Located (HsExpr id)

    -------------------------
    -- | PostTcExpr is an evidence expression attached to the syntax tree by the
    -- type checker (c.f. postTcType).
    type PostTcExpr  = HsExpr Id
    -- | We use a PostTcTable where there are a bunch of pieces of evidence, more
    -- than is convenient to keep individually.
    type PostTcTable = [(Name, PostTcExpr)]

    noPostTcExpr :: PostTcExpr
    noPostTcExpr = HsLit (HsString (fsLit "noPostTcExpr"))

    noPostTcTable :: PostTcTable
    noPostTcTable = []

    -------------------------
    -- | SyntaxExpr is like 'PostTcExpr', but it's filled in a little earlier,
    -- by the renamer.  It's used for rebindable syntax.
    --
    -- E.g. @(>>=)@ is filled in before the renamer by the appropriate 'Name' for
    --      @(>>=)@, and then instantiated by the type checker with its type args
    --      etc

    type SyntaxExpr id = HsExpr id

    noSyntaxExpr :: SyntaxExpr id -- Before renaming, and sometimes after,
                                  -- (if the syntax slot makes no sense)
    noSyntaxExpr = HsLit (HsString (fsLit "noSyntaxExpr"))

    type SyntaxTable id = [(Name, SyntaxExpr id)]
    -- ^ Currently used only for 'CmdTop' (sigh)
    --
    -- * Before the renamer, this list is 'noSyntaxTable'
    --
    -- * After the renamer, it takes the form @[(std_name, HsVar actual_name)]@
    --   For example, for the 'return' op of a monad
    --
    --    * normal case:            @(GHC.Base.return, HsVar GHC.Base.return)@
    --
    --    * with rebindable syntax: @(GHC.Base.return, return_22)@
    --              where @return_22@ is whatever @return@ is in scope
    --
    -- * After the type checker, it takes the form @[(std_name, <expression>)]@
    --      where @<expression>@ is the evidence for the method

    noSyntaxTable :: SyntaxTable id
    noSyntaxTable = []

    -------------------------
    -- | A Haskell expression.
    data HsExpr id
      = HsVar     id                        -- ^ variable
      | HsIPVar   (IPName id)               -- ^ implicit parameter
      | HsOverLit (HsOverLit id)            -- ^ Overloaded literals

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

      | HsLam     (MatchGroup id)           -- Currently always a single match

      | HsApp     (LHsExpr id) (LHsExpr id) -- Application

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

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

      | OpApp       (LHsExpr id)    -- left operand
                    (LHsExpr id)    -- operator
                    Fixity          -- Renamer adds fixity; bottom until then
                    (LHsExpr id)    -- right operand

      | NegApp      (LHsExpr id)    -- negated expr
                    (SyntaxExpr id) -- Name of 'negate'

      | HsPar       (LHsExpr id)    -- parenthesised expr

      | SectionL    (LHsExpr id)    -- operand
                    (LHsExpr id)    -- operator
      | SectionR    (LHsExpr id)    -- operator
                    (LHsExpr id)    -- operand

      | ExplicitTuple		-- Used for explicit tuples and sections thereof
            [HsTupArg id]
            Boxity

      | HsCase      (LHsExpr id)
                    (MatchGroup id)

      | HsIf        (Maybe (SyntaxExpr id)) -- cond function
        		       		        -- Nothing => use the built-in 'if'
    					-- See Note [Rebindable if]
                    (LHsExpr id)    --  predicate
                    (LHsExpr id)    --  then part
                    (LHsExpr id)    --  else part

      | HsLet       (HsLocalBinds id) -- let(rec)
                    (LHsExpr  id)

      | HsDo        (HsStmtContext Name) -- The parameterisation is unimportant
                                         -- because in this context we never use
                                         -- the PatGuard or ParStmt variant
                    [LStmt id]           -- "do":one or more stmts
                    (LHsExpr id)         -- The body; the last expression in the
                                         -- 'do' of [ body | ... ] in a list comp
                    PostTcType           -- Type of the whole expression

      | ExplicitList                -- syntactic list
                    PostTcType      -- Gives type of components of list
                    [LHsExpr id]

      | ExplicitPArr                -- syntactic parallel array: [:e1, ..., en:]
                    PostTcType      -- type of elements of the parallel array
                    [LHsExpr id]

      -- Record construction
      | RecordCon   (Located id)       -- The constructor.  After type checking
                                       -- it's the dataConWrapId of the constructor
                    PostTcExpr         -- Data con Id applied to type args
                    (HsRecordBinds id)

      -- Record update
      | RecordUpd   (LHsExpr id)
                    (HsRecordBinds id)
    --		(HsMatchGroup Id)  -- Filled in by the type checker to be
    --				   -- a match that does the job
                    [DataCon]          -- Filled in by the type checker to the
                                       -- _non-empty_ list of DataCons that have
                                       -- all the upd'd fields
                    [PostTcType]       -- Argument types of *input* record type
                    [PostTcType]       --              and  *output* record type
      -- For a type family, the arg types are of the *instance* tycon,
      -- not the family tycon

      | ExprWithTySig                       -- e :: type
                    (LHsExpr id)
                    (LHsType id)

      | ExprWithTySigOut                    -- TRANSLATION
                    (LHsExpr id)
                    (LHsType Name)          -- Retain the signature for
                                            -- round-tripping purposes

      | ArithSeq                            -- arithmetic sequence
                    PostTcExpr
                    (ArithSeqInfo id)

      | PArrSeq                             -- arith. sequence for parallel array
                    PostTcExpr              -- [:e1..e2:] or [:e1, e2..e3:]
                    (ArithSeqInfo id)

      | HsSCC       FastString              -- "set cost centre" SCC pragma
                    (LHsExpr id)            -- expr whose cost is to be measured

      | HsCoreAnn   FastString              -- hdaume: core annotation
                    (LHsExpr id)

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

      | HsBracket    (HsBracket id)

      | HsBracketOut (HsBracket Name)       -- Output of the type checker is
                                            -- the *original*
                     [PendingSplice]        -- renamed expression, plus
                                            -- _typechecked_ splices to be
                                            -- pasted back in by the desugarer

      | HsSpliceE (HsSplice id)

      | HsQuasiQuoteE (HsQuasiQuote id)
    	-- See Note [Quasi-quote overview] in TcSplice

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

      | HsProc      (LPat id)               -- arrow abstraction, proc
                    (LHsCmdTop id)          -- body of the abstraction
                                            -- always has an empty stack

      ---------------------------------------
      -- The following are commands, not expressions proper

      | HsArrApp            -- Arrow tail, or arrow application (f -< arg)
            (LHsExpr id)    -- arrow expression, f
            (LHsExpr id)    -- input expression, arg
            PostTcType      -- type of the arrow expressions f,
                            -- of the form a t t', where arg :: t
            HsArrAppType    -- higher-order (-<<) or first-order (-<)
            Bool            -- True => right-to-left (f -< arg)
                            -- False => left-to-right (arg >- f)

      | HsArrForm           -- Command formation,  (| e cmd1 .. cmdn |)
            (LHsExpr id)    -- the operator
                            -- after type-checking, a type abstraction to be
                            -- applied to the type of the local environment tuple
            (Maybe Fixity)  -- fixity (filled in by the renamer), for forms that
                            -- were converted from OpApp's by the renamer
            [LHsCmdTop id]  -- argument commands

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

      | HsTick
         Int                                -- module-local tick number
         [id]                               -- variables in scope
         (LHsExpr id)                       -- sub-expression

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

      | HsTickPragma                        -- A pragma introduced tick
         (FastString,(Int,Int),(Int,Int))   -- external span for this tick
         (LHsExpr id)

      ---------------------------------------
      -- These constructors only appear temporarily in the parser.
      -- The renamer translates them into the Right Thing.

      | EWildPat                 -- wildcard

      | EAsPat      (Located id) -- as pattern
                    (LHsExpr id)

      | EViewPat    (LHsExpr id) -- view pattern
                    (LHsExpr id)

      | ELazyPat    (LHsExpr id) -- ~ pattern

      | HsType      (LHsType id) -- Explicit type argument; e.g  f {| Int |} x y

      ---------------------------------------
      -- Finally, HsWrap appears only in typechecker output

      |  HsWrap     HsWrapper    -- TRANSLATION
                    (HsExpr id)
      deriving (Data, Typeable)

    -- HsTupArg is used for tuple sections
    --  (,a,) is represented by  ExplicitTuple [Mising ty1, Present a, Missing ty3]
    --  Which in turn stands for (\x:ty1 \y:ty2. (x,a,y))
    data HsTupArg id
      = Present (LHsExpr id)	-- The argument
      | Missing PostTcType		-- The argument is missing, but this is its type
      deriving (Data, Typeable)

    tupArgPresent :: HsTupArg id -> Bool
    tupArgPresent (Present {}) = True
    tupArgPresent (Missing {}) = False

    type PendingSplice = (Name, LHsExpr Id) -- Typechecked splices, waiting to be
                                            -- pasted back in by the desugarer

\end{code} Note [Rebindable if] ~~~~~~~~~~~~~~~~~~~~ The rebindable syntax for 'if' is a bit special, because when rebindable syntax is *off* we do not want to treat (if c then t else e) as if it was an application (ifThenElse c t e). Why not? Because we allow an 'if' to return *unboxed* results, thus if blah then 3# else 4# whereas that would not be possible using a all to a polymorphic function (because you can't call a polymorphic function at an unboxed type). So we use Nothing to mean "use the old built-in typing rule". \begin{code}

    instance OutputableBndr id => Outputable (HsExpr id) where
        ppr expr = pprExpr expr

\end{code} \begin{code}

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

    pprExpr :: OutputableBndr id => HsExpr id -> SDoc
    pprExpr e | isAtomicHsExpr e || isQuietHsExpr e =            ppr_expr e
              | otherwise                           = pprDeeper (ppr_expr e)

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

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

    -----------------------
    ppr_lexpr :: OutputableBndr id => LHsExpr id -> SDoc
    ppr_lexpr e = ppr_expr (unLoc e)

    ppr_expr :: OutputableBndr id => HsExpr id -> SDoc
    ppr_expr (HsVar v)       = pprHsVar v
    ppr_expr (HsIPVar v)     = ppr v
    ppr_expr (HsLit lit)     = ppr lit
    ppr_expr (HsOverLit lit) = ppr lit
    ppr_expr (HsPar e)       = parens (ppr_lexpr e)

    ppr_expr (HsCoreAnn s e)
      = vcat [ptext (sLit "HsCoreAnn") <+> ftext s, ppr_lexpr e]

    ppr_expr (HsApp e1 e2)
      = let (fun, args) = collect_args e1 [e2] in
        hang (ppr_lexpr fun) 2 (sep (map pprParendExpr args))
      where
        collect_args (L _ (HsApp fun arg)) args = collect_args fun (arg:args)
        collect_args fun args = (fun, args)

    ppr_expr (OpApp e1 op _ e2)
      = case unLoc op of
          HsVar v -> pp_infixly v
          _       -> pp_prefixly
      where
        pp_e1 = pprDebugParendExpr e1   -- In debug mode, add parens
        pp_e2 = pprDebugParendExpr e2   -- to make precedence clear

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

        pp_infixly v
          = sep [nest 2 pp_e1, pprHsInfix v, nest 2 pp_e2]

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

    ppr_expr (SectionL expr op)
      = case unLoc op of
          HsVar v -> pp_infixly v
          _       -> pp_prefixly
      where
        pp_expr = pprDebugParendExpr expr

        pp_prefixly = hang (hsep [text " \\ x_ ->", ppr op])
                           4 (hsep [pp_expr, ptext (sLit "x_ )")])
        pp_infixly v = (sep [pp_expr, pprHsInfix v])

    ppr_expr (SectionR op expr)
      = case unLoc op of
          HsVar v -> pp_infixly v
          _       -> pp_prefixly
      where
        pp_expr = pprDebugParendExpr expr

        pp_prefixly = hang (hsep [text "( \\ x_ ->", ppr op, ptext (sLit "x_")])
                           4 ((<>) pp_expr rparen)
        pp_infixly v
          = (sep [pprHsInfix v, pp_expr])

    ppr_expr (ExplicitTuple exprs boxity)
      = tupleParens boxity (fcat (ppr_tup_args exprs))
      where
        ppr_tup_args []               = []
        ppr_tup_args (Present e : es) = (ppr_lexpr e <> punc es) : ppr_tup_args es
        ppr_tup_args (Missing _ : es) = punc es : ppr_tup_args es

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

    --avoid using PatternSignatures for stage1 code portability
    ppr_expr exprType@(HsLam matches)
      = pprMatches (LambdaExpr `asTypeOf` idType exprType) matches
     where idType :: HsExpr id -> HsMatchContext id; idType = undefined

    ppr_expr exprType@(HsCase expr matches)
      = sep [ sep [ptext (sLit "case"), nest 4 (ppr expr), ptext (sLit "of {")],
              nest 2 (pprMatches (CaseAlt `asTypeOf` idType exprType) matches <+> char '}') ]
     where idType :: HsExpr id -> HsMatchContext id; idType = undefined

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

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

    ppr_expr (HsLet binds expr)
      = sep [hang (ptext (sLit "let")) 2 (pprBinds binds),
             hang (ptext (sLit "in"))  2 (ppr expr)]

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

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

    ppr_expr (ExplicitPArr _ exprs)
      = pa_brackets (pprDeeperList fsep (punctuate comma (map ppr_lexpr exprs)))

    ppr_expr (RecordCon con_id _ rbinds)
      = hang (ppr con_id) 2 (ppr rbinds)

    ppr_expr (RecordUpd aexp rbinds _ _ _)
      = hang (pprParendExpr aexp) 2 (ppr rbinds)

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

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

    ppr_expr EWildPat       = char '_'
    ppr_expr (ELazyPat e)   = char '~' <> pprParendExpr e
    ppr_expr (EAsPat v e)   = ppr v <> char '@' <> pprParendExpr e
    ppr_expr (EViewPat p e) = ppr p <+> ptext (sLit "->") <+> ppr e

    ppr_expr (HsSCC lbl expr)
      = sep [ ptext (sLit "_scc_") <+> doubleQuotes (ftext lbl),
              pprParendExpr expr ]

    ppr_expr (HsWrap co_fn e) = pprHsWrapper (pprExpr e) co_fn
    ppr_expr (HsType id)      = ppr id

    ppr_expr (HsSpliceE s)       = pprSplice s
    ppr_expr (HsBracket b)       = pprHsBracket b
    ppr_expr (HsBracketOut e []) = ppr e
    ppr_expr (HsBracketOut e ps) = ppr e $$ ptext (sLit "pending") <+> ppr ps
    ppr_expr (HsQuasiQuoteE qq)  = ppr qq

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

    ppr_expr (HsTick tickId vars exp)
      = pprTicks (ppr exp) $
        hcat [ptext (sLit "tick<"),
        ppr tickId,
        ptext (sLit ">("),
        hsep (map pprHsVar vars),
        ppr exp,
        ptext (sLit ")")]
    ppr_expr (HsBinTick tickIdTrue tickIdFalse exp)
      = pprTicks (ppr exp) $
        hcat [ptext (sLit "bintick<"),
              ppr tickIdTrue,
              ptext (sLit ","),
              ppr tickIdFalse,
              ptext (sLit ">("),
              ppr exp,ptext (sLit ")")]
    ppr_expr (HsTickPragma externalSrcLoc exp)
      = pprTicks (ppr exp) $
        hcat [ptext (sLit "tickpragma<"),
              ppr externalSrcLoc,
              ptext (sLit ">("),
              ppr exp,
              ptext (sLit ")")]

    ppr_expr (HsArrApp arrow arg _ HsFirstOrderApp True)
      = hsep [ppr_lexpr arrow, ptext (sLit "-<"), ppr_lexpr arg]
    ppr_expr (HsArrApp arrow arg _ HsFirstOrderApp False)
      = hsep [ppr_lexpr arg, ptext (sLit ">-"), ppr_lexpr arrow]
    ppr_expr (HsArrApp arrow arg _ HsHigherOrderApp True)
      = hsep [ppr_lexpr arrow, ptext (sLit "-<<"), ppr_lexpr arg]
    ppr_expr (HsArrApp arrow arg _ HsHigherOrderApp False)
      = hsep [ppr_lexpr arg, ptext (sLit ">>-"), ppr_lexpr arrow]

    ppr_expr (HsArrForm (L _ (HsVar v)) (Just _) [arg1, arg2])
      = sep [pprCmdArg (unLoc arg1), hsep [pprHsInfix v, pprCmdArg (unLoc arg2)]]
    ppr_expr (HsArrForm op _ args)
      = hang (ptext (sLit "(|") <> ppr_lexpr op)
             4 (sep (map (pprCmdArg.unLoc) args) <> ptext (sLit "|)"))

    pprCmdArg :: OutputableBndr id => HsCmdTop id -> SDoc
    pprCmdArg (HsCmdTop cmd@(L _ (HsArrForm _ Nothing [])) _ _ _)
      = ppr_lexpr cmd
    pprCmdArg (HsCmdTop cmd _ _ _)
      = parens (ppr_lexpr cmd)

    instance OutputableBndr id => Outputable (HsCmdTop id) where
        ppr = pprCmdArg

    -- add parallel array brackets around a document
    --
    pa_brackets :: SDoc -> SDoc
    pa_brackets p = ptext (sLit "[:") <> p <> ptext (sLit ":]")

\end{code} HsSyn records exactly where the user put parens, with HsPar. So generally speaking we print without adding any parens. However, some code is internally generated, and in some places parens are absolutely required; so for these places we use pprParendExpr (but don't print double parens of course). For operator applications we don't add parens, because the oprerator fixities should do the job, except in debug mode (-dppr-debug) so we can see the structure of the parse tree. \begin{code}

    pprDebugParendExpr :: OutputableBndr id => LHsExpr id -> SDoc
    pprDebugParendExpr expr
      = getPprStyle (\sty ->
        if debugStyle sty then pprParendExpr expr
                          else pprLExpr      expr)

    pprParendExpr :: OutputableBndr id => LHsExpr id -> SDoc
    pprParendExpr expr
      = let
            pp_as_was = pprLExpr expr
            -- Using pprLExpr makes sure that we go 'deeper'
            -- I think that is usually (always?) right
        in
        case unLoc expr of
          ArithSeq {}       -> pp_as_was
          PArrSeq {}        -> pp_as_was
          HsLit {}          -> pp_as_was
          HsOverLit {}      -> pp_as_was
          HsVar {}          -> pp_as_was
          HsIPVar {}        -> pp_as_was
          ExplicitTuple {}  -> pp_as_was
          ExplicitList {}   -> pp_as_was
          ExplicitPArr {}   -> pp_as_was
          HsPar {}          -> pp_as_was
          HsBracket {}      -> pp_as_was
          HsBracketOut _ [] -> pp_as_was
          HsDo sc _ _ _
           | isListCompExpr sc -> pp_as_was
          _                    -> parens pp_as_was

    isAtomicHsExpr :: HsExpr id -> Bool -- A single token
    isAtomicHsExpr (HsVar {})     = True
    isAtomicHsExpr (HsLit {})     = True
    isAtomicHsExpr (HsOverLit {}) = True
    isAtomicHsExpr (HsIPVar {})   = True
    isAtomicHsExpr (HsWrap _ e)   = isAtomicHsExpr e
    isAtomicHsExpr (HsPar e)      = isAtomicHsExpr (unLoc e)
    isAtomicHsExpr _              = False

\end{code} %************************************************************************ %* * \subsection{Commands (in arrow abstractions)} %* * %************************************************************************ We re-use HsExpr to represent these. \begin{code}

    type HsCmd id = HsExpr id

    type LHsCmd id = LHsExpr id

    data HsArrAppType = HsHigherOrderApp | HsFirstOrderApp
      deriving (Data, Typeable)

\end{code} The legal constructors for commands are: = HsArrApp ... -- as above | HsArrForm ... -- as above | HsApp (HsCmd id) (HsExpr id) | HsLam (Match id) -- kappa -- the renamer turns this one into HsArrForm | OpApp (HsExpr id) -- left operand (HsCmd id) -- operator Fixity -- Renamer adds fixity; bottom until then (HsCmd id) -- right operand | HsPar (HsCmd id) -- parenthesised command | HsCase (HsExpr id) [Match id] -- bodies are HsCmd's SrcLoc | HsIf (Maybe (SyntaxExpr id)) -- cond function (HsExpr id) -- predicate (HsCmd id) -- then part (HsCmd id) -- else part SrcLoc | HsLet (HsLocalBinds id) -- let(rec) (HsCmd id) | HsDo (HsStmtContext Name) -- The parameterisation is unimportant -- because in this context we never use -- the PatGuard or ParStmt variant [Stmt id] -- HsExpr's are really HsCmd's PostTcType -- Type of the whole expression SrcLoc Top-level command, introducing a new arrow. This may occur inside a proc (where the stack is empty) or as an argument of a command-forming operator. \begin{code}

    type LHsCmdTop id = Located (HsCmdTop id)

    data HsCmdTop id
      = HsCmdTop (LHsCmd id)
                 [PostTcType]     -- types of inputs on the command's stack
                 PostTcType       -- return type of the command
                 (SyntaxTable id) -- after type checking:
                                  -- names used in the command's desugaring
      deriving (Data, Typeable)

\end{code} %************************************************************************ %* * \subsection{Record binds} %* * %************************************************************************ \begin{code}

    type HsRecordBinds id = HsRecFields id (LHsExpr id)

\end{code} %************************************************************************ %* * \subsection{@Match@, @GRHSs@, and @GRHS@ datatypes} %* * %************************************************************************ @Match@es are sets of pattern bindings and right hand sides for functions, patterns or case branches. For example, if a function @g@ is defined as: \begin{verbatim} g (x,y) = y g ((x:ys),y) = y+1, \end{verbatim} then \tr{g} has two @Match@es: @(x,y) = y@ and @((x:ys),y) = y+1@. It is always the case that each element of an @[Match]@ list has the same number of @pats@s inside it. This corresponds to saying that a function defined by pattern matching must have the same number of patterns in each equation. \begin{code}

    data MatchGroup id
      = MatchGroup
            [LMatch id]     -- The alternatives
            PostTcType      -- The type is the type of the entire group
                            --      t1 -> ... -> tn -> tr
                            -- where there are n patterns
      deriving (Data, Typeable)

    type LMatch id = Located (Match id)

    data Match id
      = Match
            [LPat id]               -- The patterns
            (Maybe (LHsType id))    -- A type signature for the result of the match
                                    -- Nothing after typechecking
            (GRHSs id)
      deriving (Data, Typeable)

    isEmptyMatchGroup :: MatchGroup id -> Bool
    isEmptyMatchGroup (MatchGroup ms _) = null ms

    matchGroupArity :: MatchGroup id -> Arity
    matchGroupArity (MatchGroup [] _)
      = panic "matchGroupArity"     -- Precondition: MatchGroup is non-empty
    matchGroupArity (MatchGroup (match:matches) _)
      = ASSERT( all ((== n_pats) . length . hsLMatchPats) matches )
        -- Assertion just checks that all the matches have the same number of pats
        n_pats
      where
        n_pats = length (hsLMatchPats match)

    hsLMatchPats :: LMatch id -> [LPat id]
    hsLMatchPats (L _ (Match pats _ _)) = pats

    -- | GRHSs are used both for pattern bindings and for Matches
    data GRHSs id
      = GRHSs {
          grhssGRHSs :: [LGRHS id],  -- ^ Guarded RHSs
          grhssLocalBinds :: (HsLocalBinds id) -- ^ The where clause
        } deriving (Data, Typeable)

    type LGRHS id = Located (GRHS id)

    -- | Guarded Right Hand Side.
    data GRHS id = GRHS [LStmt id]   -- Guards
                        (LHsExpr id) -- Right hand side
      deriving (Data, Typeable)

\end{code} We know the list must have at least one @Match@ in it. \begin{code}

    pprMatches :: (OutputableBndr idL, OutputableBndr idR) => HsMatchContext idL -> MatchGroup idR -> SDoc
    pprMatches ctxt (MatchGroup matches _)
        = vcat (map (pprMatch ctxt) (map unLoc matches))
          -- Don't print the type; it's only a place-holder before typechecking

    -- Exported to HsBinds, which can't see the defn of HsMatchContext
    pprFunBind :: (OutputableBndr idL, OutputableBndr idR) => idL -> Bool -> MatchGroup idR -> SDoc
    pprFunBind fun inf matches = pprMatches (FunRhs fun inf) matches

    -- Exported to HsBinds, which can't see the defn of HsMatchContext
    pprPatBind :: (OutputableBndr bndr, OutputableBndr id)
               => LPat bndr -> GRHSs id -> SDoc
    pprPatBind pat ty@(grhss)
     = sep [ppr pat, nest 2 (pprGRHSs (PatBindRhs `asTypeOf` idType ty) grhss)]
    --avoid using PatternSignatures for stage1 code portability
     where idType :: GRHSs id -> HsMatchContext id; idType = undefined

    pprMatch :: (OutputableBndr idL, OutputableBndr idR) => HsMatchContext idL -> Match idR -> SDoc
    pprMatch ctxt (Match pats maybe_ty grhss)
      = sep [ sep (herald : map (nest 2 . pprParendLPat) other_pats)
            , nest 2 ppr_maybe_ty
            , nest 2 (pprGRHSs ctxt grhss) ]
      where
        (herald, other_pats)
            = case ctxt of
                FunRhs fun is_infix
                    | not is_infix -> (ppr fun, pats)
                            -- f x y z = e
                            -- Not pprBndr; the AbsBinds will
                            -- have printed the signature

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

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

                LambdaExpr -> (char '\\', pats)

                _  -> ASSERT( null pats1 )
                      (ppr pat1, [])	-- No parens around the single pat

        (pat1:pats1) = pats
        (pat2:pats2) = pats1
        ppr_maybe_ty = case maybe_ty of
                            Just ty -> dcolon <+> ppr ty
                            Nothing -> empty

    pprGRHSs :: (OutputableBndr idL, OutputableBndr idR)
             => HsMatchContext idL -> GRHSs idR -> SDoc
    pprGRHSs ctxt (GRHSs grhss binds)
      = vcat (map (pprGRHS ctxt . unLoc) grhss)
     $$ ppUnless (isEmptyLocalBinds binds)
          (text "where" $$ nest 4 (pprBinds binds))

    pprGRHS :: (OutputableBndr idL, OutputableBndr idR)
            => HsMatchContext idL -> GRHS idR -> SDoc

    pprGRHS ctxt (GRHS [] expr)
     =  pp_rhs ctxt expr

    pprGRHS ctxt (GRHS guards expr)
     = sep [char '|' <+> interpp'SP guards, pp_rhs ctxt expr]

    pp_rhs :: OutputableBndr idR => HsMatchContext idL -> LHsExpr idR -> SDoc
    pp_rhs ctxt rhs = matchSeparator ctxt <+> pprDeeper (ppr rhs)

\end{code} %************************************************************************ %* * \subsection{Do stmts and list comprehensions} %* * %************************************************************************ \begin{code}

    type LStmt id = Located (StmtLR id id)
    type LStmtLR idL idR = Located (StmtLR idL idR)

    type Stmt id = StmtLR id id

    -- The SyntaxExprs in here are used *only* for do-notation, which
    -- has rebindable syntax.  Otherwise they are unused.
    data StmtLR idL idR
      = BindStmt (LPat idL)
                 (LHsExpr idR)
                 (SyntaxExpr idR) -- The (>>=) operator
                 (SyntaxExpr idR) -- The fail operator
                 -- The fail operator is noSyntaxExpr
                 -- if the pattern match can't fail

      | ExprStmt (LHsExpr idR)     -- See Note [ExprStmt]
                 (SyntaxExpr idR) -- The (>>) operator
                 PostTcType       -- Element type of the RHS (used for arrows)

      | LetStmt  (HsLocalBindsLR idL idR)

      -- ParStmts only occur in a list comprehension
      | ParStmt  [([LStmt idL], [idR])]
      -- After renaming, the ids are the binders bound by the stmts and used
      -- after them

      -- "qs, then f by e" ==> TransformStmt qs binders f (Just e)
      -- "qs, then f"      ==> TransformStmt qs binders f Nothing
      | TransformStmt
             [LStmt idL]	-- Stmts are the ones to the left of the 'then'

             [idR] 		-- After renaming, the IDs are the binders occurring
    		        -- within this transform statement that are used after it

             (LHsExpr idR)		-- "then f"

             (Maybe (LHsExpr idR))	-- "by e" (optional)

      | GroupStmt
             [LStmt idL]      -- Stmts to the *left* of the 'group'
    	 	       	  -- which generates the tuples to be grouped

             [(idR, idR)]	  -- See Note [GroupStmt binder map]

             (Maybe (LHsExpr idR)) 	-- "by e" (optional)

             (Either		-- "using f"
                 (LHsExpr idR)	--   Left f  => explicit "using f"
                 (SyntaxExpr idR))	--   Right f => implicit; filled in with 'groupWith'

      -- Recursive statement (see Note [How RecStmt works] below)
      | RecStmt
         { recS_stmts :: [LStmtLR idL idR]

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

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

    	-- Rebindable syntax
         , recS_bind_fn :: SyntaxExpr idR -- The bind function
         , recS_ret_fn  :: SyntaxExpr idR -- The return function
         , recS_mfix_fn :: SyntaxExpr idR -- The mfix function

            -- These fields are only valid after typechecking
         , recS_rec_rets :: [PostTcExpr] -- These expressions correspond 1-to-1 with
                                         -- recS_rec_ids, and are the
                                         -- expressions that should be returned by
                                         -- the recursion.
                                         -- They may not quite be the Ids themselves,
                                         -- because the Id may be *polymorphic*, but
                                         -- the returned thing has to be *monomorphic*,
    				     -- so they may be type applications

          , recS_dicts :: TcEvBinds    -- Method bindings of Ids bound by the
                                       -- RecStmt, and used afterwards
          }
      deriving (Data, Typeable)

\end{code} Note [GroupStmt binder map] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ The [(idR,idR)] in a GroupStmt behaves as follows: * Before renaming: [] * After renaming: [ (x27,x27), ..., (z35,z35) ] These are the variables bound by the stmts to the left of the 'group' and used either in the 'by' clause, or in the stmts following the 'group' Each item is a pair of identical variables. * After typechecking: [ (x27:Int, x27:[Int]), ..., (z35:Bool, z35:[Bool]) ] Each pair has the same unique, but different *types*. Note [ExprStmt] ~~~~~~~~~~~~~~~ ExprStmts are a bit tricky, because what they mean depends on the context. Consider the following contexts: A do expression of type (m res_ty) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * ExprStmt E any_ty: do { ....; E; ... } E :: m any_ty Translation: E >> ... A list comprehensions of type [elt_ty] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * ExprStmt E Bool: [ .. | .... E ] [ .. | ..., E, ... ] [ .. | .... | ..., E | ... ] E :: Bool Translation: if E then fail else ... A guard list, guarding a RHS of type rhs_ty ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * ExprStmt E Bool: f x | ..., E, ... = ...rhs... E :: Bool Translation: if E then fail else ... Array comprehensions are handled like list comprehensions -=chak Note [How RecStmt works] ~~~~~~~~~~~~~~~~~~~~~~~~ Example: HsDo [ BindStmt x ex , RecStmt { recS_rec_ids = [a, c] , recS_stmts = [ BindStmt b (return (a,c)) , LetStmt a = ...b... , BindStmt c ec ] , recS_later_ids = [a, b] , return (a b) ] Here, the RecStmt binds a,b,c; but - Only a,b are used in the stmts *following* the RecStmt, - Only a,c are used in the stmts *inside* the RecStmt *before* their bindings Why do we need *both* rec_ids and later_ids? For monads they could be combined into a single set of variables, but not for arrows. That follows from the types of the respective feedback operators: mfix :: MonadFix m => (a -> m a) -> m a loop :: ArrowLoop a => a (b,d) (c,d) -> a b c * For mfix, the 'a' covers the union of the later_ids and the rec_ids * For 'loop', 'c' is the later_ids and 'd' is the rec_ids Note [Typing a RecStmt] ~~~~~~~~~~~~~~~~~~~~~~~ A (RecStmt stmts) types as if you had written (v1,..,vn, _, ..., _) <- mfix (\~(_, ..., _, r1, ..., rm) -> do { stmts ; return (v1,..vn, r1, ..., rm) }) where v1..vn are the later_ids r1..rm are the rec_ids \begin{code}

    instance (OutputableBndr idL, OutputableBndr idR) => Outputable (StmtLR idL idR) where
        ppr stmt = pprStmt stmt

    pprStmt :: (OutputableBndr idL, OutputableBndr idR) => (StmtLR idL idR) -> SDoc
    pprStmt (BindStmt pat expr _ _)   = hsep [ppr pat, ptext (sLit "<-"), ppr expr]
    pprStmt (LetStmt binds)           = hsep [ptext (sLit "let"), pprBinds binds]
    pprStmt (ExprStmt expr _ _)       = ppr expr
    pprStmt (ParStmt stmtss)          = hsep (map doStmts stmtss)
      where doStmts stmts = ptext (sLit "| ") <> ppr stmts

    pprStmt (TransformStmt stmts bndrs using by)
      = sep (ppr_lc_stmts stmts ++ [pprTransformStmt bndrs using by])

    pprStmt (GroupStmt stmts _ by using)
      = sep (ppr_lc_stmts stmts ++ [pprGroupStmt by using])

    pprStmt (RecStmt { recS_stmts = segment, recS_rec_ids = rec_ids
                     , recS_later_ids = later_ids })
      = ptext (sLit "rec") <+>
        vcat [ braces (vcat (map ppr segment))
             , ifPprDebug (vcat [ ptext (sLit "rec_ids=") <> ppr rec_ids
                                , ptext (sLit "later_ids=") <> ppr later_ids])]

    pprTransformStmt :: OutputableBndr id => [id] -> LHsExpr id -> Maybe (LHsExpr id) -> SDoc
    pprTransformStmt bndrs using by
      = sep [ ptext (sLit "then") <+> ifPprDebug (braces (ppr bndrs))
            , nest 2 (ppr using)
            , nest 2 (pprBy by)]

    pprGroupStmt :: OutputableBndr id => Maybe (LHsExpr id)
                                      -> Either (LHsExpr id) (SyntaxExpr is)
    				  -> SDoc
    pprGroupStmt by using
      = sep [ ptext (sLit "then group"), nest 2 (pprBy by), nest 2 (ppr_using using)]
      where
        ppr_using (Right _) = empty
        ppr_using (Left e)  = ptext (sLit "using") <+> ppr e

    pprBy :: OutputableBndr id => Maybe (LHsExpr id) -> SDoc
    pprBy Nothing  = empty
    pprBy (Just e) = ptext (sLit "by") <+> ppr e

    pprDo :: OutputableBndr id => HsStmtContext any -> [LStmt id] -> LHsExpr id -> SDoc
    pprDo DoExpr      stmts body = ptext (sLit "do")  <+> ppr_do_stmts stmts body
    pprDo GhciStmt    stmts body = ptext (sLit "do")  <+> ppr_do_stmts stmts body
    pprDo (MDoExpr _) stmts body = ptext (sLit "mdo") <+> ppr_do_stmts stmts body
    pprDo ListComp    stmts body = brackets    $ pprComp stmts body
    pprDo PArrComp    stmts body = pa_brackets $ pprComp stmts body
    pprDo _           _     _    = panic "pprDo" -- PatGuard, ParStmtCxt

    ppr_do_stmts :: OutputableBndr id => [LStmt id] -> LHsExpr id -> SDoc
    -- Print a bunch of do stmts, with explicit braces and semicolons,
    -- so that we are not vulnerable to layout bugs
    ppr_do_stmts stmts body
      = lbrace <+> pprDeeperList vcat ([ppr s <> semi | s <- stmts] ++ [ppr body])
               <+> rbrace

    ppr_lc_stmts :: OutputableBndr id => [LStmt id] -> [SDoc]
    ppr_lc_stmts stmts = [ppr s <> comma | s <- stmts]

    pprComp :: OutputableBndr id => [LStmt id] -> LHsExpr id -> SDoc
    pprComp quals body	  -- Prints:  body | qual1, ..., qualn
      = hang (ppr body <+> char '|') 2 (interpp'SP quals)

\end{code} %************************************************************************ %* * Template Haskell quotation brackets %* * %************************************************************************ \begin{code}

    data HsSplice id  = HsSplice            --  $z  or $(f 4)
                            id              -- The id is just a unique name to
                            (LHsExpr id)    -- identify this splice point
      deriving (Data, Typeable)

    instance OutputableBndr id => Outputable (HsSplice id) where
      ppr = pprSplice

    pprSplice :: OutputableBndr id => HsSplice id -> SDoc
    pprSplice (HsSplice n e)
        = char '$' <> ifPprDebug (brackets (ppr n)) <> eDoc
        where
              -- We use pprLExpr to match pprParendExpr:
              --     Using pprLExpr makes sure that we go 'deeper'
              --     I think that is usually (always?) right
              pp_as_was = pprLExpr e
              eDoc = case unLoc e of
                     HsPar _ -> pp_as_was
                     HsVar _ -> pp_as_was
                     _ -> parens pp_as_was

    data HsBracket id = ExpBr (LHsExpr id)   -- [|  expr  |]
                      | PatBr (LPat id)      -- [p| pat   |]
                      | DecBrL [LHsDecl id]	 -- [d| decls |]; result of parser
                      | DecBrG (HsGroup id)  -- [d| decls |]; result of renamer
                      | TypBr (LHsType id)   -- [t| type  |]
                      | VarBr id             -- 'x, ''T
      deriving (Data, Typeable)

    instance OutputableBndr id => Outputable (HsBracket id) where
      ppr = pprHsBracket

    pprHsBracket :: OutputableBndr id => HsBracket id -> SDoc
    pprHsBracket (ExpBr e) 	 = thBrackets empty (ppr e)
    pprHsBracket (PatBr p) 	 = thBrackets (char 'p') (ppr p)
    pprHsBracket (DecBrG gp) = thBrackets (char 'd') (ppr gp)
    pprHsBracket (DecBrL ds) = thBrackets (char 'd') (vcat (map ppr ds))
    pprHsBracket (TypBr t) 	 = thBrackets (char 't') (ppr t)
    pprHsBracket (VarBr n) 	 = char '\'' <> ppr n
    -- Infelicity: can't show ' vs '', because
    -- we can't ask n what its OccName is, because the
    -- pretty-printer for HsExpr doesn't ask for NamedThings
    -- But the pretty-printer for names will show the OccName class

    thBrackets :: SDoc -> SDoc -> SDoc
    thBrackets pp_kind pp_body = char '[' <> pp_kind <> char '|' <+>
                                 pp_body <+> ptext (sLit "|]")

\end{code} %************************************************************************ %* * \subsection{Enumerations and list comprehensions} %* * %************************************************************************ \begin{code}

    data ArithSeqInfo id
      = From            (LHsExpr id)
      | FromThen        (LHsExpr id)
                        (LHsExpr id)
      | FromTo          (LHsExpr id)
                        (LHsExpr id)
      | FromThenTo      (LHsExpr id)
                        (LHsExpr id)
                        (LHsExpr id)
      deriving (Data, Typeable)

\end{code} \begin{code}

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

    pp_dotdot :: SDoc
    pp_dotdot = ptext (sLit " .. ")

\end{code} %************************************************************************ %* * \subsection{HsMatchCtxt} %* * %************************************************************************ \begin{code}

    data HsMatchContext id  -- Context of a Match
      = FunRhs id Bool              -- Function binding for f; True <=> written infix
      | LambdaExpr                  -- Patterns of a lambda
      | CaseAlt                     -- Patterns and guards on a case alternative
      | ProcExpr                    -- Patterns of a proc
      | PatBindRhs                  -- Patterns in the *guards* of a pattern binding
      | RecUpd                      -- Record update [used only in DsExpr to
                                    --    tell matchWrapper what sort of
                                    --    runtime error message to generate]
      | StmtCtxt (HsStmtContext id) -- Pattern of a do-stmt or list comprehension
      | ThPatQuote			-- A Template Haskell pattern quotation [p| (a,b) |]
      deriving (Data, Typeable)

    data HsStmtContext id
      = ListComp
      | DoExpr
      | GhciStmt				 -- A command-line Stmt in GHCi pat <- rhs
      | MDoExpr PostTcTable                  -- Recursive do-expression
                                             -- (tiresomely, it needs table
                                             --  of its return/bind ops)
      | PArrComp                             -- Parallel array comprehension
      | PatGuard (HsMatchContext id)         -- Pattern guard for specified thing
      | ParStmtCtxt (HsStmtContext id)       -- A branch of a parallel stmt
      | TransformStmtCtxt (HsStmtContext id) -- A branch of a transform stmt
      deriving (Data, Typeable)

\end{code} \begin{code}

    isDoExpr :: HsStmtContext id -> Bool
    isDoExpr DoExpr      = True
    isDoExpr (MDoExpr _) = True
    isDoExpr _           = False

    isListCompExpr :: HsStmtContext id -> Bool
    isListCompExpr ListComp = True
    isListCompExpr PArrComp = True
    isListCompExpr _        = False

\end{code} \begin{code}

    matchSeparator :: HsMatchContext id -> SDoc
    matchSeparator (FunRhs {})  = ptext (sLit "=")
    matchSeparator CaseAlt      = ptext (sLit "->")
    matchSeparator LambdaExpr   = ptext (sLit "->")
    matchSeparator ProcExpr     = ptext (sLit "->")
    matchSeparator PatBindRhs   = ptext (sLit "=")
    matchSeparator (StmtCtxt _) = ptext (sLit "<-")
    matchSeparator RecUpd       = panic "unused"
    matchSeparator ThPatQuote   = panic "unused"

\end{code} \begin{code}

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

    pprMatchContextNoun :: Outputable id => HsMatchContext id -> SDoc
    pprMatchContextNoun (FunRhs fun _)  = ptext (sLit "equation for")
                                          <+> quotes (ppr fun)
    pprMatchContextNoun CaseAlt         = ptext (sLit "case alternative")
    pprMatchContextNoun RecUpd          = ptext (sLit "record-update construct")
    pprMatchContextNoun ThPatQuote      = ptext (sLit "Template Haskell pattern quotation")
    pprMatchContextNoun PatBindRhs      = ptext (sLit "pattern binding")
    pprMatchContextNoun LambdaExpr      = ptext (sLit "lambda abstraction")
    pprMatchContextNoun ProcExpr        = ptext (sLit "arrow abstraction")
    pprMatchContextNoun (StmtCtxt ctxt) = ptext (sLit "pattern binding in")
                                          $$ pprStmtContext ctxt

    pprStmtContext :: Outputable id => HsStmtContext id -> SDoc
    pprStmtContext (ParStmtCtxt c)
     = sep [ptext (sLit "a parallel branch of"), pprStmtContext c]
    pprStmtContext (TransformStmtCtxt c)
     = sep [ptext (sLit "a transformed branch of"), pprStmtContext c]
    pprStmtContext (PatGuard ctxt)
     = ptext (sLit "a pattern guard for") $$ pprMatchContext ctxt
    pprStmtContext GhciStmt        = ptext (sLit "an interactive GHCi command")
    pprStmtContext DoExpr          = ptext (sLit "a 'do' expression")
    pprStmtContext (MDoExpr _)     = ptext (sLit "an 'mdo' expression")
    pprStmtContext ListComp        = ptext (sLit "a list comprehension")
    pprStmtContext PArrComp        = ptext (sLit "an array comprehension")

    {-
    pprMatchRhsContext (FunRhs fun) = ptext (sLit "a right-hand side of function") <+> quotes (ppr fun)
    pprMatchRhsContext CaseAlt      = ptext (sLit "the body of a case alternative")
    pprMatchRhsContext PatBindRhs   = ptext (sLit "the right-hand side of a pattern binding")
    pprMatchRhsContext LambdaExpr   = ptext (sLit "the body of a lambda")
    pprMatchRhsContext ProcExpr     = ptext (sLit "the body of a proc")
    pprMatchRhsContext other        = panic "pprMatchRhsContext"    -- RecUpd, StmtCtxt

    -- Used for the result statement of comprehension
    -- e.g. the 'e' in      [ e | ... ]
    --      or the 'r' in   f x = r
    pprStmtResultContext (PatGuard ctxt) = pprMatchRhsContext ctxt
    pprStmtResultContext other           = ptext (sLit "the result of") <+> pprStmtContext other
    -}

    -- Used to generate the string for a *runtime* error message
    matchContextErrString :: Outputable id => HsMatchContext id -> SDoc
    matchContextErrString (FunRhs fun _)             = ptext (sLit "function") <+> ppr fun
    matchContextErrString CaseAlt                    = ptext (sLit "case")
    matchContextErrString PatBindRhs                 = ptext (sLit "pattern binding")
    matchContextErrString RecUpd                     = ptext (sLit "record update")
    matchContextErrString LambdaExpr                 = ptext (sLit "lambda")
    matchContextErrString ProcExpr                   = ptext (sLit "proc")
    matchContextErrString ThPatQuote                 = panic "matchContextErrString"  -- Not used at runtime
    matchContextErrString (StmtCtxt (ParStmtCtxt c)) = matchContextErrString (StmtCtxt c)
    matchContextErrString (StmtCtxt (TransformStmtCtxt c)) = matchContextErrString (StmtCtxt c)
    matchContextErrString (StmtCtxt (PatGuard _))    = ptext (sLit "pattern guard")
    matchContextErrString (StmtCtxt GhciStmt)        = ptext (sLit "interactive GHCi command")
    matchContextErrString (StmtCtxt DoExpr)          = ptext (sLit "'do' expression")
    matchContextErrString (StmtCtxt (MDoExpr _))     = ptext (sLit "'mdo' expression")
    matchContextErrString (StmtCtxt ListComp)        = ptext (sLit "list comprehension")
    matchContextErrString (StmtCtxt PArrComp)        = ptext (sLit "array comprehension")

\end{code} \begin{code}

    pprMatchInCtxt :: (OutputableBndr idL, OutputableBndr idR)
    	       => HsMatchContext idL -> Match idR -> SDoc
    pprMatchInCtxt ctxt match  = hang (ptext (sLit "In") <+> pprMatchContext ctxt <> colon)
    			     4 (pprMatch ctxt match)

    pprStmtInCtxt :: (OutputableBndr idL, OutputableBndr idR)
       	       => HsStmtContext idL -> StmtLR idL idR -> SDoc
    pprStmtInCtxt ctxt stmt = hang (ptext (sLit "In a stmt of") <+> pprStmtContext ctxt <> colon)
    		    	  4 (ppr_stmt stmt)
      where
        -- For Group and Transform Stmts, don't print the nested stmts!
        ppr_stmt (GroupStmt _ _ by using)         = pprGroupStmt by using
        ppr_stmt (TransformStmt _ bndrs using by) = pprTransformStmt bndrs using by
        ppr_stmt stmt                             = pprStmt stmt

\end{code}