%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1994
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[HsExpr]{Abstract Haskell syntax: expressions}
#include "HsVersions.h"
-- friends:
-import {-# SOURCE #-} HsMatches ( pprMatches, pprMatch, Match )
-
-import HsBinds ( HsBinds )
-import HsBasic ( HsLit )
-import BasicTypes ( Fixity(..), FixityDirection(..) )
-import HsTypes ( HsType )
+import HsDecls ( HsGroup )
+import HsPat ( LPat )
+import HsLit ( HsLit(..), HsOverLit )
+import HsTypes ( LHsType, PostTcType )
+import HsImpExp ( isOperator, pprHsVar )
+import HsBinds ( HsLocalBinds, DictBinds, ExprCoFn, isEmptyLocalBinds )
-- others:
-import Name ( NamedThing )
-import Id ( Id )
+import Type ( Type, pprParendType )
+import Var ( TyVar, Id )
+import Name ( Name )
+import BasicTypes ( IPName, Boxity, tupleParens, Arity, Fixity(..) )
+import SrcLoc ( Located(..), unLoc )
import Outputable
-import PprType ( pprType, pprParendType )
-import Type ( GenType )
-import TyVar ( GenTyVar )
-import SrcLoc ( SrcLoc )
+import FastString
\end{code}
+
%************************************************************************
%* *
\subsection{Expressions proper}
%************************************************************************
\begin{code}
-data HsExpr flexi id pat
- = HsVar id -- variable
- | HsLit HsLit -- literal
- | HsLitOut HsLit -- TRANSLATION
- (GenType flexi) -- (with its type)
+type LHsExpr id = Located (HsExpr id)
+
+-------------------------
+-- PostTcExpr is an evidence expression attached to the
+-- syntax tree by the type checker (c.f. postTcType)
+-- We use a PostTcTable where there are a bunch of pieces of
+-- evidence, more than is convenient to keep individually
+type PostTcExpr = HsExpr Id
+type PostTcTable = [(Name, Id)]
+
+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 tec
+
+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"))
- | HsLam (Match flexi id pat) -- lambda
- | HsApp (HsExpr flexi id pat) -- application
- (HsExpr flexi id pat)
+
+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 = []
+
+
+-------------------------
+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) -- Application
+ (LHsExpr id)
-- 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 (HsExpr flexi id pat) -- left operand
- (HsExpr flexi id pat) -- operator
- Fixity -- Renamer adds fixity; bottom until then
- (HsExpr flexi id pat) -- right operand
+ | OpApp (LHsExpr id) -- left operand
+ (LHsExpr id) -- operator
+ Fixity -- Renamer adds fixity; bottom until then
+ (LHsExpr id) -- right operand
- -- We preserve prefix negation and parenthesis for the precedence parser.
- -- They are eventually removed by the type checker.
+ | NegApp (LHsExpr id) -- negated expr
+ (SyntaxExpr id) -- Name of 'negate'
- | NegApp (HsExpr flexi id pat) -- negated expr
- (HsExpr flexi id pat) -- the negate id (in a HsVar)
+ | HsPar (LHsExpr id) -- parenthesised expr
- | HsPar (HsExpr flexi id pat) -- parenthesised expr
-
- | SectionL (HsExpr flexi id pat) -- operand
- (HsExpr flexi id pat) -- operator
- | SectionR (HsExpr flexi id pat) -- operator
- (HsExpr flexi id pat) -- operand
+ | SectionL (LHsExpr id) -- operand
+ (LHsExpr id) -- operator
+ | SectionR (LHsExpr id) -- operator
+ (LHsExpr id) -- operand
- | HsCase (HsExpr flexi id pat)
- [Match flexi id pat] -- must have at least one Match
- SrcLoc
+ | HsCase (LHsExpr id)
+ (MatchGroup id)
- | HsIf (HsExpr flexi id pat) -- predicate
- (HsExpr flexi id pat) -- then part
- (HsExpr flexi id pat) -- else part
- SrcLoc
-
- | HsLet (HsBinds flexi id pat) -- let(rec)
- (HsExpr flexi id pat)
+ | HsIf (LHsExpr id) -- predicate
+ (LHsExpr id) -- then part
+ (LHsExpr id) -- else part
- | HsDo DoOrListComp
- [Stmt flexi id pat] -- "do":one or more stmts
- SrcLoc
+ | HsLet (HsLocalBinds id) -- let(rec)
+ (LHsExpr id)
- | HsDoOut DoOrListComp
- [Stmt flexi id pat] -- "do":one or more stmts
- id -- id for return
- id -- id for >>=
- id -- id for zero
- (GenType flexi) -- Type of the whole expression
- SrcLoc
+ | 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
- [HsExpr flexi id pat]
- | ExplicitListOut -- TRANSLATION
- (GenType flexi) -- Gives type of components of list
- [HsExpr flexi id pat]
+ 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]
| ExplicitTuple -- tuple
- [HsExpr flexi id pat]
+ [LHsExpr id]
-- NB: Unit is ExplicitTuple []
-- for tuples, we can get the types
-- direct from the components
+ Boxity
- | HsCon Id -- TRANSLATION; a saturated constructor application
- [GenType flexi]
- [HsExpr flexi id pat]
-- Record construction
- | RecordCon id -- The constructor
- (HsExpr flexi id pat) -- Always (HsVar id) until type checker,
- -- but the latter adds its type args too
- (HsRecordBinds flexi id pat)
+ | 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 (HsExpr flexi id pat)
- (HsRecordBinds flexi id pat)
-
- | RecordUpdOut (HsExpr flexi id pat) -- TRANSLATION
- (GenType flexi) -- Type of *result* record (may differ from
- -- type of input record)
- [id] -- Dicts needed for construction
- (HsRecordBinds flexi id pat)
-
- | ExprWithTySig -- signature binding
- (HsExpr flexi id pat)
- (HsType id)
- | ArithSeqIn -- arithmetic sequence
- (ArithSeqInfo flexi id pat)
- | ArithSeqOut
- (HsExpr flexi id pat) -- (typechecked, of course)
- (ArithSeqInfo flexi id pat)
-
- | CCall FAST_STRING -- call into the C world; string is
- [HsExpr flexi id pat] -- the C function; exprs are the
- -- arguments to pass.
- Bool -- True <=> might cause Haskell
- -- garbage-collection (must generate
- -- more paranoid code)
- Bool -- True <=> it's really a "casm"
- -- NOTE: this CCall is the *boxed*
- -- version; the desugarer will convert
- -- it into the unboxed "ccall#".
- (GenType flexi) -- The result type; will be *bottom*
- -- until the typechecker gets ahold of it
-
- | HsSCC FAST_STRING -- "set cost centre" (_scc_) annotation
- (HsExpr flexi id pat) -- expr whose cost is to be measured
+ | RecordUpd (LHsExpr id)
+ (HsRecordBinds id)
+ PostTcType -- Type of *input* record
+ PostTcType -- Type of *result* record (may differ from
+ -- type of input record)
+
+ | 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_) annotation
+ (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)
+
+ -----------------------------------------------------------
+ -- 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
+\end{code}
+
+
+These constructors only appear temporarily in the parser.
+The renamer translates them into the Right Thing.
+
+\begin{code}
+ | EWildPat -- wildcard
+
+ | EAsPat (Located id) -- as pattern
+ (LHsExpr id)
+
+ | ELazyPat (LHsExpr id) -- ~ pattern
+
+ | HsType (LHsType id) -- Explicit type argument; e.g f {| Int |} x y
\end{code}
Everything from here on appears only in typechecker output.
\begin{code}
| TyLam -- TRANSLATION
- [GenTyVar flexi]
- (HsExpr flexi id pat)
+ [TyVar]
+ (LHsExpr id)
| TyApp -- TRANSLATION
- (HsExpr flexi id pat) -- generated by Spec
- [GenType flexi]
+ (LHsExpr id) -- generated by Spec
+ [Type]
-- DictLam and DictApp are "inverses"
| DictLam
[id]
- (HsExpr flexi id pat)
+ (LHsExpr id)
| DictApp
- (HsExpr flexi id pat)
+ (LHsExpr id)
[id]
-type HsRecordBinds flexi id pat
- = [(id, HsExpr flexi id pat, Bool)]
- -- True <=> source code used "punning",
- -- i.e. {op1, op2} rather than {op1=e1, op2=e2}
+ | HsCoerce ExprCoFn -- TRANSLATION
+ (HsExpr id)
+
+type PendingSplice = (Name, LHsExpr Id) -- Typechecked splices, waiting to be
+ -- pasted back in by the desugarer
\end{code}
A @Dictionary@, unless of length 0 or 1, becomes a tuple. A
\end{verbatim}
\begin{code}
-instance (NamedThing id, Outputable id, Outputable pat) =>
- Outputable (HsExpr flexi id pat) where
+instance OutputableBndr id => Outputable (HsExpr id) where
ppr expr = pprExpr expr
\end{code}
\begin{code}
-pprExpr :: (NamedThing id, Outputable id, Outputable pat)
- => HsExpr flexi id pat -> SDoc
+pprExpr :: OutputableBndr id => HsExpr id -> SDoc
-pprExpr e = pprDeeper (ppr_expr e)
+pprExpr e = pprDeeper (ppr_expr e)
-ppr_expr (HsVar v) = ppr v
+pprBinds :: OutputableBndr id => HsLocalBinds id -> SDoc
+pprBinds b = pprDeeper (ppr b)
-ppr_expr (HsLit lit) = ppr lit
-ppr_expr (HsLitOut lit _) = ppr lit
+ppr_lexpr :: OutputableBndr id => LHsExpr id -> SDoc
+ppr_lexpr e = ppr_expr (unLoc e)
-ppr_expr (HsLam match)
- = hsep [char '\\', nest 2 (pprMatch True match)]
+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 expr@(HsApp e1 e2)
- = let (fun, args) = collect_args expr [] in
- (ppr_expr fun) <+> (sep (map ppr_expr args))
+ppr_expr (HsApp e1 e2)
+ = let (fun, args) = collect_args e1 [e2] in
+ (ppr_lexpr fun) <+> (sep (map pprParendExpr args))
where
- collect_args (HsApp fun arg) args = collect_args fun (arg:args)
- collect_args fun args = (fun, args)
+ collect_args (L _ (HsApp fun arg)) args = collect_args fun (arg:args)
+ collect_args fun args = (fun, args)
ppr_expr (OpApp e1 op fixity e2)
- = case op of
+ = case unLoc op of
HsVar v -> pp_infixly v
_ -> pp_prefixly
where
pp_e2 = pprParendExpr e2
pp_prefixly
- = hang (pprExpr op) 4 (sep [pp_e1, pp_e2])
+ = hang (ppr op) 4 (sep [pp_e1, pp_e2])
pp_infixly v
- = sep [pp_e1, hsep [ppr v, pp_e2]]
+ = sep [pp_e1, hsep [pprInfix v, pp_e2]]
-ppr_expr (NegApp e _)
- = (<>) (char '-') (pprParendExpr e)
+ppr_expr (NegApp e _) = char '-' <+> pprParendExpr e
-ppr_expr (HsPar e)
- = parens (ppr_expr e)
+ppr_expr (HsPar e) = parens (ppr_lexpr e)
ppr_expr (SectionL expr op)
- = case op of
+ = case unLoc op of
HsVar v -> pp_infixly v
_ -> pp_prefixly
where
pp_prefixly = hang (hsep [text " \\ x_ ->", ppr op])
4 (hsep [pp_expr, ptext SLIT("x_ )")])
- pp_infixly v = parens (sep [pp_expr, ppr v])
+ pp_infixly v = parens (sep [pp_expr, pprInfix v])
ppr_expr (SectionR op expr)
- = case op of
+ = case unLoc op of
HsVar v -> pp_infixly v
_ -> pp_prefixly
where
pp_prefixly = hang (hsep [text "( \\ x_ ->", ppr op, ptext SLIT("x_")])
4 ((<>) pp_expr rparen)
pp_infixly v
- = parens (sep [ppr v, pp_expr])
+ = parens (sep [pprInfix v, pp_expr])
+
+ppr_expr (HsLam matches)
+ = pprMatches LambdaExpr matches
-ppr_expr (HsCase expr matches _)
- = sep [ sep [ptext SLIT("case"), nest 4 (pprExpr expr), ptext SLIT("of")],
- nest 2 (pprMatches (True, empty) matches) ]
+ppr_expr (HsCase expr matches)
+ = sep [ sep [ptext SLIT("case"), nest 4 (ppr expr), ptext SLIT("of")],
+ nest 2 (pprMatches CaseAlt matches) ]
-ppr_expr (HsIf e1 e2 e3 _)
- = sep [hsep [ptext SLIT("if"), nest 2 (pprExpr e1), ptext SLIT("then")],
- nest 4 (pprExpr e2),
+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 (pprExpr e3)]
+ nest 4 (ppr e3)]
-- special case: let ... in let ...
-ppr_expr (HsLet binds expr@(HsLet _ _))
- = sep [hang (ptext SLIT("let")) 2 (hsep [ppr binds, ptext SLIT("in")]),
- pprExpr expr]
+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 (ppr binds),
+ = sep [hang (ptext SLIT("let")) 2 (pprBinds binds),
hang (ptext SLIT("in")) 2 (ppr expr)]
-ppr_expr (HsDo do_or_list_comp stmts _) = pprDo do_or_list_comp stmts
-ppr_expr (HsDoOut do_or_list_comp stmts _ _ _ _ _) = pprDo do_or_list_comp stmts
+ppr_expr (HsDo do_or_list_comp stmts body _) = pprDo do_or_list_comp stmts body
-ppr_expr (ExplicitList exprs)
- = brackets (fsep (punctuate comma (map ppr_expr exprs)))
-ppr_expr (ExplicitListOut ty exprs)
- = hcat [ brackets (fsep (punctuate comma (map ppr_expr exprs))),
- ifNotPprForUser ((<>) space (parens (pprType ty))) ]
+ppr_expr (ExplicitList _ exprs)
+ = brackets (fsep (punctuate comma (map ppr_lexpr exprs)))
-ppr_expr (ExplicitTuple exprs)
- = parens (sep (punctuate comma (map ppr_expr exprs)))
+ppr_expr (ExplicitPArr _ exprs)
+ = pa_brackets (fsep (punctuate comma (map ppr_lexpr exprs)))
-ppr_expr (HsCon con_id tys args)
- = ppr con_id <+> sep (map pprParendType tys ++
- map pprParendExpr args)
+ppr_expr (ExplicitTuple exprs boxity)
+ = tupleParens boxity (sep (punctuate comma (map ppr_lexpr exprs)))
-ppr_expr (RecordCon con_id con rbinds)
- = pp_rbinds (ppr con) rbinds
+ppr_expr (RecordCon con_id con_expr rbinds)
+ = pp_rbinds (ppr con_id) rbinds
-ppr_expr (RecordUpd aexp rbinds)
- = pp_rbinds (pprParendExpr aexp) rbinds
-ppr_expr (RecordUpdOut aexp _ _ rbinds)
+ppr_expr (RecordUpd aexp rbinds _ _)
= pp_rbinds (pprParendExpr aexp) rbinds
ppr_expr (ExprWithTySig expr sig)
- = hang (nest 2 (ppr_expr expr) <+> ptext SLIT("::"))
+ = 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 (ArithSeqIn info)
- = brackets (ppr info)
-ppr_expr (ArithSeqOut expr info)
- = brackets (ppr info)
+ppr_expr (ArithSeq expr info) = brackets (ppr info)
+ppr_expr (PArrSeq expr info) = pa_brackets (ppr info)
-ppr_expr (CCall fun args _ is_asm result_ty)
- = hang (if is_asm
- then ptext SLIT("_casm_ ``") <> ptext fun <> ptext SLIT("''")
- else ptext SLIT("_ccall_") <+> ptext fun)
- 4 (sep (map pprParendExpr args))
+ppr_expr EWildPat = char '_'
+ppr_expr (ELazyPat e) = char '~' <> pprParendExpr e
+ppr_expr (EAsPat v e) = ppr v <> char '@' <> pprParendExpr e
-ppr_expr (HsSCC label expr)
- = sep [ ptext SLIT("_scc_") <+> doubleQuotes (ptext label), pprParendExpr expr ]
+ppr_expr (HsSCC lbl expr)
+ = sep [ ptext SLIT("_scc_") <+> doubleQuotes (ftext lbl), pprParendExpr expr ]
ppr_expr (TyLam tyvars expr)
- = hang (hsep [ptext SLIT("/\\"), interppSP tyvars, ptext SLIT("->")])
- 4 (ppr_expr expr)
+ = hang (hsep [ptext SLIT("/\\"),
+ hsep (map (pprBndr LambdaBind) tyvars),
+ ptext SLIT("->")])
+ 4 (ppr_lexpr expr)
ppr_expr (TyApp expr [ty])
- = hang (ppr_expr expr) 4 (pprParendType ty)
+ = hang (ppr_lexpr expr) 4 (pprParendType ty)
ppr_expr (TyApp expr tys)
- = hang (ppr_expr expr)
+ = hang (ppr_lexpr expr)
4 (brackets (interpp'SP tys))
ppr_expr (DictLam dictvars expr)
- = hang (hsep [ptext SLIT("\\{-dict-}"), interppSP dictvars, ptext SLIT("->")])
- 4 (ppr_expr expr)
+ = hang (hsep [ptext SLIT("\\{-dict-}"),
+ hsep (map (pprBndr LambdaBind) dictvars),
+ ptext SLIT("->")])
+ 4 (ppr_lexpr expr)
ppr_expr (DictApp expr [dname])
- = hang (ppr_expr expr) 4 (ppr dname)
+ = hang (ppr_lexpr expr) 4 (ppr dname)
ppr_expr (DictApp expr dnames)
- = hang (ppr_expr expr)
+ = hang (ppr_lexpr expr)
4 (brackets (interpp'SP dnames))
+ppr_expr (HsCoerce co_fn e) = ppr_expr e
+
+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 (HsProc pat (L _ (HsCmdTop cmd _ _ _)))
+ = hsep [ptext SLIT("proc"), ppr pat, ptext SLIT("->"), ppr cmd]
+
+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 [pprInfix 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)
+
+-- Put a var in backquotes if it's not an operator already
+pprInfix :: Outputable name => name -> SDoc
+pprInfix v | isOperator ppr_v = ppr_v
+ | otherwise = char '`' <> ppr_v <> char '`'
+ where
+ ppr_v = ppr v
+
+-- add parallel array brackets around a document
+--
+pa_brackets :: SDoc -> SDoc
+pa_brackets p = ptext SLIT("[:") <> p <> ptext SLIT(":]")
\end{code}
Parenthesize unless very simple:
\begin{code}
-pprParendExpr :: (NamedThing id, Outputable id, Outputable pat)
- => HsExpr flexi id pat -> SDoc
-
+pprParendExpr :: OutputableBndr id => LHsExpr id -> SDoc
pprParendExpr expr
= let
- pp_as_was = pprExpr expr
+ pp_as_was = ppr_lexpr expr
+ -- Using ppr_expr here avoids the call to 'deeper'
+ -- Not sure if that's always right.
in
- case expr of
- HsLit l -> ppr l
- HsLitOut l _ -> ppr l
+ case unLoc expr of
+ HsLit l -> ppr l
+ HsOverLit l -> ppr l
+
+ HsVar _ -> pp_as_was
+ HsIPVar _ -> pp_as_was
+ ExplicitList _ _ -> pp_as_was
+ ExplicitPArr _ _ -> pp_as_was
+ ExplicitTuple _ _ -> pp_as_was
+ HsPar _ -> pp_as_was
+ HsBracket _ -> pp_as_was
+ HsBracketOut _ [] -> pp_as_was
+
+ _ -> parens pp_as_was
+\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
+\end{code}
- HsVar _ -> pp_as_was
- ExplicitList _ -> pp_as_was
- ExplicitListOut _ _ -> pp_as_was
- ExplicitTuple _ -> pp_as_was
- HsPar _ -> pp_as_was
+The legal constructors for commands are:
- _ -> parens pp_as_was
+ = 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 (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
\end{code}
%************************************************************************
%************************************************************************
\begin{code}
-pp_rbinds :: (NamedThing id, Outputable id, Outputable pat)
- => SDoc
- -> HsRecordBinds flexi id pat -> SDoc
+type HsRecordBinds id = [(Located id, LHsExpr id)]
+recBindFields :: HsRecordBinds id -> [id]
+recBindFields rbinds = [unLoc field | (field,_) <- rbinds]
+
+pp_rbinds :: OutputableBndr id => SDoc -> HsRecordBinds id -> SDoc
pp_rbinds thing rbinds
= hang thing
- 4 (braces (hsep (punctuate comma (map (pp_rbind) rbinds))))
+ 4 (braces (sep (punctuate comma (map (pp_rbind) rbinds))))
where
- pp_rbind (v, e, pun_flag)
- = getPprStyle $ \ sty ->
- if pun_flag && userStyle sty then
- ppr v
- else
- hsep [ppr v, char '=', ppr e]
+ pp_rbind (v, e) = hsep [pprBndr LetBind (unLoc v), char '=', ppr e]
\end{code}
+
+
%************************************************************************
%* *
-\subsection{Do stmts and list comprehensions}
+\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 DoOrListComp = DoStmt | ListComp | Guard
-
-pprDo DoStmt stmts
- = hang (ptext SLIT("do")) 2 (vcat (map ppr stmts))
-pprDo ListComp stmts
- = brackets $
- hang (pprExpr expr <+> char '|')
- 4 (interpp'SP quals)
+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
+
+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)
+
+matchGroupArity :: MatchGroup id -> Arity
+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
- ReturnStmt expr = last stmts -- Last stmt should be a ReturnStmt for list comps
- quals = init stmts
+ 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 [LGRHS id] -- Guarded RHSs
+ (HsLocalBinds id) -- The where clause
+
+type LGRHS id = Located (GRHS id)
+
+data GRHS id = GRHS [LStmt id] -- Guards
+ (LHsExpr id) -- Right hand side
\end{code}
+We know the list must have at least one @Match@ in it.
+
\begin{code}
-data Stmt flexi id pat
- = BindStmt pat
- (HsExpr flexi id pat)
- SrcLoc
+pprMatches :: (OutputableBndr id) => HsMatchContext id -> MatchGroup id -> SDoc
+pprMatches ctxt (MatchGroup matches _) = vcat (map (pprMatch ctxt) (map unLoc matches))
- | LetStmt (HsBinds flexi id pat)
+-- Exported to HsBinds, which can't see the defn of HsMatchContext
+pprFunBind :: (OutputableBndr id) => id -> MatchGroup id -> SDoc
+pprFunBind fun matches = pprMatches (FunRhs fun) matches
- | GuardStmt (HsExpr flexi id pat) -- List comps only
- SrcLoc
+-- Exported to HsBinds, which can't see the defn of HsMatchContext
+pprPatBind :: (OutputableBndr bndr, OutputableBndr id)
+ => LPat bndr -> GRHSs id -> SDoc
+pprPatBind pat grhss = sep [ppr pat, nest 4 (pprGRHSs PatBindRhs grhss)]
- | ExprStmt (HsExpr flexi id pat) -- Do stmts only
- SrcLoc
- | ReturnStmt (HsExpr flexi id pat) -- List comps only, at the end
+pprMatch :: OutputableBndr id => HsMatchContext id -> Match id -> SDoc
+pprMatch ctxt (Match pats maybe_ty grhss)
+ = pp_name ctxt <+> sep [sep (map ppr pats),
+ ppr_maybe_ty,
+ nest 2 (pprGRHSs ctxt grhss)]
+ where
+ pp_name (FunRhs fun) = ppr fun -- Not pprBndr; the AbsBinds will
+ -- have printed the signature
+ pp_name LambdaExpr = char '\\'
+ pp_name other = empty
+
+ ppr_maybe_ty = case maybe_ty of
+ Just ty -> dcolon <+> ppr ty
+ Nothing -> empty
+
+
+pprGRHSs :: OutputableBndr id => HsMatchContext id -> GRHSs id -> SDoc
+pprGRHSs ctxt (GRHSs grhss binds)
+ = vcat (map (pprGRHS ctxt . unLoc) grhss)
+ $$
+ (if isEmptyLocalBinds binds then empty
+ else text "where" $$ nest 4 (pprBinds binds))
+
+pprGRHS :: OutputableBndr id => HsMatchContext id -> GRHS id -> 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 ctxt rhs = matchSeparator ctxt <+> pprDeeper (ppr rhs)
\end{code}
+%************************************************************************
+%* *
+\subsection{Do stmts and list comprehensions}
+%* *
+%************************************************************************
+
\begin{code}
-instance (NamedThing id, Outputable id, Outputable pat) =>
- Outputable (Stmt flexi id pat) where
+type LStmt id = Located (Stmt id)
+
+-- The SyntaxExprs in here are used *only* for do-notation, which
+-- has rebindable syntax. Otherwise they are unused.
+data Stmt id
+ = BindStmt (LPat id)
+ (LHsExpr id)
+ (SyntaxExpr id) -- The (>>=) operator
+ (SyntaxExpr id) -- The fail operator
+ -- The fail operator is noSyntaxExpr
+ -- if the pattern match can't fail
+
+ | ExprStmt (LHsExpr id)
+ (SyntaxExpr id) -- The (>>) operator
+ PostTcType -- Element type of the RHS (used for arrows)
+
+ | LetStmt (HsLocalBinds id)
+
+ -- ParStmts only occur in a list comprehension
+ | ParStmt [([LStmt id], [id])] -- After renaming, the ids are the binders
+ -- bound by the stmts and used subsequently
+
+ -- Recursive statement (see Note [RecStmt] below)
+ | RecStmt [LStmt id]
+ --- The next two fields are only valid after renaming
+ [id] -- The ids are a subset of the variables bound by the stmts
+ -- that are used in stmts that follow the RecStmt
+
+ [id] -- Ditto, but these variables are the "recursive" ones, that
+ -- are used before they are bound in the stmts of the RecStmt
+ -- From a type-checking point of view, these ones have to be monomorphic
+
+ --- These fields are only valid after typechecking
+ [PostTcExpr] -- These expressions correspond
+ -- 1-to-1 with the "recursive" [id], and are the expresions 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*.
+ (DictBinds id) -- Method bindings of Ids bound by the RecStmt,
+ -- and used afterwards
+\end{code}
+
+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 [RecStmt]
+~~~~~~~~~~~~~~
+Example:
+ HsDo [ BindStmt x ex
+
+ , RecStmt [a::forall a. a -> a, b]
+ [a::Int -> Int, c]
+ [ BindStmt b (return x)
+ , LetStmt a = ea
+ , BindStmt c ec ]
+
+ , return (a b) ]
+
+Here, the RecStmt binds a,b,c; but
+ - Only a,b are used in the stmts *following* the RecStmt,
+ This 'a' is *polymorphic'
+ - Only a,c are used in the stmts *inside* the RecStmt
+ *before* their bindings
+ This 'a' is monomorphic
+
+Nota Bene: the two a's have different types, even though they
+have the same Name.
+
+
+\begin{code}
+instance OutputableBndr id => Outputable (Stmt id) where
ppr stmt = pprStmt stmt
-pprStmt (BindStmt pat expr _)
- = hsep [ppr pat, ptext SLIT("<-"), ppr expr]
-pprStmt (LetStmt binds)
- = hsep [ptext SLIT("let"), ppr binds]
-pprStmt (ExprStmt expr _)
- = ppr expr
-pprStmt (GuardStmt expr _)
- = ppr expr
-pprStmt (ReturnStmt expr)
- = hsep [ptext SLIT("return"), ppr expr]
+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 (\stmts -> ptext SLIT("| ") <> ppr stmts) stmtss)
+pprStmt (RecStmt segment _ _ _ _) = ptext SLIT("rec") <+> braces (vcat (map ppr segment))
+
+pprDo :: OutputableBndr id => HsStmtContext any -> [LStmt id] -> LHsExpr id -> SDoc
+pprDo DoExpr stmts body = hang (ptext SLIT("do")) 2 (vcat (map ppr stmts) $$ ppr body)
+pprDo (MDoExpr _) stmts body = hang (ptext SLIT("mdo")) 3 (vcat (map ppr stmts) $$ ppr body)
+pprDo ListComp stmts body = pprComp brackets stmts body
+pprDo PArrComp stmts body = pprComp pa_brackets stmts body
+
+pprComp :: OutputableBndr id => (SDoc -> SDoc) -> [LStmt id] -> LHsExpr id -> SDoc
+pprComp brack quals body
+ = brack $
+ hang (ppr body <+> char '|')
+ 4 (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
+
+instance OutputableBndr id => Outputable (HsSplice id) where
+ ppr = pprSplice
+
+pprSplice :: OutputableBndr id => HsSplice id -> SDoc
+pprSplice (HsSplice n e) = char '$' <> brackets (ppr n) <> pprParendExpr e
+
+
+data HsBracket id = ExpBr (LHsExpr id) -- [| expr |]
+ | PatBr (LPat id) -- [p| pat |]
+ | DecBr (HsGroup id) -- [d| decls |]
+ | TypBr (LHsType id) -- [t| type |]
+ | VarBr id -- 'x, ''T
+
+instance OutputableBndr id => Outputable (HsBracket id) where
+ ppr = pprHsBracket
+
+
+pprHsBracket (ExpBr e) = thBrackets empty (ppr e)
+pprHsBracket (PatBr p) = thBrackets (char 'p') (ppr p)
+pprHsBracket (DecBr d) = thBrackets (char 'd') (ppr d)
+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 pp_kind pp_body = char '[' <> pp_kind <> char '|' <+>
+ pp_body <+> ptext SLIT("|]")
\end{code}
%************************************************************************
%************************************************************************
\begin{code}
-data ArithSeqInfo flexi id pat
- = From (HsExpr flexi id pat)
- | FromThen (HsExpr flexi id pat)
- (HsExpr flexi id pat)
- | FromTo (HsExpr flexi id pat)
- (HsExpr flexi id pat)
- | FromThenTo (HsExpr flexi id pat)
- (HsExpr flexi id pat)
- (HsExpr flexi id pat)
+data ArithSeqInfo id
+ = From (LHsExpr id)
+ | FromThen (LHsExpr id)
+ (LHsExpr id)
+ | FromTo (LHsExpr id)
+ (LHsExpr id)
+ | FromThenTo (LHsExpr id)
+ (LHsExpr id)
+ (LHsExpr id)
\end{code}
\begin{code}
-instance (NamedThing id, Outputable id, Outputable pat) =>
- Outputable (ArithSeqInfo flexi id pat) where
+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]
pp_dotdot = ptext SLIT(" .. ")
\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{HsMatchCtxt}
+%* *
+%************************************************************************
+
+\begin{code}
+data HsMatchContext id -- Context of a Match
+ = FunRhs id -- Function binding for f
+ | CaseAlt -- Guard on a case alternative
+ | LambdaExpr -- Pattern of a lambda
+ | ProcExpr -- Pattern of a proc
+ | PatBindRhs -- 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
+ deriving ()
+
+data HsStmtContext id
+ = ListComp
+ | DoExpr
+ | 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
+\end{code}
+
+\begin{code}
+isDoExpr :: HsStmtContext id -> Bool
+isDoExpr DoExpr = True
+isDoExpr (MDoExpr _) = True
+isDoExpr other = False
+\end{code}
+
+\begin{code}
+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"
+\end{code}
+
+\begin{code}
+pprMatchContext (FunRhs fun) = ptext SLIT("the definition of") <+> quotes (ppr fun)
+pprMatchContext CaseAlt = ptext SLIT("a case alternative")
+pprMatchContext RecUpd = ptext SLIT("a record-update construct")
+pprMatchContext PatBindRhs = ptext SLIT("a pattern binding")
+pprMatchContext LambdaExpr = ptext SLIT("a lambda abstraction")
+pprMatchContext ProcExpr = ptext SLIT("an arrow abstraction")
+pprMatchContext (StmtCtxt ctxt) = ptext SLIT("a pattern binding in") $$ pprStmtContext ctxt
+
+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 RecUpd = panic "pprMatchRhsContext"
+
+pprStmtContext (ParStmtCtxt c) = sep [ptext SLIT("a parallel branch of"), pprStmtContext c]
+pprStmtContext (PatGuard ctxt) = ptext SLIT("a pattern guard for") $$ pprMatchContext ctxt
+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")
+
+-- 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 (FunRhs fun) = "function " ++ showSDoc (ppr fun)
+matchContextErrString CaseAlt = "case"
+matchContextErrString PatBindRhs = "pattern binding"
+matchContextErrString RecUpd = "record update"
+matchContextErrString LambdaExpr = "lambda"
+matchContextErrString ProcExpr = "proc"
+matchContextErrString (StmtCtxt (ParStmtCtxt c)) = matchContextErrString (StmtCtxt c)
+matchContextErrString (StmtCtxt (PatGuard _)) = "pattern guard"
+matchContextErrString (StmtCtxt DoExpr) = "'do' expression"
+matchContextErrString (StmtCtxt (MDoExpr _)) = "'mdo' expression"
+matchContextErrString (StmtCtxt ListComp) = "list comprehension"
+matchContextErrString (StmtCtxt PArrComp) = "array comprehension"
+\end{code}
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