2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[DsGRHSs]{Matching guarded right-hand-sides (GRHSs)}
7 module DsGRHSs ( dsGuarded, dsGRHSs ) where
9 #include "HsVersions.h"
11 import {-# SOURCE #-} DsExpr ( dsLExpr, dsLocalBinds )
12 import {-# SOURCE #-} Match ( matchSinglePat )
14 import HsSyn ( Stmt(..), HsExpr(..), GRHSs(..), GRHS(..),
15 LHsExpr, HsMatchContext(..), Pat(..) )
16 import CoreSyn ( CoreExpr )
22 import Unique ( Uniquable(..) )
23 import PrelInfo ( nON_EXHAUSTIVE_GUARDS_ERROR_ID )
24 import TysWiredIn ( trueDataConId )
25 import PrelNames ( otherwiseIdKey, hasKey )
27 import SrcLoc ( unLoc, Located(..) )
30 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
38 producing an expression with a runtime error in the corner if
39 necessary. The type argument gives the type of the @ei@.
42 dsGuarded :: GRHSs Id -> Type -> DsM CoreExpr
44 dsGuarded grhss rhs_ty
45 = dsGRHSs PatBindRhs [] grhss rhs_ty `thenDs` \ match_result ->
46 mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty "" `thenDs` \ error_expr ->
47 extractMatchResult match_result error_expr
50 In contrast, @dsGRHSs@ produces a @MatchResult@.
53 dsGRHSs :: HsMatchContext Name -> [Pat Id] -- These are to build a MatchContext from
54 -> GRHSs Id -- Guarded RHSs
55 -> Type -- Type of RHS
58 dsGRHSs hs_ctx pats (GRHSs grhss binds) rhs_ty
59 = mappM (dsGRHS hs_ctx pats rhs_ty) grhss `thenDs` \ match_results ->
61 match_result1 = foldr1 combineMatchResults match_results
62 match_result2 = adjustMatchResultDs (dsLocalBinds binds) match_result1
63 -- NB: nested dsLet inside matchResult
65 returnDs match_result2
67 dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
68 = matchGuards (map unLoc guards) hs_ctx rhs rhs_ty
72 %************************************************************************
74 %* matchGuard : make a MatchResult from a guarded RHS *
76 %************************************************************************
79 matchGuards :: [Stmt Id] -- Guard
80 -> HsMatchContext Name -- Context
82 -> Type -- Type of RHS of guard
85 -- See comments with HsExpr.Stmt re what an ExprStmt means
86 -- Here we must be in a guard context (not do-expression, nor list-comp)
88 matchGuards [] ctx rhs rhs_ty
89 = do { core_rhs <- dsLExpr rhs
90 ; return (cantFailMatchResult core_rhs) }
92 -- ExprStmts must be guards
93 -- Turn an "otherwise" guard is a no-op. This ensures that
94 -- you don't get a "non-exhaustive eqns" message when the guards
95 -- finish in "otherwise".
96 -- NB: The success of this clause depends on the typechecker not
97 -- wrapping the 'otherwise' in empty HsTyApp or HsCoerce constructors
98 -- If it does, you'll get bogus overlap warnings
99 matchGuards (ExprStmt (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
100 | v `hasKey` otherwiseIdKey
101 || v `hasKey` getUnique trueDataConId
102 -- trueDataConId doesn't have the same unique as trueDataCon
103 = matchGuards stmts ctx rhs rhs_ty
105 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
106 = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
107 dsLExpr expr `thenDs` \ pred_expr ->
108 returnDs (mkGuardedMatchResult pred_expr match_result)
110 matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty
111 = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
112 returnDs (adjustMatchResultDs (dsLocalBinds binds) match_result)
113 -- NB the dsLet occurs inside the match_result
114 -- Reason: dsLet takes the body expression as its argument
115 -- so we can't desugar the bindings without the
116 -- body expression in hand
118 matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty
119 = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
120 dsLExpr bind_rhs `thenDs` \ core_rhs ->
121 matchSinglePat core_rhs ctx pat rhs_ty match_result
124 Should {\em fail} if @e@ returns @D@
126 f x | p <- e', let C y# = e, f y# = r1