2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Matching guarded right-hand-sides (GRHSs)
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module DsGRHSs ( dsGuarded, dsGRHSs ) where
18 #include "HsVersions.h"
20 import {-# SOURCE #-} DsExpr ( dsLExpr, dsLocalBinds )
21 import {-# SOURCE #-} Match ( matchSinglePat )
38 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
46 producing an expression with a runtime error in the corner if
47 necessary. The type argument gives the type of the @ei@.
50 dsGuarded :: GRHSs Id -> Type -> DsM CoreExpr
52 dsGuarded grhss rhs_ty = do
53 match_result <- dsGRHSs PatBindRhs [] grhss rhs_ty
54 error_expr <- mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty ""
55 extractMatchResult match_result error_expr
58 In contrast, @dsGRHSs@ produces a @MatchResult@.
61 dsGRHSs :: HsMatchContext Name -> [Pat Id] -- These are to build a MatchContext from
62 -> GRHSs Id -- Guarded RHSs
63 -> Type -- Type of RHS
65 dsGRHSs hs_ctx pats grhssa@(GRHSs grhss binds) rhs_ty = do
66 match_results <- mapM (dsGRHS hs_ctx pats rhs_ty) grhss
68 match_result1 = foldr1 combineMatchResults match_results
69 match_result2 = adjustMatchResultDs
70 (\e -> dsLocalBinds binds e)
72 -- NB: nested dsLet inside matchResult
76 dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
77 = matchGuards (map unLoc guards) hs_ctx rhs rhs_ty
81 %************************************************************************
83 %* matchGuard : make a MatchResult from a guarded RHS *
85 %************************************************************************
88 matchGuards :: [Stmt Id] -- Guard
89 -> HsMatchContext Name -- Context
91 -> Type -- Type of RHS of guard
94 -- See comments with HsExpr.Stmt re what an ExprStmt means
95 -- Here we must be in a guard context (not do-expression, nor list-comp)
97 matchGuards [] ctx rhs rhs_ty
98 = do { core_rhs <- dsLExpr rhs
99 ; return (cantFailMatchResult core_rhs) }
101 -- ExprStmts must be guards
102 -- Turn an "otherwise" guard is a no-op. This ensures that
103 -- you don't get a "non-exhaustive eqns" message when the guards
104 -- finish in "otherwise".
105 -- NB: The success of this clause depends on the typechecker not
106 -- wrapping the 'otherwise' in empty HsTyApp or HsWrap constructors
107 -- If it does, you'll get bogus overlap warnings
108 matchGuards (ExprStmt e _ _ : stmts) ctx rhs rhs_ty
109 | Just addTicks <- isTrueLHsExpr e = do
110 match_result <- matchGuards stmts ctx rhs rhs_ty
111 return (adjustMatchResultDs addTicks match_result)
112 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty = do
113 match_result <- matchGuards stmts ctx rhs rhs_ty
114 pred_expr <- dsLExpr expr
115 return (mkGuardedMatchResult pred_expr match_result)
117 matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty = do
118 match_result <- matchGuards stmts ctx rhs rhs_ty
119 return (adjustMatchResultDs (dsLocalBinds binds) match_result)
120 -- NB the dsLet occurs inside the match_result
121 -- Reason: dsLet takes the body expression as its argument
122 -- so we can't desugar the bindings without the
123 -- body expression in hand
125 matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty = do
126 match_result <- matchGuards stmts ctx rhs rhs_ty
127 core_rhs <- dsLExpr bind_rhs
128 matchSinglePat core_rhs ctx pat rhs_ty match_result
130 isTrueLHsExpr :: LHsExpr Id -> Maybe (CoreExpr -> DsM CoreExpr)
132 -- Returns Just {..} if we're sure that the expression is True
133 -- I.e. * 'True' datacon
135 -- * Trivial wappings of these
136 -- The arguments to Just are any HsTicks that we have found,
137 -- because we still want to tick then, even it they are aways evaluted.
138 isTrueLHsExpr (L _ (HsVar v)) | v `hasKey` otherwiseIdKey
139 || v `hasKey` getUnique trueDataConId
141 -- trueDataConId doesn't have the same unique as trueDataCon
142 isTrueLHsExpr (L loc (HsTick ix frees e))
143 | Just ticks <- isTrueLHsExpr e = Just (\x -> ticks x >>= mkTickBox ix frees)
144 -- This encodes that the result is constant True for Hpc tick purposes;
145 -- which is specifically what isTrueLHsExpr is trying to find out.
146 isTrueLHsExpr (L loc (HsBinTick ixT _ e))
147 | Just ticks <- isTrueLHsExpr e = Just (\x -> ticks x >>= mkTickBox ixT [])
148 isTrueLHsExpr (L _ (HsPar e)) = isTrueLHsExpr e
149 isTrueLHsExpr other = Nothing
152 Should {\em fail} if @e@ returns @D@
154 f x | p <- e', let C y# = e, f y# = r1