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 )
36 import Control.Monad ((>=>))
40 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
48 producing an expression with a runtime error in the corner if
49 necessary. The type argument gives the type of the @ei@.
52 dsGuarded :: GRHSs Id -> Type -> DsM CoreExpr
54 dsGuarded grhss rhs_ty = do
55 match_result <- dsGRHSs PatBindRhs [] grhss rhs_ty
56 error_expr <- mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty ""
57 extractMatchResult match_result error_expr
60 In contrast, @dsGRHSs@ produces a @MatchResult@.
63 dsGRHSs :: HsMatchContext Name -> [Pat Id] -- These are to build a MatchContext from
64 -> GRHSs Id -- Guarded RHSs
65 -> Type -- Type of RHS
67 dsGRHSs hs_ctx pats grhssa@(GRHSs grhss binds) rhs_ty = do
68 match_results <- mapM (dsGRHS hs_ctx pats rhs_ty) grhss
70 match_result1 = foldr1 combineMatchResults match_results
71 match_result2 = adjustMatchResultDs
72 (\e -> dsLocalBinds binds e)
74 -- NB: nested dsLet inside matchResult
78 dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
79 = matchGuards (map unLoc guards) hs_ctx rhs rhs_ty
83 %************************************************************************
85 %* matchGuard : make a MatchResult from a guarded RHS *
87 %************************************************************************
90 matchGuards :: [Stmt Id] -- Guard
91 -> HsMatchContext Name -- Context
93 -> Type -- Type of RHS of guard
96 -- See comments with HsExpr.Stmt re what an ExprStmt means
97 -- Here we must be in a guard context (not do-expression, nor list-comp)
99 matchGuards [] ctx rhs rhs_ty
100 = do { core_rhs <- dsLExpr rhs
101 ; return (cantFailMatchResult core_rhs) }
103 -- ExprStmts must be guards
104 -- Turn an "otherwise" guard is a no-op. This ensures that
105 -- you don't get a "non-exhaustive eqns" message when the guards
106 -- finish in "otherwise".
107 -- NB: The success of this clause depends on the typechecker not
108 -- wrapping the 'otherwise' in empty HsTyApp or HsWrap constructors
109 -- If it does, you'll get bogus overlap warnings
110 matchGuards (ExprStmt e _ _ : stmts) ctx rhs rhs_ty
111 | Just addTicks <- isTrueLHsExpr e = do
112 match_result <- matchGuards stmts ctx rhs rhs_ty
113 return (adjustMatchResultDs addTicks match_result)
114 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty = do
115 match_result <- matchGuards stmts ctx rhs rhs_ty
116 pred_expr <- dsLExpr expr
117 return (mkGuardedMatchResult pred_expr match_result)
119 matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty = do
120 match_result <- matchGuards stmts ctx rhs rhs_ty
121 return (adjustMatchResultDs (dsLocalBinds binds) match_result)
122 -- NB the dsLet occurs inside the match_result
123 -- Reason: dsLet takes the body expression as its argument
124 -- so we can't desugar the bindings without the
125 -- body expression in hand
127 matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty = do
128 match_result <- matchGuards stmts ctx rhs rhs_ty
129 core_rhs <- dsLExpr bind_rhs
130 matchSinglePat core_rhs ctx pat rhs_ty match_result
132 isTrueLHsExpr :: LHsExpr Id -> Maybe (CoreExpr -> DsM CoreExpr)
134 -- Returns Just {..} if we're sure that the expression is True
135 -- I.e. * 'True' datacon
137 -- * Trivial wappings of these
138 -- The arguments to Just are any HsTicks that we have found,
139 -- because we still want to tick then, even it they are aways evaluted.
140 isTrueLHsExpr (L _ (HsVar v)) | v `hasKey` otherwiseIdKey
141 || v `hasKey` getUnique trueDataConId
143 -- trueDataConId doesn't have the same unique as trueDataCon
144 isTrueLHsExpr (L loc (HsTick ix frees e))
145 | Just ticks <- isTrueLHsExpr e = Just (ticks >=> mkTickBox ix frees)
146 -- This encodes that the result is constant True for Hpc tick purposes;
147 -- which is specifically what isTrueLHsExpr is trying to find out.
148 isTrueLHsExpr (L loc (HsBinTick ixT _ e))
149 | Just ticks <- isTrueLHsExpr e = Just (ticks >=> mkTickBox ixT [])
150 isTrueLHsExpr (L _ (HsPar e)) = isTrueLHsExpr e
151 isTrueLHsExpr other = Nothing
154 Should {\em fail} if @e@ returns @D@
156 f x | p <- e', let C y# = e, f y# = r1