2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Matching guarded right-hand-sides (GRHSs)
9 module DsGRHSs ( dsGuarded, dsGRHSs ) where
11 #include "HsVersions.h"
13 import {-# SOURCE #-} DsExpr ( dsLExpr, dsLocalBinds )
14 import {-# SOURCE #-} Match ( matchSinglePat )
33 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
41 producing an expression with a runtime error in the corner if
42 necessary. The type argument gives the type of the @ei@.
45 dsGuarded :: GRHSs Id -> Type -> DsM CoreExpr
47 dsGuarded grhss rhs_ty
48 = dsGRHSs PatBindRhs [] grhss rhs_ty `thenDs` \ match_result ->
49 mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty "" `thenDs` \ error_expr ->
50 extractMatchResult match_result error_expr
53 In contrast, @dsGRHSs@ produces a @MatchResult@.
56 dsGRHSs :: HsMatchContext Name -> [Pat Id] -- These are to build a MatchContext from
57 -> GRHSs Id -- Guarded RHSs
58 -> Type -- Type of RHS
60 dsGRHSs hs_ctx pats (GRHSs grhss binds) rhs_ty =
61 bindLocalsDs (bindsBinders ++ patsBinders) $
62 mappM (dsGRHS hs_ctx pats rhs_ty) grhss `thenDs` \ match_results ->
64 match_result1 = foldr1 combineMatchResults match_results
65 match_result2 = adjustMatchResultDs
66 (\e -> bindLocalsDs patsBinders $ dsLocalBinds binds e)
68 -- NB: nested dsLet inside matchResult
70 returnDs match_result2
71 where bindsBinders = map unLoc (collectLocalBinders binds)
72 patsBinders = collectPatsBinders (map (L undefined) pats)
74 dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
75 = matchGuards (map unLoc guards) hs_ctx rhs rhs_ty
79 %************************************************************************
81 %* matchGuard : make a MatchResult from a guarded RHS *
83 %************************************************************************
86 matchGuards :: [Stmt Id] -- Guard
87 -> HsMatchContext Name -- Context
89 -> Type -- Type of RHS of guard
92 -- See comments with HsExpr.Stmt re what an ExprStmt means
93 -- Here we must be in a guard context (not do-expression, nor list-comp)
95 matchGuards [] ctx rhs rhs_ty
96 = do { core_rhs <- dsLExpr rhs
97 ; return (cantFailMatchResult core_rhs) }
99 -- ExprStmts must be guards
100 -- Turn an "otherwise" guard is a no-op. This ensures that
101 -- you don't get a "non-exhaustive eqns" message when the guards
102 -- finish in "otherwise".
103 -- NB: The success of this clause depends on the typechecker not
104 -- wrapping the 'otherwise' in empty HsTyApp or HsWrap constructors
105 -- If it does, you'll get bogus overlap warnings
106 matchGuards (ExprStmt (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
107 | v `hasKey` otherwiseIdKey
108 || v `hasKey` getUnique trueDataConId
109 -- trueDataConId doesn't have the same unique as trueDataCon
110 = matchGuards stmts ctx rhs rhs_ty
112 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
113 = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
114 dsLExpr expr `thenDs` \ pred_expr ->
115 returnDs (mkGuardedMatchResult pred_expr match_result)
117 matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty
118 = bindLocalsDs (map unLoc $ collectLocalBinders binds) $
119 matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
120 returnDs (adjustMatchResultDs (dsLocalBinds binds) match_result)
121 -- NB the dsLet occurs inside the match_result
122 -- Reason: dsLet takes the body expression as its argument
123 -- so we can't desugar the bindings without the
124 -- body expression in hand
126 matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty
127 = bindLocalsDs (collectPatBinders pat) $
128 matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
129 dsLExpr bind_rhs `thenDs` \ core_rhs ->
130 matchSinglePat core_rhs ctx pat rhs_ty match_result
133 Should {\em fail} if @e@ returns @D@
135 f x | p <- e', let C y# = e, f y# = r1