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 )
34 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
42 producing an expression with a runtime error in the corner if
43 necessary. The type argument gives the type of the @ei@.
46 dsGuarded :: GRHSs Id -> Type -> DsM CoreExpr
48 dsGuarded grhss rhs_ty
49 = dsGRHSs PatBindRhs [] grhss rhs_ty `thenDs` \ match_result ->
50 mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty "" `thenDs` \ error_expr ->
51 extractMatchResult match_result error_expr
54 In contrast, @dsGRHSs@ produces a @MatchResult@.
57 dsGRHSs :: HsMatchContext Name -> [Pat Id] -- These are to build a MatchContext from
58 -> GRHSs Id -- Guarded RHSs
59 -> Type -- Type of RHS
61 dsGRHSs hs_ctx pats (GRHSs grhss binds) rhs_ty =
62 bindLocalsDs (bindsBinders ++ patsBinders) $
63 mappM (dsGRHS hs_ctx pats rhs_ty) grhss `thenDs` \ match_results ->
65 match_result1 = foldr1 combineMatchResults match_results
66 match_result2 = adjustMatchResultDs
67 (\e -> bindLocalsDs patsBinders $ dsLocalBinds binds e)
69 -- NB: nested dsLet inside matchResult
71 returnDs match_result2
72 where bindsBinders = map unLoc (collectLocalBinders binds)
73 patsBinders = collectPatsBinders (map (L undefined) pats)
75 dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
76 = do rhs' <- maybeInsertBreakpoint rhs rhs_ty
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 (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
109 | v `hasKey` otherwiseIdKey
110 || v `hasKey` getUnique trueDataConId
111 -- trueDataConId doesn't have the same unique as trueDataCon
112 = matchGuards stmts ctx rhs rhs_ty
114 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
115 = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
116 dsLExpr expr `thenDs` \ pred_expr ->
117 returnDs (mkGuardedMatchResult pred_expr match_result)
119 matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty
120 = bindLocalsDs (map unLoc $ collectLocalBinders binds) $
121 matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
122 returnDs (adjustMatchResultDs (dsLocalBinds binds) match_result)
123 -- NB the dsLet occurs inside the match_result
124 -- Reason: dsLet takes the body expression as its argument
125 -- so we can't desugar the bindings without the
126 -- body expression in hand
128 matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty
129 = bindLocalsDs (collectPatBinders pat) $
130 matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
131 dsLExpr bind_rhs `thenDs` \ core_rhs ->
132 matchSinglePat core_rhs ctx pat rhs_ty match_result
135 Should {\em fail} if @e@ returns @D@
137 f x | p <- e', let C y# = e, f y# = r1