minor cleanup; remove one use of fromJust

[ghc-hetmet.git] / ghc / compiler / deSugar / DsGRHSs.lhs

diff --git a/ghc/compiler/deSugar/DsGRHSs.lhs b/ghc/compiler/deSugar/DsGRHSs.lhs
index 60c67bc..eea61ba 100644 (file)
--- a/ghc/compiler/deSugar/DsGRHSs.lhs
+++ b/ghc/compiler/deSugar/DsGRHSs.lhs
@@ -8,14 +8,14 @@ module DsGRHSs ( dsGuarded, dsGRHSs ) where
 
 #include "HsVersions.h"
 
-import {-# SOURCE #-} DsExpr  ( dsLExpr, dsLet )
+import {-# SOURCE #-} DsExpr  ( dsLExpr, dsLocalBinds )
 import {-# SOURCE #-} Match   ( matchSinglePat )
 
 import HsSyn           ( Stmt(..), HsExpr(..), GRHSs(..), GRHS(..), 
-                         HsMatchContext(..), Pat(..), LStmt )
+                         LHsExpr, HsMatchContext(..), Pat(..) )
 import CoreSyn         ( CoreExpr )
-import Type            ( Type )
 import Var             ( Id )
+import Type            ( Type )
 
 import DsMonad
 import DsUtils
@@ -39,11 +39,11 @@ producing an expression with a runtime error in the corner if
 necessary.  The type argument gives the type of the @ei@.
 
 \begin{code}
-dsGuarded :: GRHSs Id -> DsM CoreExpr
+dsGuarded :: GRHSs Id -> Type -> DsM CoreExpr
 
-dsGuarded grhss
-  = dsGRHSs PatBindRhs [] grhss                                `thenDs` \ (err_ty, match_result) ->
-    mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID err_ty ""      `thenDs` \ error_expr ->
+dsGuarded grhss rhs_ty
+  = dsGRHSs PatBindRhs [] grhss rhs_ty                                 `thenDs` \ match_result ->
+    mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty ""      `thenDs` \ error_expr ->
     extractMatchResult match_result error_expr
 \end{code}
 
@@ -52,19 +52,20 @@ In contrast, @dsGRHSs@ produces a @MatchResult@.
 \begin{code}
 dsGRHSs :: HsMatchContext Name -> [Pat Id]     -- These are to build a MatchContext from
        -> GRHSs Id                             -- Guarded RHSs
-       -> DsM (Type, MatchResult)
+       -> Type                                 -- Type of RHS
+       -> DsM MatchResult
 
-dsGRHSs kind pats (GRHSs grhss binds ty)
-  = mappM (dsGRHS kind pats) grhss             `thenDs` \ match_results ->
+dsGRHSs hs_ctx pats (GRHSs grhss binds) rhs_ty
+  = mappM (dsGRHS hs_ctx pats rhs_ty) grhss    `thenDs` \ match_results ->
     let 
        match_result1 = foldr1 combineMatchResults match_results
-       match_result2 = adjustMatchResultDs (dsLet binds) match_result1
+       match_result2 = adjustMatchResultDs (dsLocalBinds binds) match_result1
                -- NB: nested dsLet inside matchResult
     in
-    returnDs (ty, match_result2)
+    returnDs match_result2
 
-dsGRHS kind pats (L loc (GRHS guard))
-  = matchGuard (map unLoc guard) (DsMatchContext kind pats loc)
+dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
+  = matchGuards (map unLoc guards) hs_ctx rhs rhs_ty
 \end{code}
 
 
@@ -75,40 +76,49 @@ dsGRHS kind pats (L loc (GRHS guard))
 %************************************************************************
 
 \begin{code}
-matchGuard :: [Stmt Id]        -- Guard
-           -> DsMatchContext            -- Context
-          -> DsM MatchResult
+matchGuards :: [Stmt Id]               -- Guard
+            -> HsMatchContext Name     -- Context
+           -> LHsExpr Id               -- RHS
+           -> Type                     -- Type of RHS of guard
+           -> DsM MatchResult
 
 -- See comments with HsExpr.Stmt re what an ExprStmt means
 -- Here we must be in a guard context (not do-expression, nor list-comp)       
 
-matchGuard [ResultStmt expr] ctx 
-  = dsLExpr expr       `thenDs` \ core_expr ->
-    returnDs (cantFailMatchResult core_expr)
+matchGuards [] ctx rhs rhs_ty
+  = do { core_rhs <- dsLExpr rhs
+       ; return (cantFailMatchResult core_rhs) }
 
        -- ExprStmts must be guards
-       -- Turn an "otherwise" guard is a no-op
-matchGuard (ExprStmt (L _ (HsVar v)) _ : stmts) ctx
+       -- Turn an "otherwise" guard is a no-op.  This ensures that 
+       -- you don't get a "non-exhaustive eqns" message when the guards 
+       -- finish in "otherwise".
+       -- NB:  The success of this clause depends on the typechecker not
+       --      wrapping the 'otherwise' in empty HsTyApp or HsCoerce constructors
+       --      If it does, you'll get bogus overlap warnings
+matchGuards (ExprStmt (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
   |  v `hasKey` otherwiseIdKey
   || v `hasKey` getUnique trueDataConId        
-       -- trueDataConId doesn't have the same 
-       -- unique as trueDataCon
-  = matchGuard stmts ctx
+       -- trueDataConId doesn't have the same unique as trueDataCon
+  = matchGuards stmts ctx rhs rhs_ty
 
-matchGuard (ExprStmt expr _ : stmts) ctx
-  = matchGuard stmts ctx       `thenDs` \ match_result ->
-    dsLExpr expr               `thenDs` \ pred_expr ->
+matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
+  = matchGuards stmts ctx rhs rhs_ty   `thenDs` \ match_result ->
+    dsLExpr expr                       `thenDs` \ pred_expr ->
     returnDs (mkGuardedMatchResult pred_expr match_result)
 
-matchGuard (LetStmt binds : stmts) ctx
-  = matchGuard stmts ctx       `thenDs` \ match_result ->
-    returnDs (adjustMatchResultDs (dsLet binds) match_result)
+matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty
+  = matchGuards stmts ctx rhs rhs_ty   `thenDs` \ match_result ->
+    returnDs (adjustMatchResultDs (dsLocalBinds binds) match_result)
        -- NB the dsLet occurs inside the match_result
-
-matchGuard (BindStmt pat rhs : stmts) ctx
-  = matchGuard stmts ctx               `thenDs` \ match_result ->
-    dsLExpr rhs                                `thenDs` \ core_rhs ->
-    matchSinglePat core_rhs ctx pat match_result
+       -- Reason: dsLet takes the body expression as its argument
+       --         so we can't desugar the bindings without the
+       --         body expression in hand
+
+matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty
+  = matchGuards stmts ctx rhs rhs_ty   `thenDs` \ match_result ->
+    dsLExpr bind_rhs                   `thenDs` \ core_rhs ->
+    matchSinglePat core_rhs ctx pat rhs_ty match_result
 \end{code}
 
 Should {\em fail} if @e@ returns @D@