[project @ 2003-10-09 11:58:39 by simonpj]

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

diff --git a/ghc/compiler/deSugar/DsGRHSs.lhs b/ghc/compiler/deSugar/DsGRHSs.lhs
index 3134b9e..75c76d6 100644 (file)
--- a/ghc/compiler/deSugar/DsGRHSs.lhs
+++ b/ghc/compiler/deSugar/DsGRHSs.lhs
@@ -11,17 +11,17 @@ module DsGRHSs ( dsGuarded, dsGRHSs ) where
 import {-# SOURCE #-} DsExpr  ( dsExpr, dsLet )
 import {-# SOURCE #-} Match   ( matchSinglePat )
 
-import HsSyn           ( GRHSsAndBinds(..), Stmt(..), HsExpr(..), GRHS(..) )
-import TcHsSyn         ( TypecheckedGRHSsAndBinds, TypecheckedGRHS,
-                         TypecheckedPat, TypecheckedStmt
-                       )
-import CoreSyn         ( CoreExpr, Bind(..) )
+import HsSyn           ( Stmt(..), HsExpr(..), GRHSs(..), GRHS(..), HsMatchContext(..) )
+import TcHsSyn         ( TypecheckedGRHSs, TypecheckedPat, TypecheckedStmt, TypecheckedMatchContext )
+import CoreSyn         ( CoreExpr )
+import Type            ( Type )
 
 import DsMonad
 import DsUtils
-import PrelVals                ( nON_EXHAUSTIVE_GUARDS_ERROR_ID )
-import Unique          ( otherwiseIdKey, trueDataConKey, Uniquable(..) )
-import Outputable
+import Unique          ( Uniquable(..) )
+import PrelInfo                ( nON_EXHAUSTIVE_GUARDS_ERROR_ID )
+import TysWiredIn      ( trueDataConId )
+import PrelNames       ( otherwiseIdKey, hasKey )
 \end{code}
 
 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
@@ -33,41 +33,32 @@ It desugars:
        where binds
 \end{verbatim}
 producing an expression with a runtime error in the corner if
-necessary.  The type argument gives the type of the ei.
+necessary.  The type argument gives the type of the @ei@.
 
 \begin{code}
-dsGuarded :: TypecheckedGRHSsAndBinds
-         -> DsM CoreExpr
+dsGuarded :: TypecheckedGRHSs -> DsM CoreExpr
 
-dsGuarded (GRHSsAndBindsOut grhss binds err_ty)
-  = dsGRHSs PatBindMatch [] grhss                              `thenDs` \ match_result ->
+dsGuarded grhss
+  = dsGRHSs PatBindRhs [] grhss                                `thenDs` \ (err_ty, match_result) ->
     mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID err_ty ""      `thenDs` \ error_expr ->
-    extractMatchResult match_result error_expr                 `thenDs` \ body ->
-    dsLet binds body
+    extractMatchResult match_result error_expr
 \end{code}
 
-Desugar a list of (grhs, expr) pairs [grhs = guarded
-right-hand-side], as in:
-\begin{verbatim}
-p | g1 = e1
-  | g2 = e2
-  ...
-  | gm = em
-\end{verbatim}
-We supply a @CoreExpr@ for the case in which all of
-the guards fail.
+In contrast, @dsGRHSs@ produces a @MatchResult@.
 
 \begin{code}
-dsGRHSs :: DsMatchKind -> [TypecheckedPat]     -- These are to build a MatchContext from
-       -> [TypecheckedGRHS]                    -- Guarded RHSs
-       -> DsM MatchResult
-
-dsGRHSs kind pats [grhs] = dsGRHS kind pats grhs
-
-dsGRHSs kind pats (grhs:grhss)
-  = dsGRHS kind pats grhs      `thenDs` \ match_result1 ->
-    dsGRHSs kind pats grhss    `thenDs` \ match_result2 ->
-    returnDs (combineMatchResults match_result1 match_result2)
+dsGRHSs :: TypecheckedMatchContext -> [TypecheckedPat] -- These are to build a MatchContext from
+       -> TypecheckedGRHSs                             -- Guarded RHSs
+       -> DsM (Type, MatchResult)
+
+dsGRHSs kind pats (GRHSs grhss binds ty)
+  = mappM (dsGRHS kind pats) grhss             `thenDs` \ match_results ->
+    let 
+       match_result1 = foldr1 combineMatchResults match_results
+       match_result2 = adjustMatchResultDs (dsLet binds) match_result1
+               -- NB: nested dsLet inside matchResult
+    in
+    returnDs (ty, match_result2)
 
 dsGRHS kind pats (GRHS guard locn)
   = matchGuard guard (DsMatchContext kind pats locn)
@@ -85,19 +76,23 @@ matchGuard :: [TypecheckedStmt]     -- Guard
            -> DsMatchContext            -- Context
           -> DsM MatchResult
 
-matchGuard (ExprStmt expr locn : should_be_null) ctx 
+-- 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 locn] ctx 
   = putSrcLocDs locn (dsExpr expr)     `thenDs` \ core_expr ->
     returnDs (cantFailMatchResult core_expr)
 
+       -- ExprStmts must be guards
        -- Turn an "otherwise" guard is a no-op
-matchGuard (GuardStmt (HsVar v) _ : stmts) ctx
-  |  uniq == otherwiseIdKey
-  || uniq == trueDataConKey
+matchGuard (ExprStmt (HsVar v) _ _ : stmts) ctx
+  |  v `hasKey` otherwiseIdKey
+  || v `hasKey` getUnique trueDataConId        
+       -- trueDataConId doesn't have the same 
+       -- unique as trueDataCon
   = matchGuard stmts ctx
-  where
-    uniq = getUnique v
 
-matchGuard (GuardStmt expr locn : stmts) ctx
+matchGuard (ExprStmt expr _ locn : stmts) ctx
   = matchGuard stmts ctx               `thenDs` \ match_result ->
     putSrcLocDs locn (dsExpr expr)     `thenDs` \ pred_expr ->
     returnDs (mkGuardedMatchResult pred_expr match_result)
@@ -113,7 +108,8 @@ matchGuard (BindStmt pat rhs locn : stmts) ctx
     matchSinglePat core_rhs ctx pat match_result
 \end{code}
 
--- Should *fail* if e returns D
-
+Should {\em fail} if @e@ returns @D@
+\begin{verbatim}
 f x | p <- e', let C y# = e, f y# = r1
     | otherwise         = r2 
+\end{verbatim}