Mostly fix Trac #2431: make empty case acceptable to (most of) GHC

[ghc-hetmet.git] / compiler / typecheck / TcMatches.lhs

diff --git a/compiler/typecheck/TcMatches.lhs b/compiler/typecheck/TcMatches.lhs
index 40e1ca0..37fbd19 100644 (file)
--- a/compiler/typecheck/TcMatches.lhs
+++ b/compiler/typecheck/TcMatches.lhs
@@ -7,7 +7,7 @@ TcMatches: Typecheck some @Matches@
 
 \begin{code}
 module TcMatches ( tcMatchesFun, tcGRHSsPat, tcMatchesCase, tcMatchLambda,
-                  matchCtxt, TcMatchCtxt(..), 
+                  TcMatchCtxt(..), 
                   tcStmts, tcDoStmts, tcBody,
                   tcDoStmt, tcMDoStmt, tcGuardStmt
        ) where
@@ -36,6 +36,8 @@ import SrcLoc
 import FastString
 
 import Control.Monad
+
+#include "HsVersions.h"
 \end{code}
 
 %************************************************************************
@@ -92,6 +94,13 @@ tcMatchesCase :: TcMatchCtxt         -- Case context
              -> TcM (MatchGroup TcId)  -- Translated alternatives
 
 tcMatchesCase ctxt scrut_ty matches res_ty
+  | isEmptyMatchGroup matches
+  =      -- Allow empty case expressions
+    do {  -- Make sure we follow the invariant that res_ty is filled in
+          res_ty' <- refineBoxToTau res_ty
+       ;  return (MatchGroup [] (mkFunTys [scrut_ty] res_ty')) }
+
+  | otherwise
   = tcMatches ctxt [scrut_ty] res_ty matches
 
 tcMatchLambda :: MatchGroup Name -> BoxyRhoType -> TcM (HsWrapper, MatchGroup TcId)
@@ -141,7 +150,8 @@ data TcMatchCtxt    -- c.f. TcStmtCtxt, also in this module
                 -> TcM (LHsExpr TcId) }        
 
 tcMatches ctxt pat_tys rhs_ty (MatchGroup matches _)
-  = do { matches' <- mapM (tcMatch ctxt pat_tys rhs_ty) matches
+  = ASSERT( not (null matches) )       -- Ensure that rhs_ty is filled in
+    do { matches' <- mapM (tcMatch ctxt pat_tys rhs_ty) matches
        ; return (MatchGroup matches' (mkFunTys pat_tys rhs_ty)) }
 
 -------------
@@ -164,17 +174,15 @@ tcMatch ctxt pat_tys rhs_ty match
       = tcGRHSs ctxt grhss rhs_ty      -- No result signature
 
        -- Result type sigs are no longer supported
-    tc_grhss ctxt (Just res_sig) grhss rhs_ty
-      = do { addErr (ptext (sLit "Ignoring (deprecated) result type signature")
-                       <+> ppr res_sig)
-          ; tcGRHSs ctxt grhss rhs_ty }
+    tc_grhss _ (Just {}) _ _
+      = panic "tc_ghrss"       -- Rejected by renamer
 
        -- For (\x -> e), tcExpr has already said "In the expresssion \x->e"
        -- so we don't want to add "In the lambda abstraction \x->e"
     add_match_ctxt match thing_inside
        = case mc_what ctxt of
            LambdaExpr -> thing_inside
-           m_ctxt     -> addErrCtxt (matchCtxt m_ctxt match) thing_inside
+           m_ctxt     -> addErrCtxt (pprMatchInCtxt m_ctxt match) thing_inside
 
 -------------
 tcGRHSs :: TcMatchCtxt -> GRHSs Name -> BoxyRhoType
@@ -303,7 +311,7 @@ tcStmts ctxt stmt_chk (L loc (LetStmt binds) : stmts) res_ty thing_inside
 tcStmts ctxt stmt_chk (L loc stmt : stmts) res_ty thing_inside
   = do         { (stmt', (stmts', thing)) <- 
                setSrcSpan loc                          $
-               addErrCtxt (stmtCtxt ctxt stmt)         $
+               addErrCtxt (pprStmtInCtxt ctxt stmt)    $
                stmt_chk ctxt stmt res_ty               $ \ res_ty' ->
                popErrCtxt                              $
                tcStmts ctxt stmt_chk stmts res_ty'     $
@@ -586,12 +594,3 @@ checkArgs fun (MatchGroup (match1:matches) _)
 checkArgs _ _ = panic "TcPat.checkArgs" -- Matches always non-empty
 \end{code}
 
-\begin{code}
-matchCtxt :: HsMatchContext Name -> Match Name -> SDoc
-matchCtxt ctxt match  = hang (ptext (sLit "In") <+> pprMatchContext ctxt <> colon) 
-                          4 (pprMatch ctxt match)
-
-stmtCtxt :: HsStmtContext Name -> StmtLR Name Name -> SDoc
-stmtCtxt ctxt stmt = hang (ptext (sLit "In a stmt of") <+> pprStmtContext ctxt <> colon)
-                       4 (ppr stmt)
-\end{code}