X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcExpr.lhs;h=3f5a258ed3648bb5ab174e01e027187300edf275;hb=43cc549d6b596a0ba33fff2b126e5149f07eca29;hp=03e06874c8430b5c89d7ee0a2f406eaad3e15dce;hpb=d2ce0f52d42edf32bb9f13796e6ba6edba8bd516;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 03e0687..3f5a258 100644
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -82,7 +82,7 @@ tcPolyExpr expr res_ty
 
 tcPolyExprNC expr res_ty
   = do { traceTc "tcPolyExprNC" (ppr res_ty)
-       ; (gen_fn, expr') <- tcGen (GenSkol res_ty) emptyVarSet res_ty $ \ _ rho ->
+       ; (gen_fn, expr') <- tcGen GenSigCtxt res_ty $ \ _ rho ->
 			    tcMonoExprNC expr rho
        ; return (mkLHsWrap gen_fn expr') }
 
@@ -191,7 +191,7 @@ tcExpr (ExprWithTySig expr sig_ty) res_ty
 
       -- Remember to extend the lexical type-variable environment
       ; (gen_fn, expr') 
-            <- tcGen (SigSkol ExprSigCtxt) emptyVarSet sig_tc_ty $ \ skol_tvs res_ty ->
+            <- tcGen ExprSigCtxt sig_tc_ty $ \ skol_tvs res_ty ->
       	       tcExtendTyVarEnv2 (hsExplicitTvs sig_ty `zip` mkTyVarTys skol_tvs) $
 	             	       	-- See Note [More instantiated than scoped] in TcBinds
       	       tcMonoExprNC expr res_ty
@@ -311,7 +311,7 @@ tcExpr (SectionR op arg2) res_ty
 tcExpr (SectionL arg1 op) res_ty
   = do { (op', op_ty) <- tcInferFun op
        ; dflags <- getDOpts	    -- Note [Left sections]
-       ; let n_reqd_args | dopt Opt_PostfixOperators dflags = 1
+       ; let n_reqd_args | xopt Opt_PostfixOperators dflags = 1
                          | otherwise                        = 2
 
        ; (co_fn, (arg1_ty:arg_tys), op_res_ty) <- unifyOpFunTys op n_reqd_args op_ty
@@ -392,11 +392,27 @@ tcExpr (HsCase scrut matches) exp_ty
     match_ctxt = MC { mc_what = CaseAlt,
 		      mc_body = tcBody }
 
-tcExpr (HsIf pred b1 b2) res_ty
-  = do	{ pred' <- tcMonoExpr pred boolTy
-	; b1' <- tcMonoExpr b1 res_ty
-	; b2' <- tcMonoExpr b2 res_ty
-	; return (HsIf pred' b1' b2') }
+tcExpr (HsIf Nothing pred b1 b2) res_ty	   -- Ordinary 'if'
+  = do { pred' <- tcMonoExpr pred boolTy
+       ; b1' <- tcMonoExpr b1 res_ty
+       ; b2' <- tcMonoExpr b2 res_ty
+       ; return (HsIf Nothing pred' b1' b2') }
+
+tcExpr (HsIf (Just fun) pred b1 b2) res_ty   -- Note [Rebindable syntax for if]
+  = do { pred_ty <- newFlexiTyVarTy openTypeKind
+       ; b1_ty   <- newFlexiTyVarTy openTypeKind
+       ; b2_ty   <- newFlexiTyVarTy openTypeKind
+       ; let if_ty = mkFunTys [pred_ty, b1_ty, b2_ty] res_ty
+       ; fun'  <- tcSyntaxOp IfOrigin fun if_ty
+       ; pred' <- tcMonoExpr pred pred_ty
+       ; b1'   <- tcMonoExpr b1 b1_ty
+       ; b2'   <- tcMonoExpr b2 b2_ty
+       -- Fundamentally we are just typing (ifThenElse e1 e2 e3)
+       -- so maybe we should use the code for function applications
+       -- (which would allow ifThenElse to be higher rank).
+       -- But it's a little awkward, so I'm leaving it alone for now
+       -- and it maintains uniformity with other rebindable syntax
+       ; return (HsIf (Just fun') pred' b1' b2') }
 
 tcExpr (HsDo do_or_lc stmts body _) res_ty
   = tcDoStmts do_or_lc stmts body res_ty
@@ -414,6 +430,22 @@ tcExpr e@(HsArrForm _ _ _) _
                       ptext (sLit "was found where an expression was expected")])
 \end{code}
 
+Note [Rebindable syntax for if]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The rebindable syntax for 'if' uses the most flexible possible type
+for conditionals:
+  ifThenElse :: p -> b1 -> b2 -> res
+to support expressions like this:
+
+ ifThenElse :: Maybe a -> (a -> b) -> b -> b
+ ifThenElse (Just a) f _ = f a  ifThenElse Nothing  _ e = e
+
+ example :: String
+ example = if Just 2
+              then \v -> show v
+              else "No value"
+
+
 %************************************************************************
 %*									*
 		Record construction and update
@@ -787,7 +819,8 @@ tcApp fun args res_ty
 	-- Typecheck the result, thereby propagating 
         -- info (if any) from result into the argument types
         -- Both actual_res_ty and res_ty are deeply skolemised
-        ; co_res <- unifyType actual_res_ty res_ty
+        ; co_res <- addErrCtxt (funResCtxt fun) $
+                    unifyType actual_res_ty res_ty
 
 	-- Typecheck the arguments
 	; args1 <- tcArgs fun args expected_arg_tys
@@ -1352,6 +1385,10 @@ funAppCtxt fun arg arg_no
 		    quotes (ppr fun) <> text ", namely"])
        2 (quotes (ppr arg))
 
+funResCtxt :: LHsExpr Name -> SDoc
+funResCtxt fun
+  = ptext (sLit "In the return type of a call of") <+> quotes (ppr fun)
+
 badFieldTypes :: [(Name,TcType)] -> SDoc
 badFieldTypes prs
   = hang (ptext (sLit "Record update for insufficiently polymorphic field")