Merge branch 'master' of http://darcs.haskell.org/ghc

1 files changed, 20 insertions, 9 deletions

diff --git a/compiler/typecheck/TcArrows.lhs b/compiler/typecheck/TcArrows.lhs
index 774cea5c51..0dfe8b0f1e 100644
--- a/compiler/typecheck/TcArrows.lhs
+++ b/compiler/typecheck/TcArrows.lhs
@@ -119,17 +119,28 @@ tc_cmd env in_cmd@(HsCase scrut matches) (stk, res_ty)
                       mc_body = mc_body }
     mc_body body res_ty' = tcCmd env body (stk, res_ty')
 
-tc_cmd env (HsIf mb_fun pred b1 b2) (stack_ty,res_ty)
+tc_cmd env (HsIf Nothing pred b1 b2) res_ty    -- Ordinary 'if'
+  = do  { pred' <- tcMonoExpr pred boolTy
+        ; b1'   <- tcCmd env b1 res_ty
+        ; b2'   <- tcCmd env b2 res_ty
+        ; return (HsIf Nothing pred' b1' b2')
+    }
+
+tc_cmd env (HsIf (Just fun) pred b1 b2) res_ty -- Rebindable syntax for if
   = do 	{ pred_ty <- newFlexiTyVarTy openTypeKind
-	; b_ty <- newFlexiTyVarTy openTypeKind
-        ; let if_ty = mkFunTys [pred_ty, b_ty, b_ty] res_ty
-	; mb_fun' <- case mb_fun of 
-              Nothing  -> return Nothing
-              Just fun -> liftM Just (tcSyntaxOp IfOrigin fun if_ty)
+        -- For arrows, need ifThenElse :: forall r. T -> r -> r -> r
+        -- because we're going to apply it to the environment, not
+        -- the return value.
+        ; [r_tv] <- tcInstSkolTyVars [alphaTyVar]
+	; let r_ty = mkTyVarTy r_tv
+        ; let if_ty = mkFunTys [pred_ty, r_ty, r_ty] r_ty
+        ; checkTc (not (r_tv `elemVarSet` tyVarsOfType pred_ty))
+                  (ptext (sLit "Predicate type of `ifThenElse' depends on result type"))
+	; fun'  <- tcSyntaxOp IfOrigin fun if_ty
   	; pred' <- tcMonoExpr pred pred_ty
-	; b1'   <- tcCmd env b1 (stack_ty,b_ty)
-	; b2'   <- tcCmd env b2 (stack_ty,b_ty)
-	; return (HsIf mb_fun' pred' b1' b2')
+	; b1'   <- tcCmd env b1 res_ty
+	; b2'   <- tcCmd env b2 res_ty
+	; return (HsIf (Just fun') pred' b1' b2')
     }
 
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