Make rebindable if-then-else a little more permissive

See Note [Rebindable syntax for if].  Fixes Trac #4798.
Thanks to Nils Schweinsberg <mail@n-sch.de>

1 files changed, 28 insertions, 6 deletions

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 5790b6a3be..297b4e884e 100644
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -398,14 +398,20 @@ tcExpr (HsIf Nothing pred b1 b2) res_ty	   -- Ordinary 'if'
        ; b2' <- tcMonoExpr b2 res_ty
        ; return (HsIf Nothing pred' b1' b2') }
 
-tcExpr (HsIf (Just fun) pred b1 b2) res_ty   -- Rebindable syntax
+tcExpr (HsIf (Just fun) pred b1 b2) res_ty   -- Note [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
-       ; fun' <- tcSyntaxOp IfOrigin fun if_ty
+       ; 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 b_ty
-       ; b2' <- tcMonoExpr b2 b_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
@@ -424,6 +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"
+
+
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