Hi Simon,
| op (Foo GadtValue :: Foo) = () :: ()
The thing is that GHC doesn't know the result type of the match *at the match point*. Suppose you had written
op (Foo GadtValue :: Foo) = id (() :: ()) or op (Foo GadtValue :: Foo) = if blah then () :: () else () or op (Foo GadtValue :: Foo) = let x = blah in () :: ()
The signature is too far "inside". We need to know it from "outside".
Ah, this is the crucial bit I've been misunderstanding! Instinctively it feels that when a GADT can't type check I should be annotating the GADT part - but that's not the case, I should be annotating the expression containing both the GADT match and its right-hand side. I think the way the error message is formatted (add a type signature, followed by giving the local pattern) seems to hint at this. With this insight I've managed to fix up all the real problems I was experiencing. My general method for solving these problems was to find an expression or statement that enclosed both the left and right hand sides, and add ":: Int" to it. I then recompiled and got an error message saying it couldn't match Int with <something>. I then replaced Int with <something> and it worked. It seems surprising that the compiler can't jump through those hoops for me, but I realise my particular cases may be uncommon or the complexity may be too great. A nice addition to the error message would be hinting at where the type signature should go, if it can reasonably be determined.
| I think this is the underlying problem I'm getting. For things like | list-comps and cases I don't seem to be able to find any combination | of annotations that works, but that is because I'm not using top-level | annotations.
Perhaps you can give an example that shows your difficulty?
The following is a fake example I was playing with: {-# LANGUAGE GADTs, ScopedTypeVariables #-} module Test where data E x = E x data Foo a where Foo :: Gadt a -> Foo a data Gadt a where GadtValue :: a -> Gadt (E a) g :: [()] g = [() | Foo (GadtValue _) <- undefined] :: [()] I couldn't figure out how to supply enough annotations to allow the code to work. Thanks Neil ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================