Hi The following code works in GHC 6.8.3, works in GHC 6.9.20071111, but doesn't work in GHC HEAD. {-# LANGUAGE GADTs, ScopedTypeVariables #-} module Test where data E x = E x data Foo where Foo :: Gadt a -> Foo data Gadt a where GadtValue :: Gadt (E a) f (Foo GadtValue) = True f _ = False Under GHC HEAD I get the error: GADT pattern match with non-rigid result type `t' Solution: add a type signature In the definition of `f': f (Foo GadtValue) = True Adding a type signature to f fixes this problem: f :: Foo -> Bool But I haven't found anywhere other than the top level f to add a type signature. When the match is in a list comprehension, it becomes much harder. While playing further I discovered this example: foos = undefined g = [() | Foo GadtValue <- foos] This code doesn't compile under GHC HEAD, but does under 6.8.3. However, adding the type signature: g :: [()] Makes the code compile. This is surprising, as [()] isn't a GADT type, so shouldn't need stating explicitly. Any help on what these errors mean, or how they can be fixed with local type annotations? Thanks Neil ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================