[Haskell-cafe] Definition of hidden instance members (bug in
GHC or Hugs+Yhc)
ross at soi.city.ac.uk
Sat Jul 5 19:10:44 EDT 2008
On Sun, Jul 06, 2008 at 12:00:07AM +0100, Neil Mitchell wrote:
> This is either a GHC bug, or a Yhc+Hugs bug - I'm not sure which, but
> the compilers disagree:
> import Prelude hiding ((==))
> data Foo = Foo
> instance Eq Foo where
> (==) a b = True
> GHC says:
> Temp.hs:14:4: `==' is not a (visible) method of class `Eq'
> Yhc and Hugs both successfully compile the module.
> Does anyone know which compiler(s) are in the wrong, and need bugs filing?
GHC is correct. Report 4.3.2: "It is illegal to give a binding for a
class method that is not in scope, but the name under which it is in
scope is immaterial; in particular, it may be a qualified name."
In this case neither == nor Prelude.== is in scope.
More information about the Haskell-Cafe