On Sat, Dec 27, 2008 at 3:09 PM, Andrew Wagner <wagner.and...@gmail.com> wrote:
> Hmm, I actually simplified my problem too much. What I actually want is:
> data Foo a = forall a. Bar a => Foo a Bool
>
> ...except I want the 'a' on the left to match the 'a' on the right, so that
> you can only construct values out of values of the parameterized type, which
> also must be of the Bar class.
Something like this?
{-# LANGUAGE ExistentialQuantification #-}
class Bar a where
bar :: a -> a
data Foo a = (Bar a) => Foo a Bool
baz :: Foo a -> a
baz (Foo a _) = bar a
This works fine for me with GHC 6.8, but I'd expect Hugs and earlier
versions of GHC to reject it.
See section 8.4.5 of the GHC manual.
<http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt-style>
--
Dave Menendez <d...@zednenem.com>
<http://www.eyrie.org/~zednenem/>
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