[Haskell-cafe] Typeclass question

[Andrew Wagner]

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.

On Dec 27, 2008, at 1:44 PM, "David Menendez" <dave at zednenem.com> wrote:

> On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner <wagner.andrew at gmail.com 
> > wrote:
>> I'm sure there's a way to do this, but it's escaping me at present.  
>> I want
>> to do something like this:
>>
>> data Foo = Bar a => Foo a Bool ...
>>
>> That is, I want to create a new type, Foo, whose constructor takes  
>> both a
>> Boolean and a value of a type of class Bar.
>
> Try this:
>
>    {-# LANGUAGE ExistentialQuantification #-}
>
>    data Foo = forall a. Bar a => Foo a Bool
>
>
> -- 
> Dave Menendez <dave at zednenem.com>
> <http://www.eyrie.org/~zednenem/>