[Haskell-cafe] Typeclass question

[Miguel Mitrofanov]

Oh! That's much simplier:

data Bar a => Foo a = Foo a Bool

On 27 Dec 2008, at 23:09, Andrew Wagner 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.
>
> 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/>
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