[Haskell-cafe] Re: Typeclass question

Jake McArthur jake at pikewerks.com
Sat Dec 27 15:54:51 EST 2008

Tom Pledger wrote:
> Andrew Wagner <wagner.andrew <at> gmail.com> writes:
>> 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.
> Sometimes it's enough to declare
>     data Foo a = Foo a Bool
> and to put the 'Bar a =>' context onto the functions and instances that
> involve Foo.

Although, if you really want to omit the `a` from the type, you can use 
the ExistentialQuantification extension or GADTs to do something like:

     -- with ExistentialQuantification
     data Foo = forall a . Bar a => Foo a Bool

     -- with GADTs
     data Foo where
             forall a . Bar a => Foo a Bool

I haven't used either extension in a few months though, so I may have 
gotten the syntax wrong. You can look them up to be sure.

  - Jake

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