On Sat, Dec 27, 2008 at 12:44 PM, David Menendez wrote:

On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner 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

Though for existentials, I find GADT more natural (actually I find GADT more natural in most cases): data Foo where Foo :: Bar a => a -> Foo Luke

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" wrote:

On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner

...

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 http://www.eyrie.org/~zednenem/

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.

Well, you can ignore my previous contribution to this thread anyway. I failed to see the numerous other responses suggesting the same thing. I recommend against what you are wanting to do. It is probably nicer to have something like this: data Foo a = Foo a Bool -- don't export this foo :: Bar a => a -> Bool -> Foo a -- export this foo = Foo You can also use GHC's new ViewPatterns extension if you would still like to be able to pattern match on Foo values in other modules and don't mind being restricted to more recent versions of GHC. - Jake

Tom Pledger wrote:

Andrew Wagner 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