On 6/28/06, David Roundy wrote:

On Wed, Jun 28, 2006 at 11:52:51AM +0200, Joel Bjrnson wrote:

...

Hi. I came a cross the following phenomena which, at least to me, occurs kind of awkward. The code below:

data MyData a where DC1 :: (Show a ) => a -> MyData a

GADTs don't yet work right with classes. :( The above, however, doesn't need to be expressed as a GADT, I believe you can write something like:

data MyData a = (forall a. Show a) => DC1 a

which (this is untested) should do what you want.

Only if "what he wants" is something that type checks, but doesn't do the same thing. ;-) In Joel's definition of MyData, values constructed with DC1 applied to a value of type b will have type MyData b. In your definition they will have type MyData a, for any a. In other words, your definition would be identical to the GADT data MyData a where DC1 :: forall a b . (Show b) => b -> MyData a As to Joel's question, this seems really really weird. In particular since adding the completely useless wrapper type solves the problem. In fact, giving DC1 any return type other than MyData a solves the problem. This has to be a bug of some sort. /Niklas