[Haskell-cafe] Deducing Show for GADTs
Niklas Broberg
niklas.broberg at gmail.com
Wed Jun 28 06:54:20 EDT 2006
On 6/28/06, David Roundy <droundy at darcs.net> 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
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