Strange error in show for datatype
Thu, 4 Oct 2001 15:19:48 +0200
Simon Peyton-Jones wrote:
> data T a = T1 Int | T2 a
> It's clear that (T1 Int) has no a's in it, not even bottom. Mark Shields
> and ruminated in the corridor about a kind system to make this apparent.
> That is, T1 would have type
> T1 :: forall a::Pure . Int -> T a
> Then if we see (forall a::Pure. Show a => <type>)
> we're justified in fixing a to Empty.
I think that even with this kind system, we can't fix it 'a' to Empty.
For example, the empty list constructor would get type:
 :: forall (a::Pure) . [a]
and as Koen showed, we still can't fix the 'a' to Empty.