Strange error in show for datatype

Dylan Thurston
Thu, 4 Oct 2001 17:52:50 +0900

On Thu, Oct 04, 2001 at 12:36:55AM -0700, Simon Peyton-Jones wrote:
> So in fact, all we need do is:
> 	for each class, find the variance of each of its parameters
> 	in an ambiguous type, zap any positive parameters to Empty
> That sounds pretty easy.  We don't need Haskell 2 for that.  I feel
> a little implementation coming on.

This is, nevertheless, an extension to the language, right?  Or is the
class system poorly enough specified that it's unclear?

> Void was a type with one element.  What we really want here is
> a type with no elements.  It's also useful to be able to introduce
> such empty types for phantom-type purposes, so GHC now lets you say
> 	data T
> and get a type T with no values.

Ah, excellent!  I've frequently wanted to do this.

	Dylan Thurston