Haskell' - class aliases

Simon Peyton-Jones simonpj at microsoft.com
Tue Apr 22 03:33:53 EDT 2008

| I tried to see the discussion that led to class aliases being rejected
| as a proposal, but could not find links on the Wiki.  In fact, in Trac
| (#101) that proposal is still a 'maybe', but with no updates.  Is there
| a competing proposal that got accepted?
| [Without a mechanism like class aliases, breaking up Num into a
| hierarchy of proper mathematical concepts becomes too unwieldly to be
| realistic.  This is a real stumbling block for anyone trying to use the
| class system to encode fine-grained mathematical concepts.]

I think this is partly my fault for failing to implement the idea -- a full implementation is really a prerequisite for Haskell'.

One reason I've failed to do so is because I have not come under enough pressure!  If I thought there were dozens of people slavering at the chops for class aliases, that would put it more firmly on my radar.  Ben is right to say that it's much easier than the type-function stuff, which is decidedly tricky.   (Of course there nothing stopping someone else doing it, but GHC's typechecker is one of it's more richly-interconnected parts.)   Perhaps you have all just been very polite, and waiting in quiet but unrequited hope.

Is this the most up-to-date description of the proposal?

I've just had another look, which threw up quite a few questions in my mind.   I wonder what would be a good list to discuss it.  Maybe this one is not bad, because it has people interested in Haskell innovation, regardless of whether it's a live Haskell' candidate?


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