Haskell' - class aliases

Simon Marlow marlowsd at gmail.com
Mon Apr 21 17:29:41 EDT 2008

Jacques Carette wrote:
> 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.]

We want to get Haskell' done, and unfortunately that may mean rejecting 
some good proposals - in this case, class aliases is a fairly large 
feature that we don't have enough experience with yet, so it was dropped 
(I proposed dropping it recently to the committee, and no-one argued 

That's not to say it isn't worthwhile, and this certainly doesn't 
prevent it from being adopted in a future version of the standard.  We'd 
  like to see it implemented in GHC, too.

BTW, by the end of this process I do want to make sure we've documented 
the rationale for all the decisions, so that future committees have a 
useful knowledge base to work from.


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