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 against). 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. Cheers, Simon