Proposal: Remove Show and Eq superclasses of Num

[John Meacham]

The good thing about the incremental step of removing the superclasses
as an intermediate step is that it lets you write forwards and
backwards compatible code. just include Eq a,Num a in your type
signatures, the haskell 98 compiler won't complain and you can still
be compatibile with the new classes.

   John

On Fri, Sep 16, 2011 at 4:37 PM, Paterson, Ross <R.Paterson at city.ac.uk> wrote:
> Ian Lynagh writes:
>> It doesn't solve everything, but I hope we can agree it is an
>> incremental step in the right direction. I don't think a revolutionary
>> change fixing all the issues is feasible. This particular blemish was
>> already being described as "largely historical" more than a decade ago:
>>     http://www.haskell.org/pipermail/haskell/2000-October/006147.html
>
> All these blemishes have been described as historical for a long time.
> But I'm suggesting that we address them by going in a different direction.
>
> Under this proposal the 7 numeric classes lose the Show superclass,
> and Num, Fractional and Floating lose the Eq superclass.  That breaks
> compatibility with Haskell 98, and will break functions that have
> numeric classes in their signatures and use equality or show in their
> implementations (e.g. the patches need to change the signatures of
> 27 functions in ghc and the core libs).  If we're considering that,
> we ought also to consider alternative trade-offs between interface
> improvement and breakage.
>
> The alternative approach is to refactor the numeric classes internally
> (as in the yap package).  That will break packages too, probably more,
> but that will leave us with much more sensible and useful classes.
> And the breakage falls on different users:
> - If we remove superclasses of Num, it falls on (some) people who put
>  numeric classes in their type signatures.
> - If we refactor the numeric classes internally, it falls on those who
>  define instances of numeric classes.  (I would argue that many of those
>  instances are kludged today because of the flawed classes.) Those
>  who merely use the numeric classes in type signatures or call their
>  methods should be unaffected.
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>