Control.Category in base

[Stefan O'Rear]

On Sun, Nov 18, 2007 at 09:04:15AM +0100, Bertram Felgenhauer wrote:
> Hi,
> 
> I played with ghc head today. I saw that its base library contains
> Control.Category, and Control.Arrow uses that. (I also saw the
> discussion about it on this mailing list. There weren't any serious
> objections; the discussion was far from focused though [*], and I found
> that the patch was applied rather abruptly. [1])
> 
> Anyway, here's what I found trying to compile HXT:
> 
> 1) Control.Arrow doesn't export (>>>) and (<<<) anymore.
> 2) All user defined instances of Control.Arrow.Arrow break.
> 
> The first one is easily fixed in the base library.
> 
> The second one is more annoying. The required changes to HXT aren't
> that big - my patch inserts 30 lines and deletes 10 - but it'll be
> harder to make it compatible between several ghc versions.
> 
> So all in all I'm rather unhappy that this patch went in, because
> it breaks existing code and personally I see little gain from it.
> 
> Personally I'd like to see that patch reverted. However I realize
> that my complaint comes rather late, so if we decide to keep the
> patch we have to fix the first issue above to minimize breakage of
> other packages.

Simple-ish solution: Implement the class system extension proposal[1].

Stefan

[1] http://haskell.org/haskellwiki/Class_system_extension_proposal

Stefan
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