Proposal: Add &&& and *** to Data.Tuple

[Stefan O'Rear]

On Thu, Sep 20, 2007 at 03:23:05PM +0100, Jon Fairbairn wrote:
> Henning Thielemann <lemming at henning-thielemann.de> writes:
> 
> > On Mon, 17 Sep 2007, Jon Fairbairn wrote:
> >
> >> instance Arrow (->) where
> >>   ...
> >>   (***) = (Data.Tuple.***)
> >>   (&&&) = (Data.Tuple.&&&)
> >>
> >> and modules that imported only Arrow (not Tuple) would see
> >> no difference from the present state of affairs.  Where
> >> things would be different would be if a module imported both
> >> Tuple and Arrow, when, instead of a name clash, *** and &&&
> >> would get their Arrow meanings (albeit with a specialised
> >> instance for ->).
> >
> > If you import Tuple and Arrow, why not just import Arrow?
> 
> Presumably because there are functions in Tuple that aren't
> in Arrow (if that isn't the case, this just means that
> Tuple/Arrow isn't a very good example).
> 
> Another example would be mplus.  I happen to think that ++
> is a good name for mplus, but unfortunately it's used up for
> append on lists -- which, curiously enough is mplus for
> lists. Examples of functions that are used at a specialised
> case and as class members abound, and I'd like to see a way
> round using lots of different names for the same concept.
> While one /can/ use different names, or manage the problem
> by using related names, to my mind use of a convention to
> tame the Babel of names indicates a deficiency in the
> language.

The reason ++ and mplus is so similar is that they used to be the same;
(++) *was* the MonadPlus class member.  Cale Gibbard explains this as
{{{Quite a few people on the Haskell 98 committee had an irrational fear
of polymorphism.}}}

Stefan
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