Moving basic functions

[David Luposchainsky]

Hello GHC HQ, hello mailing list,

there are a couple of basic functions that I think aren't where
they should be.

1. void is currently in Control.Monad. However, it is defined
   only in terms of fmap (and therefore only has a Functor
   constraint). Although this function is often used in a
   monadic setting as ">> return ()", I really don't think
   Control.Monad is the right place for it.

2. a) swap is the only function from Data.Tuple that is not
      exported to Prelude. On #haskell, people are sometimes
      even surprised there /is/ a Data.Tuple, and redefine
      their own version of swap at need. I therefore suggest
      including Data.Tuple.swap in the Prelude.

      The obvious downside of this change would of course be
      that it breaks code if there is a top-level user-defined
      version of it. Fixing this is of course trivial, but
      necessary.

   b) A related suggestion would be the addition of an
      irrefutable swap, (swap'?), defined as
      "swap ~(a,b) = (b,a)", and its addition to Prelude for
      the same reasons.

3. $>, a flipped version of <$, currently resides in
   Control.Comonad, but should be in Data.Functor. Applicative
   has <* and *>, Monad has >>= and =<<, and I personally keep
   redefining (or specifically importing) $> quite often, and I
   don't think I'm the only one.


As these are quite small changes I think 2 weeks should be a
sufficiently long discussion period.


Greetings,
David