Re: [Haskell-cafe] Plain lambda inside banana brackets in the arrow notation

13 Jul 2012

      On Thu, Jul 12, 2012 at 02:47:57PM +0100, Ross Paterson wrote:
...
Though one possibility that might get
us most of the way there would be to refactor the Arrow class as
class PreArrow a where
    premap :: (b -> b') -> a b' c -> a b c
class (Category a, PreArrow a) => Arrow a where
    arr :: (b -> c) -> a b c
    arr f = premap f id
first :: a b c -> a (b,d) (c,d)
I've done this and the associated GHC changes locally; it yields a simple
rule for determining which instances are needed, based on the keywords used:

* all commands ("proc" and operator arguments) need PreArrow
  * "do" needs Arrow
    * "rec" needs ArrowLoop
  * "case" or "if" need ArrowChoice

I'm warming to it as a worthwhile generalization (though not exactly what
was asked for).