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

[Ross Paterson]

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).