[Haskell-cafe] Pointfree composition for higher arity

[Daniel Fischer]

Am Mittwoch 17 Februar 2010 16:31:16 schrieb Sean Leather:
> I find myself often writing this pattern:
>
> someFun x y z = ...
>
>
>
> fun y z = runFun $ someFun someDefault y z
>
>
> or, alternatively:
>
> fun y = runFun . someFun someDefault y
>
>
> The second option approaches the ideal pointfreeness (or pointlessness
> if you prefer), but I'd like to go farther:
>
> (...) :: (c -> d) -> (a -> b -> c) -> a -> b -> d
>
> > (...) f g x y = f (g x y)

(...) = (.) . (.)

> > infixr 9 ...
>
> fun = runFun ... someFun someDefault
>
>
> There, that's better. More points for fewer points (which means I should
> really change the name from fun to pun).
>
> Does anybody else care about this? What are some alternative solutions?

o = (.)
oo = (.) . (.)
ooo = (.) . (.) . (.)
-- etc.

runFun `oo` someFun someDefault

I've also seen

(.:) = (.) . (.)

runFun .: someFun someDefault

I don't particularly like (...), it's too much like an ellipsis (and bad to 
count if you continue on that route), I prefer the 'spectacles' or

(∘) = (.)
(∘∘) = (.) . (.)

> I'd love to have something like this available in the Prelude or a
> library. (I have no strong feelings about the particular operator.)
>
> Regards,
> Sean