[Haskell-cafe] A handy little consequence of the Cont monad

[Cale Gibbard]

On 04/02/2008, Philip Armstrong <phil at kantaka.co.uk> wrote:
>
> I've always liked $ for this kind of code, if you want to keep the
> arguments around:
>
>    next xs = runCont $ sequence $ map Cont xs
>
> seems quite natural to me.
>

I'd probably write that as

nest xs = runCont . sequence . map Cont $ xs

or else as:

nest xs = runCont . sequence $ map Cont xs

so as not to abuse the fact that ($) really has the wrong
associativity. (I didn't really give that aspect of the code a
moment's thought, or else I'd probably have made it either points-free
or used the first form above. I've been bitten by the MR enough times
that I'm wary of eta-reducing the last parameter out of functions --
of course, the explicit type signature means it doesn't matter.)

It would be nice to flip the associativity of ($) someday. It loses
little in the way of expressiveness, since one can generally replace
the first (n-1) instances of ($) with (.) straightforwardly (the one
exception to this being when there are other operator symbols like
(***) acting on the functions involved, but these cases are fairly
rare, and it's arguably clearer to leave those parens in).

What it would buy us to make ($) left associative is that we could,
for instance, remove the parens from an expression like:

f (g x) (h y) (k z)

getting:

f $ g x $ h y $ k z

Perhaps for Haskell 2. :)

 - Cale