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

On Mon, 2008-02-04 at 16:56 -0500, Cale Gibbard wrote:

On 04/02/2008, Philip Armstrong 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

and also, pointedly, f $! g x $! h y $! k z or even just f $! x $! y

Perhaps for Haskell 2. :)

We'll get rid of the monomorphism restriction then too and you won't have to be wary.