On Sat, May 4, 2013 at 1:33 PM, Costello, Roger L. <costello@mitre.org> wrote:

($) odd 3 -- returns True
odd $ 3 -- returns True

Good.

But then I saw this in an article:

($ 3) odd

What does ($ 3) mean? I thought the first argument to ($) is a function?

If you have an infix operator, you can turn it into a function by wrapping it in parens, as you know. But you can also go further: you can include one of the parameters in the parens, producing a partially applied function; Haskell calls this a section.

($ 3) is a section which has partially applied the right-hand parameter to ($), resulting in a function which expects the left-hand parameter (the function). If you were writing this out "longhand", it would be

\x -> x $ 3

Similarly, (+ 3) is a section on (+) (and (3 +) is the opposite section; since (+) is commutative, it is indistinguishable from (+3) in practice. ($) is not commutative, since one parameter is a function and the other is a value to apply the function to, so (x $) and ($ x) are different operations.

let list=[1,2,3]
(map list) odd

But that fails. Why? Why does that fail whereas a very similar looking form succeeds when ($) is used?

map is not infix, so that means something different; you have simply applied the first parameter, but that wants to be a function, not a list. If you rephrase it as infix:

(`map` list) odd -- `foo` is the function foo as an infix, that is, (x `foo` y) is (foo x y). note ` not ' !

it will work.

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net