Is the o composition operator (eg. val x = foo o bar, where foo and bar are both functions), only usable on single-argument functions and/or functions with equal numbers of arguments? If not, what is the syntax for, say, composing foo(x,y) with bar(x).
As Michael already said, yes, SML only has single argument functions. I want to elaborate a bit, though.
The following function:
fun foo (x,y) = x + y
Has the type:
fn : int * int -> int
Which means that the first argument is a tuple of two ints. So you could do something like:
(sign o foo) (4,~5)
Which would give you the same as sign (foo (4,~5)).
Okay, but what about something like this?
fun bar x y = x + y
It has the type:
fn : int -> int -> int
Which means that bar actually takes just one integer, and returns a function. So you can't do this:
(sign o bar) 4 ~5
Because bar returns a function, and sign takes an integer. You can do this, though:
(sign o bar 4) ~5
Because bar 4 is a function that adds 4 to a number.