问题
I have the function below and it works:
(fn x => x * 2) 2;
but this one doesn't work:
(fn x y => x + y ) 2 3;
Can anyone tell me why? Or give me some hint to get it to work?
回答1:
(fn x => fn y => x+y) 2 3; works. fn simply doesn't have the same syntactic sugar to define curried functions that fun has.
回答2:
In Standard ML, a function can have only one argument, so use
(fn (x,y) => x + y) (2,3)
and the type is
fn: int * int -> int
in this time (x,y) and (2,3) is a list structure,
回答3:
The answers posted above are correct. SML functions take only one argument. As a result, SML functions can have only one of two input types :
1) t = (t1 * t2 * ... * tN) , for some N
2) t = a, for some a.
So, technically speaking, SML only takes product types or unary types as arguments to functions. One can more generally think of this as an Unary-Type or a projection of some product Type.
In order to have currying inside anonymous functions, feel free to nest them inside each other as :
fn x1 => fn x2 => ... fn xN => ...
I think it's also important to know that :
fun a = fn x1 => fn x2 => ... fn xN => ... is the full expansion of the syntact sugar : fun a x1 x2 .. xN
来源：https://stackoverflow.com/questions/2437019/curried-anonymous-function-in-sml