I have this function:
fun x (u,v,w) = (u::[v])::w;
Which gets a return type of
fn: 'a * 'a * 'a list list -> 'a list list
Could anyone explain to me how this type is generated from the function? Thank you!
Edit: Also, how would I call this function?
:: takes two arguments -- one on the left and one on the right. If its left argument is of type t, then its right argument must be of type t list.
Thus, :: has type ('a * 'a list) -> 'a list, as the t is arbitrary, and is therefore represented by an 'a.
When you have the code (u::[v]), therefore, you are telling SML that u : t and 'v : t for some type t. This expression is then a t list, and so to use it as the left argument to ::, the right argument, w, must have type t list list.
This then gives that the type of (u::[v])::w is t list list for some t.
In summary:
u : t
v : t
w : t list
(u::[v])::w : t list list,
all for some type t.
Thus, the type of x is ('a * 'a * 'a list) -> 'a list list, as t is arbitrary.
To call this function, you could do something like x(1,2,[3,4]), which would give the list [[1,2],[3,4]].