Functions are fairly straightforward! They are declared by using the keyword fun and you specify parameters with their types.
fun myFunction (x: int, y: int) = x + y + 1
It takes an expression after the equals sign and gets evaluated when you call it.
myFunction(4,5);
Functions very often will be recursive in SML to get things done--there are no loops in SML.
fun pow(x:int, y:int) = if y = 0 then 1 else x * pow(x,y-1)
The body, or the expression, gets type-checked in a static environment. It maps each piece to its type. Function types look like an argument type with an arrow into a result type.
t1 * t2 -> t
A function is a value. When we declare it, it gets added to the dynamic environment.
When we call it with parameters, the expression in it gets plugged in and eval'd.