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Functions are bindings whose value is a function:

let square x = x * x

square 5    // 25 (no parens needed for application)

Multiple parameters are space-separated:

let add x y = x + y
add 3 4     // 7

This is curried by default — add 3 returns a function expecting one more arg:

let add5 = add 5
add5 10     // 15

Anonymous functions use fun:

let double = fun x -> x * 2
let add = fun x y -> x + y

Multi-line bodies use indentation:

let describe n =
    let absN = abs n
    let sign = if n >= 0 then "+" else "-"
    sprintf "%s%d" sign absN

The last expression is the return value. No return keyword.