SML/NJ Write a filter function which takes a anonymous function as an argument

Question

I'm a asked to write a filter function in SMLNJ

filter:  ('a  ->  bool)  *  'a  list  ->  'a  list

that takes a function f and a list L as parameters and returns a new list containing the elements l of L such that f(l) is true.

For example:

filter(fn(x  =>  x  mod  3  =  0),  [1,2,3,4,5,6,7,8,9,10])
returns  [3,  6,  9]

Due to lack of documentation of ML I'm stuck for hours on this function,

I'm wondering how to pass the second argument to the first argument.

Answer 1

fun filter (f, []) = []
  | filter (f, x::xs) = if f x
                        then x::(filter (f , xs))
                        else filter (f , xs);

I'm not really sure what you mean about passing the second argument to the first argument.