Question

The following function gives a compilation error at the point I try to match an empty list:

let rec tuplesToList (acc: int list) (remaining: int*int list) =
    match remaining with
    | [] -> acc
    | (a, b) :: tail -> tuplesToList (a :: b :: acc)

The error is:

This expression was expected to have type int * int list but here has type 'a list

This works fine when remaining is a simple list of ints rather than tuples. How can I match an empty list of tuples?

Answer 1

[] is just fine to match an empty list of tuples. But according to your type annotation, remaining is not a list of tuples, it’s a tuple containing an int and an int list. A list of tuples would be (int*int) list. int * int list is parsed as int * (int list), not (int * int) list.

If you fix the type, your code should work fine.