recursion - Merge two lists

Question

I am looking to merge 2 lists in F# in a purely functional way. I am having a hard time understanding the syntax.

Let say I have a tuple ([5;3;8],[2;9;4])

When I call the function, it should return [5;2;3;9;8;4]

Here is why I have so far, which is wrong I am sure. If someone could explain it in a simple way I would be grateful.

let rec interleave (xs,ys) = function
|([], ys) -> ys
|(x::xs, y::ys) -> x :: y::  interleave (xs,ys) 

Answer 1

Your function is almost right. let f = function is shorthand for let f x = match x with so you don't need explicit args. Also, your algorithm needs some tweaking.

let rec interleave = function //same as: let rec interleave (xs, ys) = match xs, ys with
  |([], ys) -> ys
  |(xs, []) -> xs
  |(x::xs, y::ys) -> x :: y :: interleave (xs,ys)

interleave ([5;3;8],[2;9;4]) //output: [5; 2; 3; 9; 8; 4]