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])

Answer 1

One important point is that the function is not correct. It fails with the input ([1;2;3], []) since you missed the case of (xs, []) in pattern matching. Moreover, arguments are better in the curried form in order that it's easier to use with partial application. Here is the corrected version:

let rec interleave xs ys =
    match xs, ys with
    | [], ys -> ys
    | xs, [] -> xs
    | x::xs', y::ys' -> x::y::interleave xs' ys'

You can see that the function is not tail-recursive since it applies cons (::) constructor twice after the recursive call returned. One interesting way to make it tail-recursive is using sequence expression:

let interleave xs ys =
    let rec loop xs ys = 
       seq {
             match xs, ys with
             | [], ys -> yield! ys
             | xs, [] -> yield! xs
             | x::xs', y::ys' -> 
                   yield x
                   yield y
                   yield! loop xs' ys'
            }
    loop xs ys |> List.ofSeq