Function to create string "between" two other strings

between.fs

open System

let rec between a z = 
    if a = z then a
    elif a > z then between z a
    else
        let rec between a z acc =
            match a, z with
            | [], z -> between ['a'] z acc
            | a, [] -> between a ['z'] acc
            | a0 :: a, z0 :: z ->
                match int z0 - int a0 with
                | 0 -> between a z (a0 :: acc)
                | 1 -> between a ['z'] (a0 :: acc)
                | d -> let m = char (int a0 + (d / 2)) in m :: acc
        between (a |> List.ofSeq) (z |> List.ofSeq) [] |> Array.ofList |> Array.rev |> String

assert (between "a" "z" = "m")
assert (between "a" "b" = "am")
assert (between "z" "a" = between "a" "z")