F#: Computing Length of a List (Tail Recursive)

The code snippets listed below defines a function to compute the length of a give list using F#. Note that these functions are also known polymorphic function, as they work with any type of list (as shown in the output).

// naive implementation
let rec length list =
    match list with
        | [] -> 0
        | _::tail -> 1 + computeLength tail

A tail recursive implementation is outlined next.

// Tail recursive computation of List's length
let length list =
    // Auxiliary function to compute length
    // It store intermediate result in acc.
    let rec lengthAux acc list =
        match list with
        | [] -> acc
        | _::tail -> lengthAux (acc+1) tail
    lengthAux 0 list // invoking lengthAux with acc = 0

Output:

> length [];;
val it : int = 0
> length [1;2;3;4];;
val it : int = 4
> length ['a';'b'];;
val it : int = 2