Question

I am trying to write a function that returns the index of the passed value v in a given list x; -1 if not found. My attempt at the solution:

let rec index (x, v) =
    let i = 0 in
        match x with
        [] -> -1
        | (curr::rest) -> if(curr == v) then
                            i
                          else
                            succ i; (* i++ *)
                            index(rest, v)
;;

This is obviously wrong to me (it will return -1 every time) because it redefines i at each pass. I have some obscure ways of doing it with separate functions in my head, none which I can write down at the moment. I know this is a common pattern in all programming, so my question is, what’s the best way to do this in OCaml?

Answer 1

You can’t mutate variables in OCaml (well, there is a way but you really shouldn’t for simple things like this)

A basic trick you can do is create a helper function that receives extra arguments corresponding to the variables you want to “mutate”. Note how I added an extra parameter for the i and also “mutate” the current list head in a similar way.

let rec index_helper (x, vs, i) =
    match vs with
      [] -> -1
      | (curr::rest) ->
          if(curr == x) then
              i
          else
              index_helper (x, rest, i+1)
;;

let index (x, vs) = index_helper (x, vs, 0) ;;

This kind of tail-recursive transformation is a way to translate loops to functional programming but to be honest it is kind of low level (you have full power but the manual recursion looks like programming with gotos…).

For some particular patterns what you can instead try to do is take advantage of reusable higher order functions, such as map or folds.