Terminal recursive call with counter in ocaml

Question

I'm doing a school project in OCaml and I have to use a maximum of terminal recursive call as possible when doing recursive calls, but I don't know how to do that with a counter even tho the pedagos think it's possible. Any help with this please?

 let getContactId cl f p = match cl with
    | []                                        -> exit -1
    | (fn, ln, age, mail, tel)::tl when f = All -> if p = fn || p = ln || p = age || p = mail || p = tel then 0 else 1 + getContactId tl f p
    | (fn, _, _, _, _)::tl when f = Firstname   -> if p = fn then 0 else 1 + getContactId tl f p 
    | (_, ln, _, _, _)::tl when f = Lastname    -> if p = ln then 0 else 1 + getContactId tl f p
    | (_, _, age, _, _)::tl when f = Age        -> if p = age then 0 else 1 + getContactId tl f p
    | (_, _, _, mail, _)::tl when f = Email     -> if p = mail then 0 else 1 + getContactId tl f p
    | (_, _, _, _, tel)::tl when f = Phone      -> if p = tel then 0 else 1 + getContactId tl f p
    | (_, _, _, _, _)::tl when f = Id           -> 

Answer 1

The standard trick is to pass the counter as an additional parameter.

This is a key bit of knowledge for FP programmers.

Here's a non-tail-recursive function for determining the length of a list:

let rec ntr_length list =
    match list with
    | [] -> 0
    | _ :: tail -> 1 + ntr_length tail

Here is the tail-recursive transformation that uses an extra parameter:

let tr_length list =
    let rec i_length accum list =
        match list with
        | [] -> accum
        | _ :: tail -> i_length (accum + 1) tail
    in
    i_length 0 list