Trying to use match statements in OCaml to write a function that checks if an element is in a list

Question

I am new to OCaml and struggling to work with matches. I want to write a function that takes a list and a value and then returns true if the value is in that list and false if it is not. Here is my idea but I am struggling to get it to work.

    let rec contains xs x =
      match xs with
      | [] -> false
      | z :: zs ->
        match x with
        | z -> true
        | _ -> contains zs x

glennsl glennsl · Accepted Answer · 2018-01-27T16:31:15

When you use an identifier as a pattern, you will bind the value you match on to that identifier. I.e

match x with
| z -> true

will bind the value of x to the name z. You will also get a warning about z and the _ branch being unused.

You also don't need a second pattern match since it can be folded into the first:

let rec contains xs x =
  match xs with
  | [] -> false
  | z :: _ when z = x -> true
  | _ :: zs -> contains zs x

Trying to use match statements in OCaml to write a function that checks if an element is in a list

1 Answers