Question

I try to write a simple OCaml program that returns true if the a list contains all even integers and false if it does not.

let rec allEven l = 
List.hd l mod 2 = 0 && allEven (List.tl l);;

It didn’t give me any error when I typed in the code. But whenever I enter a list that starts with an even number like allEven [2;3] it give me the error message “Failure “hd””. Not really sure why. Thanks!!

Answer 1

List.hd will raise Failure “hd” on the empty list. To correct your function, use pattern matching:

let rec allEven l = 
  match l with
  | [] -> true
  | h::t -> if h mod 2 = 1 then false else allEven t  

Also, the modulo operator in OCaml is “mod” not “%”