OCaml Tips: Converting a String to a List of Characters

While playing with OCaml I was surprised to learn there’s no built-in function the convert a string to a list of its characters. Admittedly, that’s not something you need very often, but it does come handy from time to time. There are many ways to implement such a function ourselves and the one I like the most makes use of List.init:

let explode_string s = List.init (String.length s) (String.get s);;
val explode_string : string -> char list = <fun>

explode_string "hello, world!";;
- : char list = ['h'; 'e'; 'l'; 'l'; 'o'; ','; ' '; 'w'; 'o'; 'r'; 'l'; 'd'; '!']

I went with the name explode_string as the name explode is often used to describe this type of operation (with implode being the name of the inverse operation).

Searching for the signature string -> char list on Sherlodoc reveals that many libraries offer some version of the above function. Perhaps one day we’ll have something like String.to_list in the standard library.

By the way, it’s also pretty easy to define the inverse operation - namely creating a string out of a list of characters:

let implode_char_list l = String.of_seq (List.to_seq l);;
val implode_char_list : char list -> string = <fun>

implode_char_list (explode_string "hello");;
- : string = "hello"

Note that List.init was added in OCaml 4.06 and String.of_seq was added in OCaml 4.07.

Keep in mind that lists of chars are quite memory inefficient, as each character takes 4 bytes (64 bits) on a modern 64 bit CPU.¹ When you factor the pointers to the next element in the list, each character is effectively taking 8 bytes, which is pretty far from the memory efficiency of a string. The take away is that you should avoid using them if you’re dealing with large data sets.

That’s all I have for you today. Keep hacking!

1. You can learn more about OCaml’s runtime memory layout here. ↩