How to convert char list to string in OCaml?

Question

I have a char list ['a';'b';'c']

How do I convert this to the string "abc"?

thanks x

Wisdom

Answer 1

You can create a string of a length, equal to the length of the list, and then fold over the list, with a counter and initialize the string with the contents of the list... But, since OCaml 4.02, the string type started to shift in the direction of immutability (and became immutable in 4.06), you should start to treat strings, as an immutable data structure. So, let's try another solution. There is the Buffer module that is use specifically for the string building:

# let buf = Buffer.create 16;;
val buf : Buffer.t = <abstr>
# List.iter (Buffer.add_char buf) ['a'; 'b'; 'c'];;
- : unit = ()
# Buffer.contents buf;;
- : string = "abc"

Or, as a function:

let string_of_chars chars = 
  let buf = Buffer.create 16 in
  List.iter (Buffer.add_char buf) chars;
  Buffer.contents buf

Bigeye

Answer 2

Since OCaml 4.07, you can use sequences to easily do that.

let l = ['a';'b';'c'] in
let s = String.of_seq (List.to_seq l) in
assert ( s = "abc" )

Tisiphone

Answer 3

let cl2s cl = String.concat "" (List.map (String.make 1) cl)

Reunion

This solution produces a stack overflow if the list is too long. In such cases, use ivg's solution. BTW, there's a String.of_list in batteries included. – Martamartaban

Answer 4

Commonly used Base library also offers Base.String.of_char_list

Eric