char_string_slice.ml

open Printf
let (=>) left right = printf "%c" (if left = right then '.' else 'F')

module CharString = struct

  type t = Empty | Char of char * t


  let rec fold ~empty ~char = function
    | Empty -> empty
    | Char (c, rest) -> char c (fold ~empty ~char rest)

  let to_char_list =
    fold ~empty:[] ~char:(fun char rest -> char :: rest)

end

module String = struct

  type t = string


  module Slice = struct
    type t = {index: int; string: string}

    let rec fold ~empty ~char {index; string} =
      if index = String.length string then
        empty
      else
        char (String.get string index)
          (fold ~empty ~char {index=index + 1; string})
  end

(*
  let fold ~empty ~char string =
    Slice.fold ~empty ~char Slice.{index=0; string}
*)

  let fold ~empty ~char string =
    let length = String.length string in
    let rec go index =
      if index = length then empty
      else char (String.get string index) (go (index + 1))
    in
    go 0

  let to_char_list =
    fold ~empty:[] ~char:List.cons


  module Test = struct
    to_char_list "abcd" => ['a'; 'b'; 'c'; 'd']

  end
end