Fischer Random Chess setup generator written in OCaml

chess960.ml

(** s combinator *)
let (<*>) f g x = g (f x) x;;

(** b combinator, i.e. function composition *)
let (<$>) f g x = x |> f |> g;;

(** replaces a single element in a list at index `pos` *)
let amend a pos lst = List.mapi (fun i x -> if i = pos then a else x) lst;;

(** picks a random item from a list *)
let pick lst =
  let len = List.length lst in
  let i = Random.int len in
  List.nth lst i;;

(** get the indices of all the elements that match the input `what` *)
let get_indices what where =
  List.mapi (fun i x -> (i, x)) where
  |> List.filter (fun (_, x) -> x = what)
  |> List.map (fun (i, _) -> i);;

(** the blank board *)
let board = Array.to_list(Array.make 8 ' ');;

(** random starting position for the white bishop *)
let white_bishop = pick [1; 3; 5; 7] |> amend 'b';;
    
(** random starting position for the black bishop *)
let black_bishop  = pick [0; 2; 4; 6] |> amend 'b';;

(** random starting position for the knight *)
let knight = ((get_indices ' ') <$> pick) <*> (amend 'n');;

(** random starting position for the queen *)
let queen = ((get_indices ' ') <$> pick) <*> (amend 'q');;

(** fill out the three remaining spaces with the two rooks and the king *)
let rooks_and_king board = 
  match get_indices ' ' board with 
  | [r1;k;r2] -> board |> amend 'r' r1 |> amend 'k' k |> amend 'r' r2
  | _ -> raise (Failure "The remaining empty spaces must be exactly 3 long");;

(** because ocaml doesn't already have this... *)
let char_list_to_string = (List.map (Printf.sprintf "%c")) <$> (String.concat "");;

(** chain it all together *)
board |> white_bishop |> black_bishop |> knight |> knight |> queen |> rooks_and_king |> char_list_to_string