Execute Brain****/OCaml

From Rosetta Code
Execute Brain****/OCaml is an implementation of Brainf***. Other implementations of Brainf***.
Execute Brain****/OCaml is part of RCBF. You may find other members of RCBF at Category:RCBF.

Quick implementation of a Brainfuck interpreter in OCaml.

Like the Haskell version but without the lazy lists:

Pairs of lists are used to implement both the two-side infinite band of cells, and the program storage.

run interprets a Brainfuck program as a list of characters and reads integers from stdin and outputs them to stdout.

A more efficient implementation could for example only admit well-bracketed brainfuck programs, and parse bracket blocks first, to replace the match_left and match_right which need linear time.

let move_left  (x::l, r) = (l, x::r)
let move_right (l, x::r) = (x::l, r)
 
let rec match_left d =
match d with
'['::_, _ ->
d
| ']'::_, _ ->
match_left (move_left (match_left (move_left d)))
| _ ->
match_left (move_left d)
 
let rec match_right d =
match d with
_, '['::_ ->
move_right d
| _, '['::_ ->
match_right (match_right (move_right d))
| _ ->
match_right (move_right d)
 
let pad = function
[], [] -> [0], [0]
| [], r -> [0], r
| l, [] -> l, [0]
| d -> d;;
 
let modify f (l, x::r) = l, f x :: r
 
let rec exec p d =
match p, d with
(_, []), _ -> ()
| (_, '>'::_), _ ->
exec (move_right p) (pad (move_right d))
| (_, '<'::_), _ ->
exec (move_right p) (pad (move_left d))
| (_, '+'::_), _ ->
exec (move_right p) (modify succ d)
| (_, '-'::_), _ ->
exec (move_right p) (modify pred d)
| (_, ','::_), _ ->
let c = read_int () in
exec (move_right p) (modify (fun _ -> c) d)
| (_, '.'::_), (_, x::_) ->
print_int x;
print_newline ();
exec (move_right p) d
| (_, '['::_), (_, 0::_) ->
exec (match_right (move_right p)) d
| (_, '['::_), _ ->
exec (move_right p) d
| (_, ']'::_), (_, 0::_) ->
exec (move_right p) d
| (_, ']'::_), _ ->
exec (match_left (move_left p)) d
 
let run s = exec ([], s) ([0], [0])

Example output:

# let char_list_of_string s =
  let result = ref [] in
    String.iter (fun c -> result := c :: !result) s;
    List.rev !result;;
val char_list_of_string : string -> char list = <fun>

# run (char_list_of_string ",[>+<-].>.");;
''5''
0
5
- : unit = ()