Brainfuck interpreter in OCaml

bf.ml

let eval str =
  let cells = Array.make 4096 0 in
	let ptr = ref 0 in
	let len = String.length str in

	let rec evalbf c =
		if c >= len then
			c
		else
			match String.get str c with
				| '>' -> ptr := !ptr + 1; evalbf (c+1)
				| '<' -> ptr := !ptr - 1; evalbf (c+1)
				| '+' -> cells.(!ptr) <- cells.(!ptr) + 1; evalbf (c+1)
				| '-' -> cells.(!ptr) <- cells.(!ptr) - 1; evalbf (c+1)
				| '.' -> Printf.printf "%c" (Char.chr cells.(!ptr)); evalbf (c+1)
				| '[' ->
					(* evalbf returns the point in the input where it returns, so we use it to jump past the loop *)
					let newp = ref c in
					while cells.(!ptr) != 0 do
						newp := evalbf (c+1)
					done;
					evalbf (!newp+1)
				| ']' -> c
				| _ -> failwith "Unknown op"
	in
	ignore (evalbf 0)

let () =
	(* Hello World! *)
	eval "++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>."