Question

let rec move_robot (pos: int) (dir: string) (num_moves: int) : int =

    let new_forward_position = pos + num_moves in
    if (new_forward_position > 99) then failwith "cannot move beyond 99 steps"
    else new_forward_position

    let new_backward_position = pos - num_moves in
    if (new_backward_position pos < 0) then failwith "cannot move less than 0 steps"
    else new_backward_position

    begin match dir with 
    | "forward" -> new_forward position
    | "backward" -> new_backward_position
    end

I keep on getting “unexpected token in” for the let new_backward_position line. What is my error?

Answer 1

Here is a code that compiles:

let rec move_robot pos dir num_moves =
    let new_forward_position = pos + num_moves in
    if new_forward_position > 99 then failwith "cannot move beyond 99 steps";

    let new_backward_position = pos - num_moves in
    if new_backward_position < 0 then failwith "cannot move less than 0 steps";

    begin match dir with
    | "forward" -> new_forward_position
    | "backward" -> new_backward_position
    end

I modified several things:

• Important: if foo then bar else qux is an expression in OCaml, which takes either the value bar or qux. Thus bar and qux needs to have the same type.
• new_backward_position instead of new_backward_position pos
• you don’t need type annotations : OCaml has type inference
• no need for parentheses around the if clause
• typo in new_forward position

Also, with your code’s logic, let _ = move_robot 0 "forward" 5 fails. Shouldn’t it return 5 instead? I suggest you define a sum type for pos and do a pattern matching on it first.