Why is this OCaml code receiving "Unexpected token"? - ocaml

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.

Answer 2

Your code has this basic structure if you assume the failures won't happen:
let f () =
let p = 3 in p
let q = 5 in q
...
It's not clear what you're trying to do, but this isn't well formed OCaml (as the compiler tells you). Maybe want you want is something more like this:
let f () =
let p = 3 in
let q = 5 in
match ...
If so, you need to move your ifs before your ins:
let f () =
let p = if badp then failwith "" else 3 in
let q = if badq then failwith "" else 5 in
match ...
Or maybe this is more what you want:
let f () =
let p = 3 in
let () = if badp p then failwith "" in
let q = 5 in
let () = if badq q then failwith "" in
match ...
(I hope this is helpful.)