Try via CLI

You can compile a file directly via bsc MyFile.res:

RES
// MyFile.res
let rec fib = n => {
  switch n {
  | 0 | 1 => 1
  | n => fib(n -1) + fib(n-2)
  }
}
Js.log(fib(0))

SH
❯ bsc MyFile.res
// Generated by BUCKLESCRIPT, PLEASE EDIT WITH CARE
'use strict';
function fib(n) {
  if (n === 0 || n === 1) {
    return 1;
  } else {
    return fib(n - 1 | 0) + fib(n - 2 | 0) | 0;
  }
}
console.log(fib(0));
exports.fib = fib;
/*  Not a pure module */

You can also get the inferred signature directly via bsc -i MyFile.res

let fib: int => int