Edit

Try

Since 5.1.0, you can try bucklescript in a single file or via command line

bucklescript>bsc -i -e 'let id = x => x'
let id: 'a => 'a;

Here -i flags tell the compiler to infer the signature.

bucklescript.github.io>bsc -e 'let id = x => x'
// Generated by BUCKLESCRIPT, PLEASE EDIT WITH CARE
'use strict';


function id(x) {
  return x;
}

exports.id = id;
/* No side effect */

You can also compile it directly via bsc test.re.

Suppose you have a file called test.re:

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

You can compile it directly via bsc test.re, producing the following output:

bucklescript.github.io>bsc test.re
// 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 test.re

let fib: int => int;