Ocaml unboud value

Question

I'm trying to write a simple Ocaml function but im getting this error:

Error: This expression has type unit but an expression was expected of type int

let rec euclid a b = 
  if a = b then a
  else if a < b then 1
  else if a > b then 2

Answer 1

To fix the immediate problem, you need else clauses in your function:

let rec euclid a b =
  if a = b then a
  else if a < b then 1
  else 2 (* You know a > b is the only possible alternative *)

You may realize this, but this function is not recursive, nor, I think, is it doing what you want it to do.

However, there is an error in the way you're conceptualizing how a function works in Ocaml. The way you've written the function is imperative in nature; it is a series of if/then statements which are acted upon sequentially. Rather, the return value of euclid should be simply the result of one broad if/then statement (an integer value). Nesting, as I have done above, can be acceptable, but the essential thing to take away is that a function is just a single expression which is evaluated, not a series of imperative actions.

EDIT for updated question:

All OCaml if/then statements should have else clauses. Your very last nested if/then statement has no else clause. If OCaml detects an if/then statement with no else clause, an else clause is assumed returning () (unit). Essentially, if a > b is false, what should OCaml return? It assumes nothing, but returning () conflicts with the supposed type of your function (an integer).

Of course, that a > b is false is impossible in your function, since if not a = b and not a < b, the only other choice is a > b. Thus, you don't need another if statement at the end of your function; at that point, you know without a doubt that a > b, so you can simply say else 2.