OCaml Optional Argument

Question

How can I write a function in OCaml in which one or more argument are optional?

let foo x y z = if(x+y > z) then true else false;;

If foo do

Answer 1

OCaml has optional arguments, but it's quite a bit trickier than you would expect because OCaml functions fundamentally have exactly one argument. In your case, foo is a function that expects an int and returns a function.

If you leave off trailing arguments, normally this means that you're interested in the function that will be returned; this is sometimes called partial application.

The result is that trailing optional arguments (as you are asking for) do not work.

Optional arguments are always associated with a name, which is used to tell whether the argument is being supplied or not.

If you make z the first argument of your function rather than the last, you can get something like the following:

# let foo ?(z = 0) x y = x + y > z;;
val foo : ?z:int -> int -> int -> bool = 
# foo 3 3 ~z: 2;;
- : bool = true
# foo 3 3 ~z: 10;;
- : bool = false
# foo 2 1;;
- : bool = true

In general I'd say that optional (and named) arguments in OCaml don't solve the same problems as in some other languages.

I personally never define functions with optional arguments; so, there may be better ways to achieve what you're asking for.