Learning OCaml: Functions without Parameters

A couple of days ago I noticed on OCaml’s Discord server that someone was confused by OCaml function applications (invocations) like these:

print_newline ()

read_input ()

To people coming from “conventional” programming languages this might look like calling a function/method without any arguments. (e.g. foo() in Python) Of course, function application in OCaml is quite different from JavaScript, Python and the like - the function arguments are space separated and simply follow the function’s name:

foo arg1 arg1 arg3

So what are those () then and why are they needed? I’ll start with the second part of the question. In OCaml you can’t really define a function without any parameters - if we have to be super precise, every function takes exactly one parameter, no matter how it might look at a glance.¹ If you try to do something like:

let my_print = print_endline "Hello"
val my_print : unit = ()

You’ll just end up with static binding to nothing. Or not quite nothing, as it’s time to talk about (), which happens to be the single instance of the unit type, used to represent the absence of a meaningful value. So, when you want to define a function that doesn’t need any parameters the convention is to use a single unit parameter:

(* This defines a function that takes unit *)
let say_hello () =
  print_endline "Hello, world!"

(* same here *)
let random_int () =
  Random.int 100

I hope this also explains why’d have to call such functions with () as their argument. If you don’t do this - you’d just receive the underlying function object as the result:

# say_hello;;
- : unit -> unit = <fun>
# say_hello ();;
Hello, world!
- : unit = ()
# random_int;;
- : unit -> int = <fun>
# random_int ();;
- : int = 10

Basically, you need to pass the argument to have an actual function application. Remember that in OCaml (and most functional programming languages), functions are first-class objects that you can pass around like any other value.

And that’s a wrap. I hope you learned something useful today. Keep hacking!

1. See https://dev.realworldocaml.org/variables-and-functions.html#multi-argument-functions. ↩