"recursion" in a function argument via callback

So ignore has the following signature: 'a => unit, it will only work for functions that have one arbitrary parameter and return unit. In your case, updateSW has the signature unit => unit which can be unified by the type checker without errors.

If you had a more complex function like

@val external registerSW: opts => (string, int) => unit = "registerSW"

You would probably initialize it wrapped in an option:

let updateSWRef = ref(None)

updateSWRef :=
  Some(
    registerSW({
      foo: <button
        onClick={_ => {
          updateSWRef.contents->Option.forEach(updateSWRef =>
            updateSWRef("abc", 123)
          )
        }}
      />,
    }),
  )

And the ignore function is equivalent to this:

let ignore = _param1 => ()

So you could also create your own ignore functions if the signature of the built-in one does not match.

let ignore2 = (_param1, _param2) => ()

Caveat: The compiler won’t optimize them away like the built-in one.

Does that clarify it?