Comment author: @nojb

This behaviour is as expected. Briefly, functions with optional arguments must take at least one non-optional argument so that the compiler can decide which arguments have been omitted, and order matters.

See section 4.1.1 of the manual https://caml.inria.fr/pub/docs/manual-ocaml/lablexamples.html for more information.

Comment author: jtsang

The criterion for deciding whether an optional argument has been omitted is the non-labeled application of an argument appearing after this optional argument in the function type. Note that if that argument is labeled, you will only be able to eliminate optional arguments through the special case for total applications.

The problem is (and I got hit by it):

let f ?a ~b = ()
let g = f ~b:()

is equivalent to

let g ?a = ()

which triggers warning 16. As I understand it, argument ?a in f cannot be erased.

Comment author: @nojb

Re-opening as I misunderstood the issue.

Comment author: @nojb

#1983

Comment author: @nojb

So, after all, this is expected behaviour. You can erase ?a by passing all other arguments (in this case, just ~b) without the labels. See the discussion in the PR for more.

Comment author: jtsang

Thanks for clearing it up.

Can we have a doc change that clarifies this?

Something like

Note that if that argument is labeled, you will only be able to eliminate optional arguments by totally applying the function, omitting all optional arguments and omitting all labels for all arguments.

I find it unexpected that in

let f a ~b ?c ~d ~e = ()

the only erasing call that succeeds is exactly

f () () () ()

with zero labels.

Comment author: @nojb

I agree the doc is a bit unclear on this point. Would you like to open a PR with your doc suggestion? Otherwise, I could do it.