Issue with labeled arguments in OCaml -

this line of ocaml:

let c ~f = f() in let () = c (fun () -> ()) in () 

leads following error on c (fun () -> ()) expression:

file "tst.ml", line 1, characters 27-43: error: expression has type f:(unit -> (unit -> unit) -> 'a) -> 'a        expression expected of type unit 

without ~, there no error. there no error if explicitly label argument c ~f:(fun -> ()).

how labeling messing things up?

here's happening, believe. you're trying use following special case functions labeled parameters:

as special case, if function has known arity, arguments unlabeled, , number matches number of non-optional parameters, labels ignored , non-optional parameters matched in definition order. optional arguments defaulted.

(section 6.7.1 of ocaml manual.)

but in definition, c doesn't have known arity. because f can return function, , hence c can return function.

since special case doesn't apply, compiler has assume labeled argument ~f: hasn't appeared yet, , does return function. hence typing see in example.

if redefine c has known arity (so can't return function), things work expect.

# let c ~f = [ f () ];; val c : f:(unit -> 'a) -> 'a list = <fun> # c (fun () -> ());; - : unit list = [()]