Re: [Caml-list] confusing type error when using function with optional arguments

["Török Edwin" <edwin+ml-ocaml@etorok.net>]

On 11/16/2012 04:50 PM, Milan Stanojević wrote:
> I was playing with %revapply primitive and ran into the following
> compiler behavior that I don't understand.
> 
> external (|!) : 'a -> ('a -> 'b) -> 'b = "%revapply"
> 
> let foo1 ?x ?y () = ()    (* val foo1 : ?x:'a -> ?y:'b -> unit -> unit
> *)
> let foo2 ?x ?y z = z     (* val foo2 : ?x:'a -> ?y:'b -> 'c -> 'c  *)
> 
> let x = () |! foo1   (* compiles just fine *)
> 
> let y = () |! foo2   (* fails with
>                              Error: This expression has type ?x:'a ->
> ?y:'b -> 'c -> 'c
>                              but an expression was expected of type
> unit -> 'd   *)
> 
> I don't understand why foo1 is fine and foo2 isn't. I would have
> thought that I can use foo2 wherever I can foo1 since it has a
> strictly more general type.
> Am I missing something obvious here?

Its probably due to the optional labels, this works:
let y = () |! foo2 ?x:None ?y:None

--Edwin