Re: [Caml-list] Applying labeled function without a label

[Alain Frisch <alain@frisch.fr>]

On 07/10/2013 03:48 PM, Ivan Gotovchits wrote:
>
>
> Please, can someone explain the reason behind the following behaviour:
>
>
> # let f ~a = a;;
> val f : a:'a -> 'a = <fun>
>
> if I apply function f, omiting the label, instead of an error I'll get:
>
> # f 12;;
> - : a:(int -> 'a) -> 'a = <fun>
>
> ... a function that accepts a labeled arguments, that is a function from int
> to 'a, and returns a result of this function:

If you apply f without the labeled argument, if means that the 'a in the 
type scheme "a:'a -> 'a" must be a function type accepting 12 (because 
you pass it the value 12).  So 'a is unified with int -> 'b, and the 
type of "f 12" is thus "a:(int -> 'b) -> 'b".

The point to be noted is that omitted labelled arguments are accepted by 
the type-checker and result in a partial application of the function. 
This is different from optional arguments (when omitted in a function 
application with non-labelled arguments, they are assumed to be missing 
and don't result in a partial application).

FWIW, LexiFi's version of OCaml changes this behavior.  We have decided 
that such omitted labelled arguments should be assumed to be really 
missing and thus result in an error, not a partial application.  So in 
our version, we have:

# let f ~a = a;;
val f : a:'a -> 'a = <fun>
# f 12;;
Error: Missing labeled argument ~a in this function call.


This change tends to give better error messages and avoid some 
confusion; it was also necessary to support another local change: 
implicit arguments, which are non-optional labelled arguments that can 
be synthesized by the compiler based on their type.

-- Alain