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partial application warning unreliable?
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Date: 2005-12-09 (15:26)
From: malc <malc@p...>
Subject: Re: [Caml-list] partial application warning unreliable?
On Fri, 9 Dec 2005, Jacques Garrigue wrote:

> From: skaller <skaller@users.sourceforge.net>
>> AHA. In trying to fill out your problem to a real test case
>> for a bug report .. I think I have discovered the problem!
>> # class cow = object(self) method moo (s:string)= print_endline s
>>   end;;
>> class cow : object method moo : string -> unit end
>> # let y o = o#moo; 1;;
>> val y : < moo : 'a; .. > -> int = <fun>
>> And there we have it .. an uncaught partial application!
>> The reason is clear .. we don't know the arity of the
>> function yet -- we don't even know its type.
>> The type of a statement is currently 'a, which is just
>> plain wrong. The correct type is void, however unit
>> will catch more errors than 'a.
> This behaviour has been known for long.
> This is for instance why, in the standard library, List.iter is
> explicitly given type
>   ('a -> unit) -> 'a list -> unit
> rather than the inferred
>   ('a -> 'b) -> 'a list -> unit
> (which actually it had a long time ago, in Caml Special Light)
> The trouble is that any change to this behaviour would not be
> principal (from the type inference point of view).
> That is, we might choose to instantiate 'a to unit when generalizing
> the type of y, but actually #moo might be of type int, which we will
> discover later, when applying it. As long as returning non-unit in a
> statement grades only a warning, we cannot do that.
> So, saying that the type of y above is wrong means that all statements
> should be forced by type checking to return unit and nothing else.
> This is not the default, but this could indeed be done with
> -warn-error S.
> Note that, for objects, there was before ocaml 3.05 a warning, turned
> on only in -labels mode, that ensured that every method was known
> before being called. This would have caught the above error. It is now
> commented out :-(

I gather all this means that the only "safe" way to call a unit method
on an implictily typed object is via:

let () = o#moo in ...