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partial application warning unreliable?
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Date: 2005-12-09 (02:56)
From: skaller <skaller@u...>
Subject: Re: [Caml-list] partial application warning unreliable?
On Fri, 2005-12-09 at 11:15 +0900, Jacques Garrigue wrote:
> From: skaller <skaller@users.sourceforge.net>

> > 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.

> The trouble is that any change to this behaviour would not be
> principal (from the type inference point of view).

I'm not sure I understand that (what a principal type is .. ;(

> 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.

Yes. The only case I can think of where this would 
cause a problem is raising an exception -- since that
is allowed ALSO in non-statement position.

Of course some legacy code may break. And that is, IMHO,
a good thing. Some C programmers even write:

	extern int f(int);
	(void) f(1);

to make the casting away of the return value explicit,
clearly believing that silently throwing away a return
value is a bug in ISO C. In Ocaml we have

	ignore (f 1);

if we really want side effects but not the return value.
Its use should be mandatory. IMHO.

> This is not the default, but this could indeed be done with
> -warn-error S.

Hmm ..

When you see some code in a statement, you cannot
report an error if the type is 'a, because it may later
be specialised to type unit, and the error would then
be premature.

And secondly I asked:

> If I turn off F, will I get S instead?

and Damien replied:

"Warnings are orthogonal: disabling F does not make you get more S
warnings.  It has no impact on S."

so you'd need to -warn-error SF not just S ;(

You surely wouldn't want -warn-error to actually change
the way the inference engine worked!

> 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 :-(

Is this particular problem specific to class methods?
It seems to be a general typing problem to me, BUT,
with objects you can call a method before seeing it,
which you cannot do with ordinary functions.

John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net