Re: closures

["Mark M. Hoffman" <mhoffman@lightlink.com>]

Hi:

* mjd-phillylambda@plover.com <mjd-phillylambda@plover.com> [2010-03-15 16:35:05 -0400]:
> 
> > So I'd like to test my understanding with this group...
> > 
> > 	int x;
> > 
> > 	void foo(int x);
> > 	void baz(void (*F)(void));
> > 
> > 	void callback(void)
> > 	{
> > 		foo(x);
> > 	}
> > 
> > 	void bar(void)
> > 	{
> > 		baz(&callback);
> > 	}
> > 
> > C doesn't "support" closures, but the code above has one.
> 
> Only in a trivial sense.  C doesn't support nested function scopes, so
> in C one can't construct an interesting example of closures or of
> their failure in C.  I wanted to construct a C example for you, but
> none of my constructions made sense as C.

Fair enough.

> To not-answer your question:
> 
> > The above is a closure, if you could do such a thing in C. 
> 
> I think the only thing you could usefully do with this question is to
> unask it.  Trying to understand closures by studying the non-behavior
> of the nonexistent closures of C is pointless.

What would a closure look like in C if it had them?  Mu.  Got it. ;)

(http://www.io.com/~snewton/zen/mu.html)

> If you like Perl, I suggest that you read chapter 3 of "Higher-Order
> Perl", available for free download at:
> 
>         http://hop.perl.plover.com/
> 
> I think the explanation there is both clear and rigorous.

Thank you; I'll look there.

> If you like some other language that has closures, I suggest you ask
> for an example in that language.
> 
> If you only like C, I suggest you either forget about closures
> (because C doesn't have them) or learn to like some language that does
> have them.

All of us C programming dinosaurs are alike eh? ;)

> > The word closure
> > is used here because we create an instance of foo() over which x is "closed".
> > 
> > Do I have it right so far?
> 
> No:
> 
> * You have not created an instance of foo, or indeed of anything.
>   Functions always have static duration in C.  Instances of functions
>   are created at compile time, and not afterward.

Yes of course.  My second example was just a hypothetical in a C-like syntax,
if you will.  It isn't my intention to re-invent them in C.  I only wanted to
see if I understand the concept itself.

> * The jargon is that the function is closed over the variable, not the
>   other way around.

Oops - thank you.

> > I would appreciate it if someone could propose a few problems which
> > are trivial to solve with closures, but difficult without.
> 
> a. Implement an object-oriented programming system.  Closures make
>    adequate objects.  This is discussed in detail in _Structure and
>    interpretation of Computer Programs_, should you want to know more.

This is trivial?  I.e. 2-3 screenfuls of code in an hour or so?

> b. Almost anything in _Higher-Order Perl_; see the table of contents.

Thanks & regards,

-- 
Mark M. Hoffman
mhoffman@lightlink.com