Re: closures

On Mon, Mar 15, 2010 at 7:32 PM, Andrew Gwozdziewycz <apgwoz@gmail.com> wrote:

On Mon, Mar 15, 2010 at 5:36 PM, Michael Bevilacqua-Linn <michael.bevilacqualinn@gmail.com> wrote:

A first-class function (or procedure) means that there's a facility in the language that allows you to create procedures at execution time, store a reference to them in a variable, basically treat them as you would any other piece of data that your program is working with.

No. This is wrong. As you can have closures in a compiled language. In a compiled language, closures might be created by doing lambda lifting, which creates a named function with extra parameters for the variables that used to be free.

In other words, a function such as:

   (define (make-counter x)
   (lambda (n)
   (begin
   (setf! x (+ x n))

   x)))

Might compile to the following procedures (illustrative only):

(define (lifted-anonymous-function-1 n FREE_X)
   (begin
   (set-box! FREE_X (+ (unbox FREE_X) n))

   (unbox FREE_X)))

(define (make-counter x)
   (make-closure lifted-anonymous-function-1 (box x)))

Applying the closure would then call the reference to the function, pass the given and the boxed parameters from the closure.

On Mon, Mar 15, 2010 at 4:35 PM, <mjd-phillylambda@plover.com> wrote:

> 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.

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.

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.

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.

> 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.

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

> 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.

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

--
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