Re: I thought this was the one that worked?

On Wed, 2 Aug 2006, Chad Perrin wrote:

because 'enclose' does not imply 'closed forever':

We're discussing "closures", not "enclosures".

they are intimately related:


http://en.wikipedia.org/wiki/Scope_(programming)

...

Static scoping

With static scoping, a variable always refers to its nearest
***enclosing** binding. This is a property of the program text and
unrelated to the runtime call stack. Because matching a variable to its
binding only requires analysis of the program text, this type of scoping
is sometimes also called lexical scoping. Static scope is standard in
modern functional languages such as ML and Haskell because it allows the
programmer to reason as if variable bindings are carried out by
substitution. Static scoping also makes it much easier to make modular
code and reason about it, since its binding structure can be understood in
isolation. In contrast, dynamic scope forces the programmer to anticipate
all possible dynamic contexts in which the module's code may be invoked.

Correct implementation of static scope in languages with first-class
nested functions can be subtle, as it requires each function value to
carry with it a record of the values of the variables that it depends on
(the pair of the function and this environment is called a ***closure***).
When first-class nested functions are not used or not available (such as
in C), this overhead is of course not incurred. Variable lookup is always
very efficient with static scope, as the location of each value is known
at compile time.

...


Two definitions of "closure" from the American Heritage Dictionary of
the English Language:

A bringing to an end; a conclusion: finally brought the project to
closure.

The property of being mathematically closed.

In fact, the term "closure" in computer science is derived from the
mathematical definition of "closure", as in "a closed system". A closure in
computer science is code and data that constitute a closed system.

no, that's simply not correct. the basis for the word 'closure' in computer
science refers to 'enclosing' the present scope.

take

http://www.perl.com/pub/a/2002/05/29/closure.html

as an example of this interpretation.


As David has suggested (without specifically and explicitly saying it
outright in such terms), the data in question might consist of a sort of
"potential data" such that a closure's referent closed scope needn't
necessarily actually contain anything, so long as the scope itself is
theoretically present and accounted for.

and 'scope' is the key here. closure in computer science refers to enclosing
the present scope.

If anything, it's you who's introducing pollutants to the otherwise clearer
(if not actually clear) subject by confusing "enclosure" with "closure".

well, at the risk of sounding inflamatory, you're example and comments so far
have shown a decided lack of understanding about ruby's scoping rules and how
they related to a (common) notion of closure which is not 'closed' but is
enclosing.

your critique leaves one asking: what do __you__ think a closure should be
since what a closure actually is in ruby, perl, and many functional languages
is not closed at all.

in any case it's my hope that people realize that, whatever you call it,
ruby's closures do, in fact, act to enclose the existing scope and never
freeze or copy it.

regards.

-a
--
we can never obtain peace in the outer world until we make peace with
ourselves.
- h.h. the 14th dali lama

.

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