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comp.lang.ruby

code blocks and methods

Eps

4/8/2007 4:59:00 PM

Hi,

I am trying to learn ruby from the following site.

http://www.rubycentral.com/book/...

I am familiar with c# and java, one thing I don't understand is the
difference between code blocks and methods, can anyone offer a better
explanation than the one on the site above ?

any help appreciated.

--
Eps
9 Answers

botp

4/8/2007 5:30:00 PM

0

On 4/9/07, andy <eps@mailinator.com> wrote:
> I am familiar with c# and java, one thing I don't understand is the
> difference between code blocks and methods, can anyone offer a better
> explanation than the one on the site above ?

to quote carlson et al, "a Ruby code block is a method that has no
name." see http://www.devarticles.com/c/a/Ruby-on-Rails/Code-Blocks-and-...
or the ruby cookbook.

kind regards -botp

Chad Perrin

4/8/2007 7:12:00 PM

0

On Mon, Apr 09, 2007 at 02:05:06AM +0900, andy wrote:
> Hi,
>
> I am trying to learn ruby from the following site.
>
> http://www.rubycentral.com/book/...
>
> I am familiar with c# and java, one thing I don't understand is the
> difference between code blocks and methods, can anyone offer a better
> explanation than the one on the site above ?
>
> any help appreciated.

As botp pointed out, there really isn't much of a difference between the
two. The major difference is how they are used. Essentially, a block
is a method that has no name, is not specifically associated with any
particular object (as far as I'm aware), and can be passed around by
itself as an object (such as "stored in" a variable).

standard method:

Class Foo
def method
# do stuff
end
end

foo = Foo.new
foo.method

block, used with an iterator:

bar.each {|baz| # do stuff }

block, "stored in" a variable:

var = lambda { # do stuff }
var.call

Of course, you could just as easily use the "do . . . end" syntax for
any of these blocks as the brace-delimited syntax. Here's the iterator
example again, with the "do . . . end" syntax:

bar.each do |baz|
# do stuff
end

If that doesn't help clear things up (along with the reading recommended
by botp), you might try being more precise in your point of confusion.
I think that should cover your question, though, if it was as general in
scope as it seemed.

--
CCD CopyWrite Chad Perrin [ http://ccd.ap... ]
Ben Franklin: "As we enjoy great Advantages from the Inventions of
others we should be glad of an Opportunity to serve others by any
Invention of ours, and this we should do freely and generously."

Chad Perrin

4/8/2007 7:26:00 PM

0

On Mon, Apr 09, 2007 at 04:11:56AM +0900, Chad Perrin wrote:
>
> As botp pointed out, there really isn't much of a difference between the
> two. The major difference is how they are used. Essentially, a block
> is a method that has no name, is not specifically associated with any
> particular object (as far as I'm aware), and can be passed around by
> itself as an object (such as "stored in" a variable).

One other thing:

To understand how blocks differ from methods, it might also help to read
about similar constructs in other languages. For instance, the
difference is analogous (but not identical, because we're talking about
methods rather than functions) to the difference between named functions
and lambdas in several languages. Another place to look might be the
discussion of named subroutines, callbacks, and closures in the O'Reilly
book Learning Perl (aka The Llama Book) by Randal Schwartz et al. In
essence, a block in Ruby is something like an anonymous subroutine in
Perl, a proc like a callback, and a proc formed with lexical context
is a closure.

I'm sure someone will disagree with my definition of closure, just based
on the common assertion that all blocks are closures in Ruby, period, so
read up on closures and decide for yourself. It's helpful in doing so
to think about where the name "closure" originates in this context.

--
CD CopyWrite Chad Perrin [ http://ccd.ap... ]
"The measure on a man's real character is what he would do
if he knew he would never be found out." - Thomas McCauley

Rick DeNatale

4/8/2007 11:51:00 PM

0

On 4/8/07, Chad Perrin <perrin@apotheon.com> wrote:
> On Mon, Apr 09, 2007 at 02:05:06AM +0900, andy wrote:
> > Hi,
> >
> > I am trying to learn ruby from the following site.
> >
> > http://www.rubycentral.com/book/...
> >
> > I am familiar with c# and java, one thing I don't understand is the
> > difference between code blocks and methods, can anyone offer a better
> > explanation than the one on the site above ?
> >
> > any help appreciated.
>
> As botp pointed out, there really isn't much of a difference between the
> two. The major difference is how they are used. Essentially, a block
> is a method that has no name,

Not really, blocks are instances of Proc, they are not methods:
irb(main):001:0> def block_class(&b)
irb(main):002:1> puts b.class
irb(main):003:1> end
=> nil
irb(main):004:0> block_class {"hello"}
Proc
=> nil
irb(main):005:0> method(:block_class).class
=> Method

Although they are close cousins, and can be substituted for each other
in many contexts.


> is not specifically associated with any
> particular object (as far as I'm aware),

Not only are blocks associated with a particular object, self is the
receiver of the method which created the block, but also with the
local variables in the execution context of that method when the block
was created.

> and can be passed around by
> itself as an object (such as "stored in" a variable).

So can methods

irb(main):002:0> a = []
=> []
irb(main):003:0> a_get = a.method(:[])
=> #<Method: Array#[]>
irb(main):004:0> a[0] = :fred
=> :fred
irb(main):005:0> a_get.call(0)
=> :fred
irb(main):006:0>

But the pedagogical explanation that a block is an unnamed method is
useful to beginners.
--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denh...

Chad Perrin

4/9/2007 6:52:00 AM

0

On Mon, Apr 09, 2007 at 08:50:39AM +0900, Rick DeNatale wrote:

[snip a bunch of disagreement with what I said]
>
> But the pedagogical explanation that a block is an unnamed method is
> useful to beginners.

Fair enough. I defer to the man with the better understanding of Ruby.
Apparently, I missed something somewhere along the way, and I appreciate
getting it cleared up.

--
CCD CopyWrite Chad Perrin [ http://ccd.ap... ]
"Real ugliness is not harsh-looking syntax, but having to
build programs out of the wrong concepts." - Paul Graham

James Gray

4/9/2007 12:10:00 PM

0

On Apr 8, 2007, at 12:29 PM, botp wrote:

> On 4/9/07, andy <eps@mailinator.com> wrote:
>> I am familiar with c# and java, one thing I don't understand is the
>> difference between code blocks and methods, can anyone offer a better
>> explanation than the one on the site above ?
>
> to quote carlson et al, "a Ruby code block is a method that has no
> name." see http://www.devarticles.com/c/a/Ruby-on-Rails/Co...
> and-Iteration/
> or the ruby cookbook.

I wrote about my take on code blocks for my blog a while back:

http://blog.grayproductions.net/articles/2006/01/05/code-as-a...

James Edward Gray II

botp

4/9/2007 5:00:00 PM

0

On 4/9/07, Rick DeNatale <rick.denatale@gmail.com> wrote:
> Not really, blocks are instances of Proc, they are not methods:
> irb(main):001:0> def block_class(&b)

i hope we don't generalize that much. Here eg, we are explicitly
converting a block to a proc.

> But the pedagogical explanation that a block is an unnamed method is
> useful to beginners.

i prefer to refer to blocks as just mere codes that cannot stand on
their own yet. They have to be proc-ified or lamba-fied or yielded as
code blocks in methods.

but I really hoped that methods, procs, and lambdas be unified. Their
functions look very similar.

kind regards -botp

Joel VanderWerf

4/9/2007 6:07:00 PM

0

botp wrote:
> i prefer to refer to blocks as just mere codes that cannot stand on
> their own yet. They have to be proc-ified or lamba-fied or yielded as
> code blocks in methods.

You are right. Blocks are not even objects, just syntactical constructs
which can potentially be associated with an object using Proc.new,
#proc, #lambda, or the & notation. Yielding to a block doesn't cause any
such object to be instantiated.

There is a small performance cost to this instantiation, so in general I
try to write methods like this:

def foo; ... yield(something) ...; end

rather than

def foo(&bl); ... bl.call(something); end

Sometimes, though, you really need to instantiate a block and store it
somewhere so it persists between method calls.

Another advantage of yield is that RDoc can automatically recognize it
and document it.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Peña, Botp

4/10/2007 4:17:00 AM

0

From: Joel VanderWerf [mailto:vjoel@path.berkeley.edu] :
# Another advantage of yield is that RDoc can automatically
# recognize it and document it.

hmmm, i did not know that.
thanks for the tip, joel.

kind regards -botp