From: Kyle Rawlins <rawlins@...>
Date: 2002-06-13T03:10:39+09:00
Subject: Re: procs and blocks


--FFoLq8A0u+X9iRU8
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Thu, Jun 13, 2002 at 02:40:01AM +0900, Evan Martin wrote:
> latter sets $cb to a proc that wraps a block.  Or are a proc and a block
> the same thing?

Someone can probably give you a more thorough answer, but my understanding =
of
it is that they are not the same thing.  However, for your purposes (when y=
ou
are storing a callback) you can really only use a proc.

The way I understand it is that a block is sort of a iconoclastic element of
ruby in that it really isn't an object, and is only sort of an argument to
whatever function it is given to.  This is for efficiency reasons, which ma=
kes
sense considering that the main method of iterating over anything in ruby i=
s to
use a block.  So it is much faster to use a combination of a block with yie=
ld
for your basic iterators.  However, I don't think you can store a block by
itself, you have to wrap a proc object around it.  I believe that all proc
objects (created the normal way) store blocks.  So a block is just a proc
that can be used directly without the overhead of creation, calling, etc.

Also, for two of your solutions, the main difference for your purposes as f=
ar
as I can see is that you get different failure modes:  (code not reproduced
exactly, sorry)

given
def a(&p)
    $cb =3D p
end

def b
    $cb =3D Proc.new
end

irb(main):009:0> a      # nil is silently stored to $cb:
nil
irb(main):032:0> $cb
nil
irb(main):033:0> b
ArgumentError: tried to create Proc object without a block
    from (irb):29:in `new'
    from (irb):29:in `b'
    from (irb):33

It seems that for a formal proc parameter there is a default value of nil,
resulting in a reference to nil rather than the ArgumentError one might exp=
ect
when it isn't given.

Thinking about it, you might simply be better off for the purpose of callba=
cks
taking the callback function as a normal argument; this way you can not spe=
cify
a default of nil if you don't want that, or you could give your own default.

def a(p)
    $cb =3D p
end

a(lambda do |arg| p arg.inspect end)=20

hope this helps
-kyle

--=20
http://mas.cs.umass.edu/~rawlins
--
There is no friend
anywhere.

--FFoLq8A0u+X9iRU8
Content-Type: application/pgp-signature
Content-Disposition: inline

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.0.7 (GNU/Linux)

iD8DBQE9B46bncsSsHSBypARAkqKAKCl899z4lmfxgwSBqxEx3kels0EugCfU3/J
l4zB5CIe2pkPR+Y7dRjNuGI=
=jLrV
-----END PGP SIGNATURE-----

--FFoLq8A0u+X9iRU8--