From: Huw Collingbourne <huw@...>
Date: 2008-05-28T02:27:59+09:00
Subject: Re: OOP in Ruby?

Robert Dober wrote:

> Anyway, I will tell you what is important to me:
> Blocks in Ruby are Proc objects, always, can you agree with this
> statement,

No, it doesn't seem to me that they are. As I said before, in Ruby a 
block does not, by default, have its own independent existence. Let me 
illustrate by comparison with Smalltalk. When we write blocks and use 
them with methods (e.g. times in Ruby or timesRepeat: in Smalltalk) they 
may appear to be more or less the same:

1) SMALLTALK

i := 0.
10 timesRepeat: [  i := i + 1 ].
i.

2) RUBY

i = 0
10.times{ i += 1 }
puts( i )


BUT, a Smalltalk Block can be its own object. It is not obliged to be 
used with some 'block-aware' method. Compare the following:

1) SMALLTALK

i := 0.
myBlock := [  i := i + 1 ].
10 timesRepeat: myBlock.

2) RUBY (a)

i = 0
myBlock = { i += 1 } # <= error: Ruby thinks this is a hash
10.times{ i += 1 }
puts( i )

2) RUBY (b)

i = 0
myBlock = { || i += 1 } # <= syntax error
10.times{ i += 1 }
puts( i )

3) RUBY (c)

i = 0
myBlock = proc{ i += 1 } #<= Hurrah! Works by explictly 'converting' to 
Proc
10.times{ i += 1 }
puts( i )


In Smalltalk, a block is always an instance of its class. e.g.

myBlock := [  i := i + 1 ].
myBlock class.
   ...Smalltalk replies: BlockClosure.


It's a subtle point, I know, and I don't want to make any big deal of 
it. After all, when a Proc object block is explicitly created from a 
Ruby block, it works in a comparable way to a Smalltalk block. However, 
that doesn't alter the fact that, in Ruby, blocks (chunks of code 
delimited by {..} or do..end) are not normally instances of the Proc 
class. They are, well, just chuncks of code but not objects. To 
demonstrate that, try evaluating a block:

1) SMALLTALK
[  i := i + 1 ] class.
   ...Answers: BlockClosure

2) RUBY
{ i += 1 }.class
   ...Answers: syntax error, unexpected '}', expecting $end { || i += 1 
}.class

do i += 1 end.class
   ...Anwsers: syntax error, unexpected kEND, expecting $end do  i += 1 
end.class


best wishes
Huw

SapphireSteel Software
Ruby and Rails In Visual Studio
http://www.sapphiresteel.com
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