From: Robert Klemme <shortcutter@...>
Date: 2008-03-08T00:07:22+09:00
Subject: Re: the value in ensure block is not returned

2008/3/6, Oliver Peng <oliver.peng@skywave.com>:
> Robert Klemme wrote:
>  > On 06.03.2008 16:46, Oliver Peng wrote:
>
> >> irb(main):009:1> end
>  >> => nil
>  >> irb(main):010:0> test
>  >> => 2
>  >>
>  >> I am expecting that 1 should be returned by is 2.
>  >
>  > Why should it?  If you think about it you will see that it's actually
>  > good the way it is.  The result of evaluating code in /ensure/ cannot be
>  > returned under all circumstances (see Gary's reply).  And for normal
>  > execution of a block of code you want the result of the code block to be
>  > returned - not the result of /ensure/.  The code in /ensure/ is intended
>  > to do some cleanup that has to occur under all circumstances but not
>  > interfere with normal processing.
>  >
>  >> So it is obviously that all codes in ensure block have been run. But the
>  >> return value is always the last value before ensure block.
>  >
>  > Yes, no problem here.
>  >
>  > Cheers
>  >
>  >   robert
>
>
> Ok. I see.
>
>  I am also curious what will happen if I change the return value in
>  ensure block. Here is my test code:
>
>  irb(main):003:0> def test
>  irb(main):004:1> begin
>  irb(main):005:2* a = 1
>  irb(main):006:2> b =2
>  irb(main):007:2> b
>  irb(main):008:2> ensure
>  irb(main):009:2* b += 3
>  irb(main):010:2> end
>  irb(main):011:1> end
>  => nil
>  irb(main):012:0> test
>  => 2
>
>  It looks that the result value has been saved to a temp place before
>  running ensure block. Is that correct?

The return value is determined the very moment the block ends, which
means that the *reference* is saved (as always in Ruby).  But since
you used Fixnums and simply reassigned to b ("b += 3") this won't be
visible.  But

irb(main):001:0> def t
irb(main):002:1> s="foo"
irb(main):003:1> ensure
irb(main):004:1* s.replace "see?"
irb(main):005:1> end
=> nil
irb(main):006:0> t
=> "see?"
irb(main):007:0>

In other words, of course you can change the object before it is returned.

>  I also use global variable to test.
>
>  irb(main):014:0> @@value = 0
>  => 0
>  irb(main):015:0> def test1
>  irb(main):016:1> begin
>  irb(main):017:2* @@value = 3
>  irb(main):018:2> ensure
>  irb(main):019:2* @@value = 8
>  irb(main):020:2> end
>  irb(main):021:1> end
>  => nil
>  irb(main):023:0> test1
>  => 3
>  irb(main):024:0> @@value
>  => 8

Same story: you reassigned.

Cheers

robert

-- 
use.inject do |as, often| as.you_can - without end