From: Peter McLain <peter.mclain@...>
Date: 2010-02-06T03:04:32+09:00
Subject: eval and implicit passing of blocks

Hi,

   I'm trying to understand if/how/when/why Ruby implicitly passes  
blocks to
   instance_eval, class_eval and eval, but not to plain methods.

   It seems that 1.9 behavior is consistent across all versions of  
eval: a
   block, passed into a method that does an eval, is available for use  
by
   that eval.  It also seems that 1.8.x has a bug in that the first  
level
   method called does not make the block available to eval, but that
   it is available to more deeply nested method calls.

       class Mystery1
         def level_1(&block)
           level_2(&block)

           # 1.8.x: LocalJumpError ;  1.9.x: Ok
           puts "Mystery1 level_1: #{instance_eval('yield')}"
         end

         def level_2(&block)
           level_3(&block)
           puts "Mystery1 level_2: #{instance_eval('yield')}" # OK
         end

         def level_3(&block)
           puts "Mystery1 level_3: #{instance_eval('yield')}" # OK
         end
       end

       o = Mystery1.new
       begin
         o.level_1 { 12 }
       rescue Exception => e
         puts "#{o.class} raised #{e.class}: #{e.message}"
         p e.backtrace
       end

   Running this against 1.8.[67] and 1.9.2 gives different results:

       ruby-1.8.6-tv1_8_6_398: ruby 1.8.6 (2010-02-04 patchlevel 398)  
[i686-darwin9.8.0]

       Mystery1 level_3: 12
       Mystery1 level_2: 12
       Mystery1 raised LocalJumpError: (eval):1:in `level_1': no block  
given
       ["/Users/pmclain/tmp/m1.rb:6:in `level_1'", "/Users/pmclain/tmp/ 
m1.rb:21"]

       ruby-1.8.7-p249: ruby 1.8.7 (2010-01-10 patchlevel 249) [i686- 
darwin9.8.0]

       Mystery1 level_3: 12
       Mystery1 level_2: 12
       Mystery1 raised LocalJumpError: (eval):1:in `level_1': no block  
given
       ["/Users/pmclain/tmp/m1.rb:6:in `level_1'", "/Users/pmclain/tmp/ 
m1.rb:21"]

       ruby-1.9.2-tv1_9_2_preview2: ruby 1.9.2dev (2009-09-07 trunk  
24787) [i386-darwin9.8.0]

       Mystery1 level_3: 12
       Mystery1 level_2: 12
       Mystery1 level_1: 12

   Question 1: Is the 1.8 behavior a bug?

   Question 2: Is the 1.9 behavior intended?  If so, is it documented
   anywhere? Perhaps the various eval methods should document that  
they pass
   the calling frame's block to the eval.

   If I replace "instance_eval" with "Mystery1.class_eval", I get the  
same
   result.  If I replace "instance_eval" with "eval", I get different
   results:

       ruby-1.8.6-tv1_8_6_398: ruby 1.8.6 (2010-02-04 patchlevel 398)  
[i686-darwin9.8.0]

       Mystery1 raised LocalJumpError: (eval):1:in `level_3': no block  
given
       ["/Users/pmclain/tmp/m1.rb:15:in `level_3'", "/Users/pmclain/ 
tmp/m1.rb:10:in `eval'", "/Users/pmclain/tmp/m1.rb:15:in `level_3'", "/ 
Users/pmclain/tmp/m1.rb:10:in `level_2'", "/Users/pmclain/tmp/m1.rb: 
3:in `level_1'", "/Users/pmclain/tmp/m1.rb:21"]

       ruby-1.8.7-p249: ruby 1.8.7 (2010-01-10 patchlevel 249) [i686- 
darwin9.8.0]

       Mystery1 raised LocalJumpError: (eval):1:in `level_3': no block  
given
       ["/Users/pmclain/tmp/m1.rb:15:in `level_3'", "/Users/pmclain/ 
tmp/m1.rb:10:in `eval'", "/Users/pmclain/tmp/m1.rb:15:in `level_3'", "/ 
Users/pmclain/tmp/m1.rb:10:in `level_2'", "/Users/pmclain/tmp/m1.rb: 
3:in `level_1'", "/Users/pmclain/tmp/m1.rb:21"]

       ruby-1.9.2-tv1_9_2_preview2: ruby 1.9.2dev (2009-09-07 trunk  
24787) [i386-darwin9.8.0]

       Mystery1 level_3: 12
       Mystery1 level_2: 12
       Mystery1 level_1: 12

   So it looks like 1.9 is consistent in making the block available to  
eval
   at all levels, but 1.8 treats eval differently than class_eval and
   instance_eval.

   It also looks like 1.8 has another bug in that it tries too hard to
   pass blocks along, whereas 1.9 gives reasonable behavior.  In  
Mystery2 I
   don't pass the block from level_2 to level_3, and 1.9 complains  
that no
   block was given; 1.8 seems to pass the block implicitly...

       class Mystery2
         def level_1(&block)
           level_2(&block)

           # 1.8.x: LocalJumpError ;  1.9.x: Ok
           #puts "Mystery2 level_1: #{instance_eval('yield')}"
         end

         def level_2(&block)
           level_3
           puts "Mystery2 level_2: #{instance_eval('yield')}" # OK
         end

         def level_3
           puts "Mystery2 level_3: #{instance_eval('yield')}" # 1.8:  
OK  1.9: LocalJumpError
         end
       end

       o = Mystery2.new
       begin
         o.level_1 { 12 }
       rescue Exception => e
         puts "#{o.class} raised #{e.class}: #{e.message}"
         p e.backtrace
       end

   And the output:

       ruby-1.8.6-tv1_8_6_398: ruby 1.8.6 (2010-02-04 patchlevel 398)  
[i686-darwin9.8.0]

       Mystery2 level_3: 12
       Mystery2 level_2: 12

       ruby-1.8.7-p249: ruby 1.8.7 (2010-01-10 patchlevel 249) [i686- 
darwin9.8.0]

       Mystery2 level_3: 12
       Mystery2 level_2: 12

       ruby-1.9.2-tv1_9_2_preview2: ruby 1.9.2dev (2009-09-07 trunk  
24787) [i386-darwin9.8.0]

       Mystery2 raised LocalJumpError: no block given (yield)
       ["/Users/pmclain/tmp/m2.rb:15:in `instance_eval'", "/Users/ 
pmclain/tmp/m2.rb:15:in `instance_eval'", "/Users/pmclain/tmp/m2.rb: 
15:in `level_3'", "/Users/pmclain/tmp/m2.rb:10:in `level_2'", "/Users/ 
pmclain/tmp/m2.rb:3:in `level_1'", "/Users/pmclain/tmp/m2.rb:21:in  
`<main>'"]


   Question 3: Is the 1.8 behavior a bug?
   Question 4: Is the 1.9 behavior correct?

   Is there any documentation (other than the code) that describes  
this in
   detail?  Neither the Pick Axe book, nor the Flanagan/Matz book give
   detail on this.  The RubySpecs also seem silent on this.


--  
Peter McLain
peter.mclain@gemstone.com