# Nuby with instance\_eval question **URL:** \ **Category:** ruby-talk\ **Created:** [27 August 2006 03:14 UTC](https://rubytalk.org/t/nuby-with-instance-eval-question/30367 "2006-08-27T03:14:53Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Geoff\_Barnes](https://avatars.discourse-cdn.com/v4/letter/g/46a35a/32.png) [@Geoff\_Barnes](https://rubytalk.org/u/Geoff_Barnes)\ **Post date:** [27 August 2006 03:14 UTC](https://rubytalk.org/t/nuby-with-instance-eval-question/30367/1 "2006-08-27T03:14:53Z")
Wondering why this works the way it does:       1 class Klass       2       3 attr\_accessor :attr1       4       5 def initialize (&block)       6 chum = "chum\_in\_Klass"       7 instance\_eval(&block) # Why does instance\_eval pick up self.attr1 from Klass but not 'chum'??       8 eval\_attr1       9 end      10 def eval\_attr1      11 @ok = self.attr1.call      12 end      13 end      14      15 chum = "chum\_in\_main"      16      17 foo=Klass.new {      18 self.attr1 = proc { chum }      19 }      20      21 puts foo.inspect # -\> foo.ok = "chum\_in\_main"      22      23 chum="chum\_in\_main\_again"      24 foo.eval\_attr1      25      26 puts foo.inspect # -\> foo.ok = "chum\_in\_main\_again" I have a constructor that uses a block to intialize the object's members. Nested on the RHS of a member assignment is a proc block. The proc block carries the value of chum from the "main" namespace, whereas I expected it would take the chum from the Klass. I figured the proc would not be evaluated and bound to a namespace until the passed block was instance\_eval'd in the constructpr, and thereby pick up a binding from Klass... This makes my head hurt! The observed behaviour is what I want but I didn't think it was possible. I want to make sure I'm not taking advantage of a "bug" or a language ambgiuity. If someone could make my eval/proc light bulb come on it would be much appreciated.. Thanks > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Matthew\_Johnson](https://avatars.discourse-cdn.com/v4/letter/m/3bc359/32.png) [@Matthew\_Johnson](https://rubytalk.org/u/Matthew_Johnson)\ **Post date:** [27 August 2006 05:19 UTC](https://rubytalk.org/t/nuby-with-instance-eval-question/30367/2 "2006-08-27T05:19:59Z")
> Wondering why this works the way it does: > >       1 class Klass >       2 >       3 attr\_accessor :attr1 >       4 >       5 def initialize (&block) >       6 chum = "chum\_in\_Klass" >       7 instance\_eval(&block) # Why does instance\_eval pick up > self.attr1 from Klass but not 'chum'?? >       8 eval\_attr1 >       9 end >      10 def eval\_attr1 >      11 @ok = self.attr1.call >      12 end >      13 end >      14 >      15 chum = "chum\_in\_main" >      16 >      17 foo=Klass.new { >      18 self.attr1 = proc { chum } >      19 } >      20 >      21 puts foo.inspect # -\> foo.ok = "chum\_in\_main" >      22 >      23 chum="chum\_in\_main\_again" >      24 foo.eval\_attr1 >      25 >      26 puts foo.inspect # -\> foo.ok = "chum\_in\_main\_again" > > I have a constructor that uses a block to intialize the object's > members. Nested on the RHS of a member assignment is a proc block. The > proc block carries the value of chum from the "main" namespace, whereas > I expected it would take the chum from the Klass. I figured the proc > would not be evaluated and bound to a namespace until the passed block > was instance\_eval'd in the constructpr, and thereby pick up a binding > from Klass... This makes my head hurt! > > The observed behaviour is what I want but I didn't think it was > possible. I want to make sure I'm not taking advantage of a "bug" or a > language ambgiuity. If someone could make my eval/proc light bulb come > on it would be much appreciated.. chum is a local variable in both places. instance\_eval only affects self and class variables, not local variables. Matthew ---
**Author:** ![E\_S](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@E\_S](https://rubytalk.org/u/E_S)\ **Post date:** [27 August 2006 16:18 UTC](https://rubytalk.org/t/nuby-with-instance-eval-question/30367/3 "2006-08-27T16:18:03Z")
Geoff Barnes wrote: > Wondering why this works the way it does: > >       1 class Klass >       2 >       3 attr\_accessor :attr1 >       4 >       5 def initialize (&block) >       6 chum = "chum\_in\_Klass" >       7 instance\_eval(&block) # Why does instance\_eval pick up > self.attr1 from Klass but not 'chum'?? >       8 eval\_attr1 >       9 end >      10 def eval\_attr1 >      11 @ok = self.attr1.call >      12 end >      13 end >      14 >      15 chum = "chum\_in\_main" >      16 >      17 foo=Klass.new { >      18 self.attr1 = proc { chum } >      19 } You are creating a closure here and, since 'chum' exists in this scope (you would get an error otherwise), it is bound here. > **···** > > >      20 > >      21 puts foo.inspect # -\> foo.ok = "chum\_in\_main" > >      22 > >      23 chum="chum\_in\_main\_again" > >      24 foo.eval\_attr1 > >      25 > >      26 puts foo.inspect # -\> foo.ok = "chum\_in\_main\_again" > > > > \ > > > > Thanks > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Geoff\_Barnes](https://avatars.discourse-cdn.com/v4/letter/g/46a35a/32.png) [@Geoff\_Barnes](https://rubytalk.org/u/Geoff_Barnes)\ **Post date:** [27 August 2006 18:03 UTC](https://rubytalk.org/t/nuby-with-instance-eval-question/30367/4 "2006-08-27T18:03:00Z")
OK, thanks for the responses. Let's see if I got this right : Blocks passed to instance\_eval are still bound to where ever they were declared but instance\_eval will use instance & class variables where ever required. > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).