# Kernel#eval taking a block **URL:** \ **Category:** ruby-talk\ **Created:** [3 October 2004 01:25 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921 "2004-10-03T01:25:37Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![T\_Onoma](https://avatars.discourse-cdn.com/v4/letter/t/c2a13f/32.png) [@T\_Onoma](https://rubytalk.org/u/T_Onoma)\ **Post date:** [3 October 2004 01:25 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/1 "2004-10-03T01:25:37Z")
Is there a reason Kernel#eval won't take a block? I wrote this to overcome:   # Evaluate a Ruby source code string (or block) in the binding context   def evalb( str=nil, binding=nil, &blk )     if block\_given?       Kernel.eval( "proc {|slf, blk| slf.instance\_eval(&blk)}", binding ).call( binding, blk )     elsif str       Kernel.eval( str, binding )     end   end Although, I'm not sure it's exactly right --I put it together by a bit of trial and error. T. > **···** > > -- > ( o \_ カラチ > // trans. > / \ [transami@runbox.com](mailto:transami@runbox.com) > > I don't give a damn for a man that can only spell a word one way. > -Mark Twain ---
**Author:** ![Nobuyoshi\_Nakada](https://avatars.discourse-cdn.com/v4/letter/n/edb3f5/32.png) [@Nobuyoshi\_Nakada](https://rubytalk.org/u/Nobuyoshi_Nakada)\ **Post date:** [4 October 2004 00:20 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/2 "2004-10-04T00:20:03Z")
Hi, At Sun, 3 Oct 2004 10:25:37 +0900, trans. (T. Onoma) wrote in [ruby-talk:114646]: > Is there a reason Kernel#eval won't take a block? I wrote this to overcome: > >   # Evaluate a Ruby source code string (or block) in the binding context >   def evalb( str=nil, binding=nil, &blk ) >     if block\_given? >       Kernel.eval( "proc {|slf, blk| slf.instance\_eval(&blk)}", > binding ).call( binding, blk ) >     elsif str >       Kernel.eval( str, binding ) >     end >   end It's evaluated in the Binding/Proc's context, but not in the context where it was created. I.E., it is equivalent to   binding.instance\_eval(&blk) except that variables `slf' and `blk' will be altered if they exist in the context of `binding'. Seems less meaningful. > **···** > > -- > Nobu Nakada ---
**Author:** ![T\_Onoma](https://avatars.discourse-cdn.com/v4/letter/t/c2a13f/32.png) [@T\_Onoma](https://rubytalk.org/u/T_Onoma)\ **Post date:** [4 October 2004 14:23 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/3 "2004-10-04T14:23:22Z")
Crud. I see. You are ( as always 😉 quite correct. Is there anyway to overcome? Back to the original question: why Kernel#eval can't take a block? Thanks, T. > **···** > > On Sunday 03 October 2004 08:20 pm, nobu.nokada@softhome.net wrote: > > > Hi, > > > > At Sun, 3 Oct 2004 10:25:37 +0900, > > > > trans. (T. Onoma) wrote in [ruby-talk:114646]: > > \> Is there a reason Kernel#eval won't take a block? I wrote this to > > \> overcome: > > \> > > \> # Evaluate a Ruby source code string (or block) in the binding context > > \> def evalb( str=nil, binding=nil, &blk ) > > \> if block\_given? > > \> Kernel.eval( "proc {|slf, blk| slf.instance\_eval(&blk)}", > > \> binding ).call( binding, blk ) > > \> elsif str > > \> Kernel.eval( str, binding ) > > \> end > > \> end > > > > It's evaluated in the Binding/Proc's context, but not in the > > context where it was created. I.E., it is equivalent to > > > >   binding.instance\_eval(&blk) > > > > except that variables `slf' and `blk' will be altered if they > > exist in the context of `binding'. > > > > Seems less meaningful. ---
**Author:** ![Yukihiro\_Matsumoto2](https://avatars.discourse-cdn.com/v4/letter/y/b9e5f3/32.png) [@Yukihiro\_Matsumoto2](https://rubytalk.org/u/Yukihiro_Matsumoto2)\ **Post date:** [4 October 2004 15:11 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/4 "2004-10-04T15:11:24Z")
Hi, > **···** > > In message "Re: Kernel#eval taking a block" on Mon, 4 Oct 2004 23:23:22 +0900, "trans. (T. Onoma)" \ writes: > > > Back to the original question: why Kernel#eval can't take a block? > > Because it's useless, for my eyes at least. What do you want to > accomplish by eval with a block? > >               matz. ---
**Author:** ![T\_Onoma](https://avatars.discourse-cdn.com/v4/letter/t/c2a13f/32.png) [@T\_Onoma](https://rubytalk.org/u/T_Onoma)\ **Post date:** [4 October 2004 18:00 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/5 "2004-10-04T18:00:29Z")
How to get locals into evaluation? class T     def initialize      @x = 1    end    attr\_reader :x    def get\_binding      cool = "matz"      return binding()    end end t = T.new tb = t.get\_binding eval "cool", tb #=\> "matz" a = proc {|c| c.upcase} eval "cool = a.call(cool)", tb NameError: undefined local variable or method `a' for #\         from (irb):15:in `get\_binding'         from (irb):15:in `get\_binding'         from (irb):19 t. > **···** > > On Monday 04 October 2004 11:11 am, Yukihiro Matsumoto wrote: > > > Hi, > > > > In message "Re: Kernel#eval taking a block" \> \> on Mon, 4 Oct 2004 23:23:22 +0900, "trans. (T. Onoma)" \ writes: > > \>Back to the original question: why Kernel#eval can't take a block? > > > > Because it's useless, for my eyes at least. What do you want to > > accomplish by eval with a block? ---
**Author:** ![David\_A\_Black3](https://avatars.discourse-cdn.com/v4/letter/d/6a8cbe/32.png) [@David\_A\_Black3](https://rubytalk.org/u/David_A_Black3)\ **Post date:** [4 October 2004 18:31 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/6 "2004-10-04T18:31:14Z")
Hi -- > **···** > > On Tue, 5 Oct 2004, trans. (T. Onoma) wrote: > > > On Monday 04 October 2004 11:11 am, Yukihiro Matsumoto wrote: > > \> Hi, > > \> > > \> In message "Re: Kernel#eval taking a block" \> \> \> \> on Mon, 4 Oct 2004 23:23:22 +0900, "trans. (T. Onoma)" \> \ writes: > > \> \>Back to the original question: why Kernel#eval can't take a block? > > \> > > \> Because it's useless, for my eyes at least. What do you want to > > \> accomplish by eval with a block? > > > > How to get locals into evaluation? > > > > class T > >     def initialize > >      @x = 1 > >    end > >    attr\_reader :x > >    def get\_binding > >      cool = "matz" > >      return binding() > >    end > > end > > t = T.new > > tb = t.get\_binding > > eval "cool", tb #=\> "matz" > > a = proc {|c| c.upcase} > > eval "cool = a.call(cool)", tb > > > > NameError: undefined local variable or method `a' for #\ > >         from (irb):15:in `get\_binding' > >         from (irb):15:in `get\_binding' > >         from (irb):19 > > But if you're eval'ing in another binding, 'a' won't mean anything in > a string or a block, will it? In other words: you're trying to mix > locals from two bindings in one expression, which somewhat defeats the > purpose of having "locals" 🙂 > > David > > -- > David A. Black > dblack@wobblini.net ---
**Author:** ![T\_Onoma](https://avatars.discourse-cdn.com/v4/letter/t/c2a13f/32.png) [@T\_Onoma](https://rubytalk.org/u/T_Onoma)\ **Post date:** [4 October 2004 18:51 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/7 "2004-10-04T18:51:20Z")
I see. So this kind of thing just can't happen I take it. (Hmm... I think we discussed this kind of thing on core once.) It would be nice if we could pass in locals to the binding:   eval(tb, a) { |a| cool = a.call(cool) } It might also be nice if this could be done without "stepping on toes" (i.e. overwriting binding's locals). T. > **···** > > On Monday 04 October 2004 02:31 pm, David A. Black wrote: > > > \> eval "cool = a.call(cool)", tb > > \> > > \> NameError: undefined local variable or method `a' for #\ > \> @x=1\> from (irb):15:in `get\_binding' > > \> from (irb):15:in `get\_binding' > > \> from (irb):19 > > > > But if you're eval'ing in another binding, 'a' won't mean anything in > > a string or a block, will it? In other words: you're trying to mix > > locals from two bindings in one expression, which somewhat defeats the > > purpose of having "locals" 🙂 ---
**Author:** ![Florian\_Gross](https://avatars.discourse-cdn.com/v4/letter/f/e9a140/32.png) [@Florian\_Gross](https://rubytalk.org/u/Florian_Gross)\ **Post date:** [4 October 2004 19:49 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/8 "2004-10-04T19:49:52Z")
trans. (T. Onoma) wrote: > I see. So this kind of thing just can't happen I take it. (Hmm... I think we discussed this kind of thing on core once.) It would be nice if we could pass in locals to the binding: > >   eval(tb, a) { |a| cool = a.call(cool) } > > It might also be nice if this could be done without "stepping on toes" (i.e. overwriting binding's locals). You can use this workaround: eval("cool = yield.call(cool)", tb) { a } And in this case also: eval("cool = yield(cool)", tb, &a) Regards, Florian Gross ---
**Author:** ![Nobuyoshi\_Nakada](https://avatars.discourse-cdn.com/v4/letter/n/edb3f5/32.png) [@Nobuyoshi\_Nakada](https://rubytalk.org/u/Nobuyoshi_Nakada)\ **Post date:** [8 October 2004 05:49 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/9 "2004-10-08T05:49:46Z")
Hi, At Tue, 5 Oct 2004 03:51:20 +0900, trans. (T. Onoma) wrote in [ruby-talk:114847]: > I see. So this kind of thing just can't happen I take it. (Hmm... I think we > discussed this kind of thing on core once.) It would be nice if we could pass > in locals to the binding: > >   eval(tb, a) { |a| cool = a.call(cool) } > > It might also be nice if this could be done without "stepping on toes" (i.e. > overwriting binding's locals). eval("proc{|cool|}", tb).call(a.call(eval("cool", tb))) > **···** > > -- > Nobu Nakada ---
**Author:** ![T\_Onoma](https://avatars.discourse-cdn.com/v4/letter/t/c2a13f/32.png) [@T\_Onoma](https://rubytalk.org/u/T_Onoma)\ **Post date:** [4 October 2004 20:30 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/10 "2004-10-04T20:30:56Z")
Oh reeeaaaalllly...... darn it! LocalJumpError: no block given         from (irb):5:in `get\_binding'         from (irb):5:in `get\_binding'         from (irb):9 Is this a 1.9 thing? T. > **···** > > On Monday 04 October 2004 03:49 pm, Florian Gross wrote: > > > trans. (T. Onoma) wrote: > > \> I see. So this kind of thing just can't happen I take it. (Hmm... I think > > \> we discussed this kind of thing on core once.) It would be nice if we > > \> could pass in locals to the binding: > > \> > > \> eval(tb, a) { |a| cool = a.call(cool) } > > \> > > \> It might also be nice if this could be done without "stepping on toes" > > \> (i.e. overwriting binding's locals). > > > > You can use this workaround: > > > > eval("cool = yield.call(cool)", tb) { a } > > > > And in this case also: > > > > eval("cool = yield(cool)", tb, &a) ---
**Author:** ![T\_Onoma](https://avatars.discourse-cdn.com/v4/letter/t/c2a13f/32.png) [@T\_Onoma](https://rubytalk.org/u/T_Onoma)\ **Post date:** [8 October 2004 10:51 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/11 "2004-10-08T10:51:21Z")
What are you? (Note: if this statement doesn't cross the language barrier well, just know that it is a big compliment!) T. > **···** > > On Friday 08 October 2004 01:49 am, nobu.nokada@softhome.net wrote: > > > Hi, > > > > At Tue, 5 Oct 2004 03:51:20 +0900, > > > > trans. (T. Onoma) wrote in [ruby-talk:114847]: > > \> I see. So this kind of thing just can't happen I take it. (Hmm... I think > > \> we discussed this kind of thing on core once.) It would be nice if we > > \> could pass in locals to the binding: > > \> > > \> eval(tb, a) { |a| cool = a.call(cool) } > > \> > > \> It might also be nice if this could be done without "stepping on toes" > > \> (i.e. overwriting binding's locals). > > > > eval("proc{|cool|}", tb).call(a.call(eval("cool", tb))) ---
**Author:** ![Florian\_Gross](https://avatars.discourse-cdn.com/v4/letter/f/e9a140/32.png) [@Florian\_Gross](https://rubytalk.org/u/Florian_Gross)\ **Post date:** [4 October 2004 21:10 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/12 "2004-10-04T21:10:05Z")
trans. (T. Onoma) wrote: > > > eval(tb, a) { |a| cool = a.call(cool) } > > > > > > It might also be nice if this could be done without "stepping on toes" > > > (i.e. overwriting binding's locals). > > > > You can use this workaround: > > > > eval("cool = yield.call(cool)", tb) { a } > > LocalJumpError: no block given >         from (irb):5:in `get\_binding' >         from (irb):9 > > Is this a 1.9 thing? Yikes, sorry. It only works when no explicit binding is given. ☹ So the only way seems using Object.\_id2ref and interpolation for now. Regards, Florian Gross ---
**Author:** ![Markus](https://avatars.discourse-cdn.com/v4/letter/m/b782af/32.png) [@Markus](https://rubytalk.org/u/Markus)\ **Post date:** [8 October 2004 15:18 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/13 "2004-10-08T15:18:15Z")
> \> Hi, > \> > \> At Tue, 5 Oct 2004 03:51:20 +0900, > \> > \> trans. (T. Onoma) wrote in [ruby-talk:114847]: > \> \> I see. So this kind of thing just can't happen I take it. (Hmm... I think > \> \> we discussed this kind of thing on core once.) It would be nice if we > \> \> could pass in locals to the binding: > \> \> > \> \> eval(tb, a) { |a| cool = a.call(cool) } > \> \> > \> \> It might also be nice if this could be done without "stepping on toes" > \> \> (i.e. overwriting binding's locals). > \> > \> eval("proc{|cool|}", tb).call(a.call(eval("cool", tb))) > > What are you? > > (Note: if this statement doesn't cross the language barrier well, just know > that it is a big compliment!) I agree. That's a very clever hack. > class T >     def initialize >      @x = 1 >    end >    attr\_reader :x >    def get\_binding >      cool = "matz" >      return binding() >    end > end =\> nil > t = T.new =\> #\ > tb = t.get\_binding =\> #\ > eval "cool",tb =\> "matz" > a = proc { |s| s.upcase } =\> #\ > eval("proc{|cool|}", tb).call(a.call(eval("cool", tb))) =\> nil > eval "cool",tb =\> "MATZ" So basically, what you have is a getter:                                                  vvvvvvvvvvvvvvv eval("proc{|cool|}", tb).call(a.call(eval("cool", tb))) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ and a setter. We could even abstract them (baby steps here; I realize this opens up all sorts of possibilities): def tb.(v)     eval(v.to\_s,self)     end def tb.=(v,x)     eval("proc{|#{v}|}",self).call(x)     end (Using = also gives us the assignment chaining semantics). Very nifty! -- Markus > **···** > > On Fri, 2004-10-08 at 03:51, trans. (T. Onoma) wrote: > > > On Friday 08 October 2004 01:49 am, nobu.nokada@softhome.net wrote: ---
**Author:** ![Markus](https://avatars.discourse-cdn.com/v4/letter/m/b782af/32.png) [@Markus](https://rubytalk.org/u/Markus)\ **Post date:** [5 October 2004 04:41 UTC](https://rubytalk.org/t/kernel-eval-taking-a-block/13921/14 "2004-10-05T04:41:47Z")
What about: class T     def initialize      @x = 1    end    attr\_reader :x    def get\_binding      cool = "matz"      return binding()    end end t = T.new tb = t.get\_binding eval "cool", tb #=\> "matz" # Does not work: # a = proc {|c| c.upcase} # so instead: class \<\ **···** > > On Mon, 2004-10-04 at 14:10, Florian Gross wrote: > > > trans. (T. Onoma) wrote: > > > > \>\>\> eval(tb, a) { |a| cool = a.call(cool) } > > \>\>\> > > \>\>\>It might also be nice if this could be done without "stepping on toes" > > \>\>\>(i.e. overwriting binding's locals). > > \>\> > > \>\>You can use this workaround: > > \>\> > > \>\>eval("cool = yield.call(cool)", tb) { a } > > \> LocalJumpError: no block given > > \> from (irb):5:in `get\_binding' > > \> from (irb):5:in `get\_binding' > > \> from (irb):9 > > \> > > \> Is this a 1.9 thing? > > > > Yikes, sorry. It only works when no explicit binding is given. ☹ > > > > So the only way seems using Object.\_id2ref and interpolation for now.