# How to check for block in an evaled method? **URL:** \ **Category:** ruby-talk\ **Created:** [27 August 2007 02:15 UTC](https://rubytalk.org/t/how-to-check-for-block-in-an-evaled-method/40206 "2007-08-27T02:15:01Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Nasir\_Khan](https://avatars.discourse-cdn.com/v4/letter/n/5daacb/32.png) [@Nasir\_Khan](https://rubytalk.org/u/Nasir_Khan)\ **Post date:** [27 August 2007 02:15 UTC](https://rubytalk.org/t/how-to-check-for-block-in-an-evaled-method/40206/1 "2007-08-27T02:15:01Z")
Consider the snippet - class B    def set      B.class\_eval do         define\_method(:a) do |arg|           if block\_given?             yield           else             "hello"           end         end       end     end   end The intention is that block\_given? would tell me if the new method a() is invoked with a block or not. Now I run this irb(main):057:0\* y = B.new =\> #\ irb(main):059:0\> y.set =\> #\ irb(main):060:0\> y.a (irb):45: warning: multiple values for a block parameter (0 for 1)         from (irb):60 =\> "hello" Now if I use with a block irb(main):061:0\> y.a {"bye"} (irb):45: warning: multiple values for a block parameter (0 for 1)         from (irb):61 =\> "hello" Oops doesnt work ! Actually the block is being evaled when the method is being created - irb(main):062:0\> y.set {"bye"} =\> #\ irb(main):063:0\> y.a (irb):45: warning: multiple values for a block parameter (0 for 1)         from (irb):63 =\> "bye" I get that. But the question is, is there a way to check for a block and then yield in a defined method, as was my naive intention? Thanks Nasir ---
**Author:** ![Logan\_Capaldo](https://avatars.discourse-cdn.com/v4/letter/l/7ea924/32.png) [@Logan\_Capaldo](https://rubytalk.org/u/Logan_Capaldo)\ **Post date:** [27 August 2007 03:30 UTC](https://rubytalk.org/t/how-to-check-for-block-in-an-evaled-method/40206/2 "2007-08-27T03:30:22Z")
> I get that. But the question is, is there a way to check for a block and > then yield in a defined method, as was my naive intention? Stage 1) Upgrade to ruby 1.9 or later. 2) define\_method(:foo) { |arg, &block|   if block then block.call else puts arg } Alternative route is to use string eval. > **···** > > On 8/26/07, Nasir Khan \ wrote: > > > Thanks > > Nasir ---
**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:** [27 August 2007 10:43 UTC](https://rubytalk.org/t/how-to-check-for-block-in-an-evaled-method/40206/3 "2007-08-27T10:43:18Z")
Hi -- > > I get that. But the question is, is there a way to check for a block and > > then yield in a defined method, as was my naive intention? > > Stage 1) Upgrade to ruby 1.9 or later. 2) define\_method(:foo) { |arg, &block| > if block then block.call else puts arg > } I'm not sure. Here's my little test:    class C    define\_method(:foo) { |arg, &block|      if block then block.call else puts arg end    }    end    c = C.new    c.foo(3) { puts "hi" } which gives me:    $ /usr/local/lib/ruby-svn/bin/ruby -v block9.rb    ruby 1.9.0 (2007-08-25 patchlevel 0) [i686-darwin8.10.1]    3 This is on 1/2-cup coffee so I may well be not seeing something I'm doing wrong. David > **···** > > On Mon, 27 Aug 2007, Logan Capaldo wrote: > > > On 8/26/07, Nasir Khan \ wrote: > > -- > \* Books: >    RAILS ROUTING (new! [http://www.awprofessional.com/title/0321509242\](http://www.awprofessional.com/title/0321509242%5C)) >    RUBY FOR RAILS ([http://www.manning.com/black\](http://www.manning.com/black%5C)) > \* Ruby/Rails training >      & consulting: Ruby Power and Light, LLC ([http://www.rubypal.com](http://www.rubypal.com)) ---
**Author:** ![Logan\_Capaldo](https://avatars.discourse-cdn.com/v4/letter/l/7ea924/32.png) [@Logan\_Capaldo](https://rubytalk.org/u/Logan_Capaldo)\ **Post date:** [27 August 2007 23:41 UTC](https://rubytalk.org/t/how-to-check-for-block-in-an-evaled-method/40206/4 "2007-08-27T23:41:33Z")
> Hi -- > > \> > \>\> > \>\> I get that. But the question is, is there a way to check for a block and > \>\> then yield in a defined method, as was my naive intention? > \>\> > \> Stage 1) Upgrade to ruby 1.9 or later. 2) define\_method(:foo) { |arg, &block| > \> if block then block.call else puts arg > \> } > > I'm not sure. Here's my little test: > >    class C >    define\_method(:foo) { |arg, &block| >      if block then block.call else puts arg end >    } >    end > >    c = C.new >    c.foo(3) { puts "hi" } > > which gives me: > >    $ /usr/local/lib/ruby-svn/bin/ruby -v block9.rb >    ruby 1.9.0 (2007-08-25 patchlevel 0) [i686-darwin8.10.1] >    3 > > This is on 1/2-cup coffee so I may well be not seeing something I'm > doing wrong. Well alrighty then... I don't know what thats about and I haven't installed 1.9ish in a while. I'm surprised that doesn't work but is valid syntax though. I wonder what is actually happening? Assigning the (non-existant, and I think impossible) block passed to class C...? > **···** > > On 8/27/07, dblack@wobblini.net \ wrote: > > > On Mon, 27 Aug 2007, Logan Capaldo wrote: > > \> On 8/26/07, Nasir Khan \ wrote: > > > > David > > > > -- > > \* Books: > >    RAILS ROUTING (new! [http://www.awprofessional.com/title/0321509242\](http://www.awprofessional.com/title/0321509242%5C)) > >    RUBY FOR RAILS ([http://www.manning.com/black\](http://www.manning.com/black%5C)) > > \* Ruby/Rails training > >      & consulting: Ruby Power and Light, LLC ([http://www.rubypal.com](http://www.rubypal.com))