# Why is Ruby dynamic? **URL:** \ **Category:** ruby-talk\ **Created:** [6 September 2008 01:16 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894 "2008-09-06T01:16:54Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Zhao\_Yi](https://avatars.discourse-cdn.com/v4/letter/z/fbc32d/32.png) [@Zhao\_Yi](https://rubytalk.org/u/Zhao_Yi)\ **Post date:** [6 September 2008 01:16 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/1 "2008-09-06T01:16:54Z")
People say Ruby is a dynamic language. I don't understand what the "dynamic" mean here. Does this mean Ruby can dynamic change itself at run-time? > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![James\_Adam2](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/james_adam2/32/1989_2.png) [@James\_Adam2](https://rubytalk.org/u/James_Adam2)\ **Post date:** [6 September 2008 16:11 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/2 "2008-09-06T16:11:09Z")
Essentially, yes. You might consider it sort-of-opposite to a 'compiled' language, though that's less strictly true. Ruby objects are completely malleable as your program runs. > **···** > > On 6 Sep 2008, at 02:16, Zhao Yi wrote: > > > People say Ruby is a dynamic language. I don't understand what the > > "dynamic" mean here. Does this mean Ruby can dynamic change itself at > > run-time? ---
**Author:** ![Phlip1](https://avatars.discourse-cdn.com/v4/letter/p/df788c/32.png) [@Phlip1](https://rubytalk.org/u/Phlip1)\ **Post date:** [9 September 2008 01:19 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/3 "2008-09-09T01:19:18Z")
Zhao Yi wrote: > People say Ruby is a dynamic language. I don't understand what the > "dynamic" mean here. Does this mean Ruby can dynamic change itself at > run-time? It means Ruby can change its objects at runtime. For example:    class Narcissus      def call\_once        eval '          class Narcissus            def call\_once              raise "nope!"            end          end'      end    end The first time you call Narcissus.new.call\_once, it will not raise. The second time, it will raise "nope!". Ruby allows class definitions to change on the fly. > **···** > > -- >    Phlip ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [6 September 2008 17:26 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/4 "2008-09-06T17:26:11Z")
There are several aspects to Ruby's dynamism: methods and classes can change at runtime, variables are dynamically typed and generally as implementing is often faster than in other languages Ruby is very dynamic (some call it "agile"). Kind regards robert > **···** > > 2008/9/6 James Adam \: > > > On 6 Sep 2008, at 02:16, Zhao Yi wrote: > > > > > People say Ruby is a dynamic language. I don't understand what the > > > "dynamic" mean here. Does this mean Ruby can dynamic change itself at > > > run-time? > > > > Essentially, yes. You might consider it sort-of-opposite to a 'compiled' > > language, though that's less strictly true. Ruby objects are completely > > malleable as your program runs. > > -- > use.inject do |as, often| as.you\_can - without end ---
**Author:** ![David\_Masover](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@David\_Masover](https://rubytalk.org/u/David_Masover)\ **Post date:** [9 September 2008 03:10 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/5 "2008-09-09T03:10:38Z")
A nitpick, but... string evals considered harmful! >    class Narcissus >      def call\_once >        eval ' >          class Narcissus >            def call\_once >              raise "nope!" >            end >          end' >      end >    end I'd write this as: class Narcissus   def call\_once     def call\_once       raise 'nope!'     end   end end But perhaps the more impressive demos are modifying system classes -- re-opening String, for instance -- or eigenclass hacks, or dynamically generating functions. I always loved define\_method. When you know exactly what kind of things are acceptable: class PunchCard   %w(fold bend mutilate).each do |action|     define\_method action do       raise "Don't #{action} me!"     end   end end And when you don't necessarily know, there's method\_missing: class TwoYearOld   def method\_missing method, \*args     puts "That's MY #{method}!"   end end And Ruby makes it very difficult to build a cheat-proof robot game, as you can always start rewriting the other robot: class MyRobot   def move     class \<\< other\_robot       def move         raise 'Explode!'       end     end   end end Of course, it's comforting to know that string evals are there -- having a working "eval" is a sign of a truly dynamic language. (Probably makes irb easier to write, too!) But there's very few legitimate uses of it anymore, I think. > **···** > > On Monday 08 September 2008 20:19:18 Phlip wrote: ---
**Author:** ![Zhao\_Yi](https://avatars.discourse-cdn.com/v4/letter/z/fbc32d/32.png) [@Zhao\_Yi](https://rubytalk.org/u/Zhao_Yi)\ **Post date:** [7 September 2008 02:04 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/6 "2008-09-07T02:04:41Z")
Robert Klemme wrote: > **···** > > > 2008/9/6 James Adam \: > > > > > On 6 Sep 2008, at 02:16, Zhao Yi wrote: > > > > > > > People say Ruby is a dynamic language. I don't understand what the > > > > "dynamic"e$B!!e(Bmean here. Does this mean Ruby can dynamic change itself at > > > > run-time? > > > > > > Essentially, yes. You might consider it sort-of-opposite to a 'compiled' > > > language, though that's less strictly true. Ruby objects are completely > > > malleable as your program runs. > > > > There are several aspects to Ruby's dynamism: methods and classes can > > change at runtime, variables are dynamically typed and generally as > > implementing is often faster than in other languages Ruby is very > > dynamic (some call it "agile"). > > > > Kind regards > > > > robert > > It is also same for other scripts like perl, python, so they are all > dynamic language? > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Phlip1](https://avatars.discourse-cdn.com/v4/letter/p/df788c/32.png) [@Phlip1](https://rubytalk.org/u/Phlip1)\ **Post date:** [9 September 2008 10:58 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/7 "2008-09-09T10:58:40Z")
David Masover wrote: > A nitpick, but... string evals considered harmful! > >        eval ' > I always loved define\_method. I recently tried define\_method and it didn't work, so I fell back to eval. (Note to the newbs: Try things the most progressive way possible, such as a fixed version of Dave's version of my example, before trying a hack...) Here's the lines, from inspect\_sql:    def self.\_decorate\_explainers(explains) #:nodoc:      def explains.find\_statements(regexp)        return AssertEfficientSql.\_decorate\_explainers(select{|explanation|          explanation[:statement] =~ regexp        })      end      (\_multiples + \_singulars).each do |rollup, explainer|        eval "def explains.#{rollup}; map{|q|q[:#{rollup}]}.flatten.uniq; end"      end      return explains    end For the eval line, I first tried explains.define\_method, and it didn't work. Instead of researching why, or finding a kindler gentler system, I whipped out eval and put a bullet thru the problem. It's the shortest possible eval, anyway! Also I think you can't say 'def foo; def bar; end; end' - the def is in method-space, not class-space. > **···** > > -- >    Phlip ---
**Author:** ![Avdi\_Grimm1](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Avdi\_Grimm1](https://rubytalk.org/u/Avdi_Grimm1)\ **Post date:** [7 September 2008 05:04 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/8 "2008-09-07T05:04:15Z")
Yup. > **···** > > 2008/9/6 Zhao Yi \: > > > It is also same for other scripts like perl, python, so they are all > > dynamic language? > > -- > Avdi > > Home: [http://avdi.org](http://avdi.org) > Developer Blog: [http://avdi.org/devblog/](http://avdi.org/devblog/) > Twitter: [http://twitter.com/avdi](http://twitter.com/avdi) > Journal: [http://avdi.livejournal.com](http://avdi.livejournal.com) ---
**Author:** ![David\_A\_Black1](https://avatars.discourse-cdn.com/v4/letter/d/bcef8e/32.png) [@David\_A\_Black1](https://rubytalk.org/u/David_A_Black1)\ **Post date:** [9 September 2008 11:53 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/9 "2008-09-09T11:53:30Z")
Hi -- > David Masover wrote: > > > A nitpick, but... string evals considered harmful! > > > >        eval ' > > > I always loved define\_method. > > I recently tried define\_method and it didn't work, so I fell back to eval. (Note to the newbs: Try things the most progressive way possible, such as a fixed version of Dave's version of my example, before trying a hack...) > > Here's the lines, from inspect\_sql: > > def self.\_decorate\_explainers(explains) #:nodoc: >    def explains.find\_statements(regexp) >      return AssertEfficientSql.\_decorate\_explainers(select{|explanation| >        explanation[:statement] =~ regexp >      }) >    end > >    (\_multiples + \_singulars).each do |rollup, explainer| >      eval "def explains.#{rollup}; map{|q|q[:#{rollup}]}.flatten.uniq; end" >    end > >    return explains > end > > For the eval line, I first tried explains.define\_method, and it didn't work. Instead of researching why, or finding a kindler gentler system, I whipped out eval and put a bullet thru the problem. define\_method is a private instance method of Module, so a typical use follow something like this path:    c.class\_eval { define\_method(m) {|x| puts x } } > It's the shortest possible eval, anyway! > > Also I think you can't say 'def foo; def bar; end; end' - the def is in method-space, not class-space. You can nest def's, but the nested one (once the outer one is executed) becomes an outer one:    class C      def x        def y          puts "Hi from y"        end      end    end    c = C.new    c.x    c.y # Hi from y    d = C.new    d.y # Hi from y David > **···** > > On Tue, 9 Sep 2008, Phlip wrote: > > -- > Rails training from David A. Black and Ruby Power and Light: >    Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL >    Advancing with Rails January 19-22 Fort Lauderdale, FL \* >    \* Co-taught with Patrick Ewing! > See [http://www.rubypal.com](http://www.rubypal.com) for details and updates! ---
**Author:** ![David\_Masover](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@David\_Masover](https://rubytalk.org/u/David_Masover)\ **Post date:** [9 September 2008 15:33 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/10 "2008-09-09T15:33:57Z")
> David Masover wrote: > > \> A nitpick, but... string evals considered harmful! This is worth bearing. I don't have a problem with eval, only with string eval. Given that: > For the eval line, I first tried explains.define\_method, and it didn't work. According to Philip, define\_method is a private method, so you can't do that from outside that object. Assuming "explains" is a class or a module, you could always go inside the object: explains.module\_eval do   define\_method :foo ...   # More methods?   define\_method :bar ... end See, I love those, because they can actually take blocks, not just strings. Since I'd mostly just be passing a variable in through string interpolation, anyway, it's not only faster, it's safer -- SQL injection is enough of a problem, we don't need Ruby injection, too! If it's only a one-off, and you find that eval to be too verbose, there's always the send hack: explains.send :define\_method, ... This works mostly because send doesn't check for whether a given method is actually reachable at that point -- I think you need public\_send for that. Obviously, send is useful for more than just that... The main legitimate solution I've found is cases like ActiveSupport's String#constantize -- this is a case where it would take quite a lot more Ruby to accomplish the same thing as a simple eval. And even here, it's surrounded by sanity checks, and it's isolated, so that you never have to know it's there (I didn't, till I looked at the source of constantize out of curiosity). And there's always the performance -- I consider evals to be acceptable when run once, during program start (or at some arbitrary point after that). It's still code smell, but constantize is an example where the alternative might be worse. But I'd never call constantize in a Rails view. > **···** > > On Tuesday 09 September 2008 05:58:40 Phlip wrote: ---
**Author:** ![Bertram\_Scharpf](https://avatars.discourse-cdn.com/v4/letter/b/43a26b/32.png) [@Bertram\_Scharpf](https://rubytalk.org/u/Bertram_Scharpf)\ **Post date:** [7 September 2008 05:39 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/11 "2008-09-07T05:39:37Z")
Hi, > \> It is also same for other scripts like perl, python, so they are all > \> dynamic language? > > Yup. Try to do something like this in Python:   class Time     def german       strftime "%d.%m.%Y, %X"     end   end   class Numeric     def MB       self/1024.0/1024.0     end   end It's dynamic because it produces dynamic programmers. Bertram > **···** > > Am Sonntag, 07. Sep 2008, 14:04:15 +0900 schrieb Avdi Grimm: > > > 2008/9/6 Zhao Yi \: > > -- > Bertram Scharpf > Stuttgart, Deutschland/Germany > [http://www.bertram-scharpf.de](http://www.bertram-scharpf.de) ---
**Author:** ![Chad\_Perrin1](https://avatars.discourse-cdn.com/v4/letter/c/977dab/32.png) [@Chad\_Perrin1](https://rubytalk.org/u/Chad_Perrin1)\ **Post date:** [9 September 2008 00:42 UTC](https://rubytalk.org/t/why-is-ruby-dynamic/48894/12 "2008-09-09T00:42:29Z")
> Hi, > > \> \> It is also same for other scripts like perl, python, so they are all > \> \> dynamic language? > \> > \> Yup. > > Try to do something like this in Python: > >   class Time >     def german >       strftime "%d.%m.%Y, %X" >     end >   end >   class Numeric >     def MB >       self/1024.0/1024.0 >     end >   end Python is "dynamic" compared to, say, C or Java. People tend to want to classify languages in a black/white manner, putting some languages on one side of a dividing line between "dynamic" and "static", and other languages on the other side. The way most people view these things, Perl, Python, and Ruby are all "dynamic". That doesn't mean they're all \*equally\* dynamic, of course -- as you point out. The fact they aren't all equally dynamic doesn't mean Python isn't "dynamic" as well. > It's dynamic because it produces dynamic programmers. That's certainly one way to look at it -- and it does indeed seem to at least facilitate, if not produce, "dynamic programmers". > **···** > > On Sun, Sep 07, 2008 at 02:39:37PM +0900, Bertram Scharpf wrote: > > > Am Sonntag, 07. Sep 2008, 14:04:15 +0900 schrieb Avdi Grimm: > > \> 2008/9/6 Zhao Yi \: > > -- > Chad Perrin [content licensed PDL: [http://pdl.apotheon.org](http://pdl.apotheon.org) ] > Olin Shivers: "I object to doing things that computers can do."