# Eval versus load and plugins **URL:** \ **Category:** Ruby\ **Created:** [July 27, 2007, 1:25pm UTC](https://www.ruby-forum.com/t/eval-versus-load-and-plugins/110552 "2007-07-27T13:25:28Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![rhubarb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/59ef9b/32.png) [@rhubarb](https://www.ruby-forum.com/u/rhubarb)\ **Post date:** [July 27, 2007, 1:25pm UTC](https://www.ruby-forum.com/t/eval-versus-load-and-plugins/110552/1 "2007-07-27T13:25:28Z")
I’m writing a program with a sort of plugin mechanism. Plugins are basically single ruby files defining classes. It’s not exactly a plugin mechanism because there an only be one plugin used. Image the program command being ruby myApp -plugin plugins/myPlugin.rb I want to load the plugin which defines its own class (which follows a certain pattern - since there are no interfaces) then instantiates it. At the moment I’m using load to load the plugin file. I don’t know the name of the class of the specific plugin, but I assume it has a _global_ method called “new\_plugin” which instantiates it. Since I’m using load I can be sure that the latest plugin loaded will redefine that global method to instantiate the plugin (I can’t call it’s constructor directly because, again, I don’t know what name the designer is going to give the plugin class - though I can expect them to follow the pattern of defining new\_plugin) Questions: 1. I notice that rails uses eval to load the init.rb files of plugins. What are the advantages disadvantages of this versus using load? (Note also this thread about using eval or load - both are suggested but I dont see why we would use eval over load [How to evaluate file as Ruby code? - Ruby - Ruby-Forum](http://www.ruby-forum.com/topic/55540#new)) 2. Is there some nicer, cleaner way of doing what I’m doing without having the plugin file define a global method? 3. Is there a neat way of forcing the plugin to match a certain pattern. I’ve seen the “interface” support library, but it seems to be shoehorning Ruby into a Java paradigm. What would be the rubyful way? Just calling respond\_to? on all of the methods I expect and raising errors? ---
**Author:** ![rhubarb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/59ef9b/32.png) [@rhubarb](https://www.ruby-forum.com/u/rhubarb)\ **Post date:** [July 27, 2007, 1:47pm UTC](https://www.ruby-forum.com/t/eval-versus-load-and-plugins/110552/2 "2007-07-27T13:47:44Z")
Rover R. wrote: > Questions: > > 1. I notice that rails uses eval to load the init.rb files of plugins. > What are the advantages disadvantages of this versus using load? > (Note also this thread about using eval or load - both are suggested but > I dont see why we would use eval over load > [How to evaluate file as Ruby code? - Ruby - Ruby-Forum](http://www.ruby-forum.com/topic/55540#new)) > Not sure off the top of my head. Someone else can probably weigh in > here. > 1. Is there some nicer, cleaner way of doing what I’m doing without > having the plugin file define a global method? > Make them subclass a base class that you provide, or make them mix in a > module that you specify. That gives you the Class#inherited and > Module#included callbacks to play with, which strikes me as cleaner than > relying on a method override. > 1. Is there a neat way of forcing the plugin to match a certain pattern. > I’ve seen the “interface” support library, but it seems to be > shoehorning Ruby into a Java paradigm. What would be the rubyful way? > Just calling respond\_to? on all of the methods I expect and raising > errors? > If the plugin subclasses a base class you specify, you can either check > it manually in the callback (which seems a little clumsy to me), or > provide all the interface members with: def interface\_member raise NotImplementedError end in the base class. The former is probably necessary if you need to sanity check the class before doing anything with it - you can get away with the latter otherwise. HTH ---
**Author:** ![rhubarb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/59ef9b/32.png) [@rhubarb](https://www.ruby-forum.com/u/rhubarb)\ **Post date:** [July 27, 2007, 7:11pm UTC](https://www.ruby-forum.com/t/eval-versus-load-and-plugins/110552/3 "2007-07-27T19:11:23Z")
Rover R. wrote: > 1. Is there some nicer, cleaner way of doing what I’m doing without > having the plugin file define a global method? You can module\_eval the file in the context of a new (anonymous, if you like) module. Here’s one way to do that: [~/tmp] cat main.rb require ‘script’ # [Script](http://redshift.sourceforge.net/script/) plugin\_module = Script.load(“plugin.rb”) # either: plugin\_class = plugin\_module::PluginClass p plugin\_class.new.foo # or: plugin\_class = plugin\_module.get\_the\_plugin\_class p plugin\_class.new.foo [~/tmp] cat plugin.rb class PluginClass def foo “FOO” end end def get\_the\_plugin\_class # NOT really global – in module scope PluginClass end [~/tmp] ruby main.rb “FOO” “FOO” ---
**Author:** ![rhubarb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/59ef9b/32.png) [@rhubarb](https://www.ruby-forum.com/u/rhubarb)\ **Post date:** [July 27, 2007, 9:32pm UTC](https://www.ruby-forum.com/t/eval-versus-load-and-plugins/110552/4 "2007-07-27T21:32:42Z")
> > Questions: > > > > 1. I notice that rails uses eval to load the init.rb files of plugins. > > What are the advantages disadvantages of this versus using load? > > (Note also this thread about using eval or load - both are suggested but > > I dont see why we would use eval over load > > [How to evaluate file as Ruby code? - Ruby - Ruby-Forum](http://www.ruby-forum.com/topic/55540#new)) > > Not sure off the top of my head. Someone else can probably weigh in here. Here’s the code I think you mean: eval(IO.read(init\_path), binding, init\_path) I think the reason it does this is to allow flexible bindings. I couldn’t find “binding” defined in the method anywhere. I really don’t know, so I paraphrased this part of your question and posted it on the Rails core list. – Giles B. Blog: [http://gilesbowkett.blogspot.com](http://gilesbowkett.blogspot.com) Portfolio: [http://www.gilesgoatboy.org](http://www.gilesgoatboy.org)