Metaprograming in Ruby Brian Sam-Bodden What s this all about? - - PowerPoint PPT Presentation

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Metaprograming in Ruby Brian Sam-Bodden What s this all about? - - PowerPoint PPT Presentation

Dec 05, 2023 214 likes •1.21k views

Metaprograming in Ruby Brian Sam-Bodden What s this all about? Well learn... mainly about Rubys Meta-programming power about a language doesnt treat you like a moron what happens when dynamic meets object-oriented

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Metaprograming in Ruby

Brian Sam-Bodden

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What’s this all about?

We’ll learn... mainly about Ruby’s Meta-programming power about a language doesn’t treat you like a moron what happens when dynamic meets object-oriented why Ruby is superb for building frameworks what happens when meta-programming ...meets someone with too much free-time

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Ruby

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Ruby Ideals: Programming should be fun! “I believe people want to express themselves when they program. They don’t want to fight with the language. Programming languages must feel natural to programmers.” Matz The language should treat you like a responsible adult!

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Ruby

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Ruby

is...

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Ruby

is...

Object-Oriented

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Ruby

is...

Object-Oriented General Purpose

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Ruby

is...

Object-Oriented Interpreted General Purpose

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Ruby

is...

Object-Oriented Dynamic Interpreted General Purpose

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Ruby

is...

Object-Oriented Dynamic Interpreted General Purpose Garbage-collected

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Ruby

is...

Object-Oriented Reflective Dynamic Interpreted General Purpose Garbage-collected

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Ruby

is...

Object-Oriented Reflective Dynamic Interpreted General Purpose Multi-paradigm Garbage-collected

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Ruby

is...

Object-Oriented Reflective Dynamic Interpreted General Purpose Multi-paradigm Garbage-collected Elegant

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Without Ruby there would be no Rails Ruby’s meta-programing is the key feature that makes Rails magical

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Open Classes

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Say we have an Array like:

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Say we have an Array like:

[1,2,3,4,5]

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Say we have an Array like:

[1,2,3,4,5]

and we want to sum of all of its elements

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Let’s try something like:

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Let’s try something like:

[1,2,3,4,5].sum

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Let’s try something like:

[1,2,3,4,5].sum

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Let’s try something like:

[1,2,3,4,5].sum

Rats! Doesn’t work in Ruby

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Let’s try something like:

[1,2,3,4,5].sum

Rats! Doesn’t work in Ruby … yet!

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Ruby classes are

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Ruby classes are

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Ruby classes are

We can teach the Array class a new behavior

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Ruby classes are

We can teach the Array class a new behavior

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Let’s try again!

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Let’s try again!

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Let’s try again!

All you need to do is load the file containing the enhancements

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Code as Data

Bend at will your code should

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When code can be manipulated as data a whole world of possibilities opens In Ruby you can evaluate the contents of a string

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Code that invokes code, that invokes code

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Code that invokes code, that invokes code

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Code that invokes code, that invokes code There you go, there you go

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Ruby provides the eval family of methods for runtime execution of code stored in strings

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Ruby provides the eval family of methods for runtime execution of code stored in strings

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Ruby provides the eval family of methods for runtime execution of code stored in strings eval will evaluate any string

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instance_eval can evaluate a string or a code block

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instance_eval can evaluate a string or a code block

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instance_eval can evaluate a string or a code block in the context of the receiver

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class_eval can evaluate a string or a code block in the context of the class or module it is called on

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class_eval can evaluate a string or a code block in the context of the class or module it is called on

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In Ruby we try to think about sending a message to an object rather than calling a method on an object

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In Ruby we try to think about sending a message to an object rather than calling a method on an object In simple cases (above) syntactic sugar hides it, but we can use it in interesting ways...

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Meta- programming

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...is about programs that write programs Meta-programming ...it’s a superb tool for building frameworks ...it’s the key ingredient for building domain-specific languages

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Ruby’s is a great vehicle for meta-programming because it is: dynamic and reflexive

  • pen and malleable

clean syntax code is data, data is code programming event model

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...uses meta-programming to bring the language closer to the problem ...is a collection of domain-specific languages (DSL) for building web applications

Ruby on Rails

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Singleton Class

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In Ruby you can enhance a particular instance of a class Ruby uses a proxy class known as the singleton class Meta-class: The singleton for Class objects

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class_eval indirectly uses the meta-class adding a singleton method to the class

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Rails relies heavily on Ruby’s ability to dynamically enhance a class

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Rails relies heavily on Ruby’s ability to dynamically enhance a class

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Shortcuts such as define_method exist to make life easier

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Accessing the singleton meta-class explicitly

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Child classes get enhanced with class methods

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Inheritance the Ruby Way

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Many Ruby DSLs create class methods on subclasses using the meta-class (singleton class)

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Rails uses this technique in ActiveRecord

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A module with a “Base” class

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Methods get added to our classes with simple declarations Getting closer to a DSL!

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Now our class has a new set of class methods

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Now our class has a new set of class methods

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AOP the Ruby Way

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With alias_method you can wrap an existing method...

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...effectively intercepting any calls and injecting any desired behavior

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...effectively intercepting any calls and injecting any desired behavior

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Meta-Programming

Events

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Ruby has a rich event model associated with static and dynamic changes of the code

Included Hook

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Method Missing

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Ruby provides several hook methods that can be used to created custom behaviors

method_missing method_added method_removed

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Ruby’s Markup Builder is an example of what can be achieved with method missing

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Let’s implement the greeter example using method_missing

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greeter using method_missing

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greeter using method_missing

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Trellis

...or what happens when you have too much time

in between consulting gigs

and your friends no longer call you
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I was bored... In the Ruby world I’ve worked with Rails and Merb ...thinking of web apps in terms of request & response meanwhile... in the Java world I was working with Tapestry and Wicket ...thinking about Pages, Components and Events

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Started to hack to see what it would be to build something like Tapestry/Wicket in Ruby ...actually it all started as an learning exercise ...but then I kept going ...somehow I ended building component-oriented web framework in Ruby ...part of a conversation among friends about the need for IoC and AOP frameworks for dynamic languages

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You might be asking yourself, but why?

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Why not?

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My goals:

Magic like Rails; less code more action Small like Camping... a micro-framework Components like Tapestry/Wicket Clean HTML templates... ...but also programatic HTML generation Non-managed components, no pooling

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The big picture:

Trellis Application Pages Components

has has

{

declares pages used declares a home page dispatches request to pages

} {

defines a template provides event handlers passes events to the components provides reusable logic responds to events can be stateless (tags)

  • r stateful
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The simplest Trellis App, one Application and one Page

The Code The Template

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What did I use to put this thing together...

A boat load of meta-programming Rack ...HTTP the Ruby way Radius ...XML tags for the templates Builder ...for building HTML/XML Hpricot, REXML ...parsing searching HTML/XML Paginator ...for duh, pagination Parts of Rails ...so far ActiveSupport (Inflectors)

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Let’s do some code spelunking on the Trellis codebase... yes, there are some things that I’m not proud of but... ...this is pre-alpha, unreleased v0.000001;-)

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Some of the example applications that I’ve put together in order of complexity

hilo guest_book hangman yui stateful_counters flickr simple_blog crud_components

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What have we learned?

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There is more to Ruby than Rails! Building a Framework? Give Ruby a try! Build the language up towards the problem

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Rack: http://rack.rubyforge.org Radius: http://radius.rubyforge.org Hpricot: http://code.whytheluckystiff.net/hpricot REXML: http://www.germane-software.com/software/rexml Builder: http://builder.rubyforge.org Paginator: http://paginator.rubyforge.org YUI (Yahoo! UI Library): http://open.yahoo.com/yui Trellis (Not Released Yet) Trellis @ RubyForge: http://rubyforge.org/projects/trellis Trellis Website: http://www.trellisframework.org

Resources

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www.integrallis.com