CSC/ECE 517 Fall 2009/wiki2 1 SA

Metaprogramming

Introduction

Metaprogramming,is the creation of procedures and programs that automatically construct the definitions of other procedures and programs. Metaprogramming automates some of the tedious and error-prone parts of the programmer's job.

Metaprogramming in various languages

There are many languages that do metaprogramming.The compiler is the simplest example of a metaprogram. It takes as input, code in some language, and creates code for another language.

In languages like C#, reflection is a form of metaprogramming since the program can examine information about itself. For example returning a list of all the properties of an object.

In languages like ActionScript, you can evaluate functions at runtime to create new programs such as eval("x" + i).DoSomething() would affect an object called x1 when i is 1 and x2 when i is 2.

Finally, another common form of metaprogramming is when the program can change itself in non-trivial fashions. LISP is well known for this and is something Paul Graham championed about a decade ago. I'll have to look up some of his specific essays. But the idea is that the program would change another part of the program based on its state. This allows a level of flexibility to make decisions at runtime that is very difficult in most popular languages today.

It is also worth noting that back in the good ol' days of programming in straight assembly, programs that altered themselves at runtime were necessary and very commonplace.

Metaprogramming in Groovy
Below example demonstrates using classes from the Jakarta Commons Lang package for metaprogramming. All of the methods in org.apache.commons.lang.StringUtils coincidentally follow the Category pattern — static methods that accept a String as the first parameter. This means that you can use the StringUtils class right out of the box as a Category.

import org.apache.commons.lang.StringUtils
 class CommonsTest extends GroovyTestCase{
  void testStringUtils(){
    def word = "Introduction"
    word.metaClass.whisper = {->
      delegate.toLowerCase()
    }
     use(StringUtils, StringHelper){
      //from org.apache.commons.lang.StringUtils
      assertEquals "Intro...", word.abbreviate(8)
       //from the StringHelper Category
      assertEquals "INTRODUCTION", word.shout()
       //from the word.metaClass
      assertEquals "introduction", word.whisper()
    }
  } 
}
class StringHelper{
  static String shout(String self){
    return self.toUpperCase()
  }
}