CSC/ECE 517 Spring 2013/ch1b 1i lh

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Metaprogramming in Dynamiclly Typed Languages

Metaprogramming

Metaprogramming is a programming technique of writing computer programs that write or manipulate other programs or themselves, as their data. [1] This technique allow us to write program that modify its own behavior at runtime (as opposes to recompiling the progam )

In metaprogam, the language in which the metaprogram is written is called the metalanguage. The language of the programs that are manipulated is called the object language.[2]

The ability of a programming language to be its own metalanguage is called reflection or reflexivity.[3]

Dynamically Typed Programming Languages

Programming languages can be split into two categories base on their type system, dynamically and statically typed. A programming language is said to use static typing when type checking is performed during compile-time. In compareson, dynamically typed languages deferring type checking until run-time as opposes to compile-time. [4]

In dynamically typed languages, the variables and parameters do not have a designated type and may take different values at different times. In all the operations, the operands must be type checked at runtime just before performing the operation. Dynamically typed languages don’t need to make a distinction between classes created at compile time and classes provided. It is possible to define classes at run time and in fact, classes are always defined at run time. These eliminate many developer constraints by avoiding the need of book keeping, declarations etc. Due to this flexibility these languages make an ideal candidate for prototyping and are widely used in agile development environments. However, dynamic languages are known to have performance issues. Static languages have code optimization features at compile time, but dynamic languages allow runtime code optimizations only. [5] In dynamically typed languages, the interpreter deduces type and type conversions, this makes development time faster, but it also can provoke runtime failures. These runtime failures are caught early on during compile time for statically typed languages.

Examples of dynamically typed languages:

• Perl
• Python
• JavaScript
• PHP
• Ruby
• SmallTalk

Implementation of Metaprograming in Dynamiclly Typed Languages

Shell Script

A shell script is a script written for command line interpreter of an operating system.

In the simplest form shell script can be use for metaprogramming by generating or source code or modifying than recompiling a existing program.

for example

Ruby

Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. It was also influenced by Eiffel and Lisp[8]. Ruby was first designed and developed in the mid-1990s by Yukihiro "Matz" Matsumoto in Japan.

Example of metaprograming in Ruby

In ruby you can write methods using metaprogramming to insert code into the program. A widely use example of such metaprogramming method is the attr_accessor method in the class named Module

In this example attr_accessor method is use to insert the accessor codes to the class

class MyClass   
  attr_accessor :info
end

Is the same as

class MyClass
 def variable=(value)
   @variable = value
 end

 def variable
   @variable
 end
end

The method method_missing() is another example of metaprograming in Ruby. In ruby if the interpreter can’t find the method anywhere up the object’s chain of inheritance, it will go back to the object and call the method named method_missing(). The interpreter looks for method_missing() in the object’s methods and up the object’s chain of inheritance until it reaches the Object class where method_missing() is defined.

class MyClass
  def method_missing(name, *args)
    puts "#{name} was called with arguments: #{args.join(', ')}"
  end
end

m = MyClass.new
m.undefined_method("one", "two") # => undefined_method was called with arguments: one, two
m.another_undefined_method("three", "four") # => another_undefined_method was called with arguments: three, four

The class MyClass does not have a method named undefined_method() or another_underfined_method() but the method_missing() method created responds to these methods

References

• [1] Wikipedia Entry on Metaprogramming
• [2] Wikipedia Entry on Dynamic programming language
• [3] Wikipedia Entry on Type System
• [4] Wikipedia Entry on Shell script
• [5] Wikipedia Entry on Shell script

• [6] Expertiza Wiki Entry on Metaprogramming in Statically Typed Languages 1
• [7] Expertiza Wiki Entry on Metaprogramming in Statically Typed Languages 2
• [8] Expertiza Wiki Entry on Metaprogramming in What is Metaprogramming