netinlet

presenation by Glenn Vanderburg

What is meta programming? It’s Programming your programming language

Rubyist have been discovering metaprogramming. Ruby style and idioms are still changing and adapting

Ruby good for metaprogramming b/c

• Dynamic and reflexive – everything is open – blocks allow writing new control structures – most declarations are executable statements – only slightly less malleable than lisp (no macros) – unobtrusive

Examples…

attr_reader, attr_writer, and attr_accessor.

if written in ruby attr_reader would be written like (actually written in C )

class Module
    def attr_reader(*syms)
        syms.each do |sym|
            class_eval %{ def #{sym}
                                @#{sym}
                              end
                            }
        end
        end

Speaker goes through several implementations over time of different ways different people did metaprogramming with ruby.

How to think about metaprogramming

• Definiting new constructs for your programming Language
  • so what do the constructs to? whatever you domain needs it to do.

Another way to think about metaprogramming is a new set of conceptual tools for eliminating duplication (and other smells) from your code.

And another way to think about it is how rails does it – almost as if you can talk you code – PersonTable has_a :name

Most DSLs also deal with other things ou don’t usually find in general-purpose languages

• Context dependence
• commands and sentences
• Units
• Large vocabularies
• Heirachy

Contexts – context for a new set of statements – a new scope (not in 1.8, but in 1.9)

Struct.new("Interval", :start, :end) do
  def length
    @start - @end
  end
end

Backend code looks like if you wanted to add it to 1.8

  class Struct
      initialize(*args, &block)
        struct_class = #define struct using args
        struct_class_class_eval(&block) if block_given?
     end
   end

Another example of context from Systir system testing tool

add_user {
  name "Charles" 
  password "secret" 
  priviliges normal
}

Commands and Sentences

Multipart complex statements

ex. field(autoinc, :reg_id, pk) Overall, it’s just a methodcall – the first parameter – the type – is a method call

def autoinc 
  return FieldType::AutoInc.instance 
end

Units

Domain specific – general purpose language deals with scalars – programs must maintain their knowledge ex 3.days.from_now Watch out for operator overloading

class Numeric
  def days
    self * 60
  end
end

Large Vocabularies

override method_missing

Usage: Roman.XXII Roman.CCIX

class Roman
  def self.missing_method(method_id)
    str = method_id.id2name
    roman_to_int(str)
  end
  def roman_to_int(string)
    ...
  end
end

Resources:

http://www.vanderburg.org/Speaking/Stuff/oscon05.pdf http://hypermetrics.com/rubyhacker/coralbook/