Creating an object the hard way, made easy with lambda


#1

#!/usr/bin/env ruby
#$LastChangedRevision: 49 $

The purpose of this sample is to demonstrate the application of

lambda

expressions to create a rudimentary ‘person object’.

Its features Smalltalk style “message passing” and "variables are

private,

methods are public" encapsulation.

This sample is an approximation, and drastic simplification, of many

of the

examples you find written in Scheme, as it does not provide an OO

system for

use by the developer.

This is OK, though, since the point of this example is only to

demonstrate

one application of lambda expressions. You may use them to write

your own

object system in Ruby, but then again why bother, Ruby has already

got

what you need!

def get_abstract_person_object
first_name = “”
last_name = “”
age = -1
my_to_s = lambda {
“FIRST NAME:’#{first_name}’, LAST NAME:’#{last_name}’,
AGE:’#{age}’\n”
}
new = lambda { |fn, ln, ag|
first_name = fn
last_name = ln
age = ag
nil
}

lambda { |msg|

result = nil

case msg[0]
        when :new
            new.call(msg[1], msg[2], msg[3])
  when :get_first_name
    result = first_name
  when :set_first_name
    result = first_name = msg[1]
  when :get_last_name
    result = last_name
  when :set_last_name
    result = last_name = msg[1]
  when :get_age
    result = lage
  when :set_age
    result = age = msg[1]
  when :to_s
    result = my_to_s.call
end

if result != nil
  result
else
  "MESSAGE '#{msg[0]}' NOT UNDERSTOOD!"
end

}
end

kristy = get_abstract_person_object
puts “Create Kristy”
puts kristy.call([:to_s])
kristy.call([:set_first_name, “Kristy”])
kristy.call([:set_last_name, “T”])
kristy.call([:set_age, 27])
puts kristy.call([:to_s])

puts “Create Steve”
steve = get_abstract_person_object
steve.call([:set_first_name, “Steve”])
steve.call([:set_last_name, “M”])
steve.call([:set_age, 48])
puts steve.call([:to_s])

puts
puts “Are the two ‘objects’ really different entities? YES.\n”
puts kristy.call([:to_s])
puts steve.call([:to_s])

puts
puts “What happens when you send a message that the object does not
understand?”
puts kristy.call([:foo])

puts
puts “A “constructor” message”
dave = get_abstract_person_object
dave.call([:new, “David”, “O”, 28])
puts dave.call([:to_s])