Class: Class

Inherits:
Object
  • Object
show all
Defined in:
lib/extensional/class.rb

Instance Method Summary collapse

Instance Method Details

#make_extensional(hash = nil, &block) ⇒ Object

Adds the ability of a Class to collect its instances. An extensional class is an Enumerable whose each method iterates over its instances, e.g.:

class Person
  make_extensional
  def initialize(ssn)
    @ssn = ssn
    Person << self
  end
  def to_s
    "#{self.class.name}{ssn=>#{@ssn}}"
  end
  ...
end
Person.new('555-55-5555')
Person.join(', ') #=> Person{ssn=>555-55-5555}

The optional callback block adds associative access to instances by key using the class #for method. The callback block is called when an instance is added to the extent. The block arguments include the key=>instance association hash and the new instance, e.g.:

class Person
  make_extensional { |hash, person| hash[person.ssn] = person }
  ...
end
Person.for('555-55-5555') #=> nil
Person.new('555-55-5555')
Person.for('555-55-5555') #=> Person{ssn=>555-55-5555}

The optional hash argument is the association hash. If the association hash is created with a block, then the instance is added to the hash on demand when the #for method is called in accordance with the standard Hash#new behavior, e.g.:

class Person
  make_extensional(Hash.new { |hash, ssn| hash[snn] = new(ssn) })
  ...
end
Person.for('555-55-5555') #=> Person{ssn=>555-55-5555}
Person.join(', ') #=> Person{ssn=>555-55-5555}
Person.new('666-66-6666') # added to the extent but not the association since there is no callback block
Person.map { |person| person.ssn }.sort #=> ["555-55-5555", "666-66-6666"]
Person.for('666-66-6666') #=> nil

The hash factory block above only adds an instance to the extent association if it is accessed using the #for class method. If an instance can be created both on demand and independently of the #for method, then specify both a factory block and a callback block, e.g.:

class Person
  make_extensional(Hash.new { |hash, ssn| new(ssn) }) {|hash, person| hash[person.snn] = person }
  ...
end
Person.for('555-55-5555') #=> Person{ssn=>555-55-5555}
Person.new('666-66-6666') # added to both the extent and the association
Person.map { |person| person.ssn }.sort #=> ["555-55-5555", "666-66-6666"]
Person.for('666-66-6666') #=> Person{ssn=>666-66-6666}

Note that hash assignment is not needed in the hash initializer method, since there is a callback block assigns the hash key to the new instance.

This latter make_extensional alternative is the most flexible strategy, since it provides for both iterative and associative access.

If neither a callback block nor a hash is given, then associative access is disabled as described in the Extensional#for method.



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# File 'lib/extensional/class.rb', line 64

def make_extensional(hash=nil, &block) # :yields: hash, instance
  @extent = Extent.new(hash, &block)
  extend(Extensional)
end