add entry for factory pattern

factory/README.md

@@ -0,0 +1,45 @@
+The Factory Pattern provides a way for a client to instantiate different object without having to know which classes they need. The objective is to provide a simple interface that doesn't need to change when new classes are added or removed.
+
+To take a trivial example, let's say I am building an app for a university. I have two classes `Student` and `Faculty` who share a common interface.
+```ruby
+class Person
+  def initialize(attrs)
+    @name = attrs[:name]
+  end
+end
+
+class Student < Person
+  def initialize(attrs)
+    super
+    @grad_class = attrs[:grad_class]
+  end
+
+  def to_s
+    "#{@name} (#{@grad_class})"
+  end
+end
+
+class Faculty < Person
+  def initialize(attrs)
+    super
+    @department = attrs[:department]
+  end
+
+  def to_s
+    "#{@name}, Professor of #{@department}"
+  end
+end
+```
+
+Let's also say that, for reasons that escape us, we don't want to make a single interface for creating instances of both classes:
+
+```ruby
+class PersonFactory
+  def self.for(type, attrs)
+    Object.const_get(type).new attrs
+  end
+end
+```
+
+Now the details of which object should be returned are hidden from the client. The client provides some data and get's back the appropriate class. This is helpful as when things get more complicated, we don't need to burden client code with logic to decide what subclass to create. The factory decides for you.
+

factory/main.rb

@@ -0,0 +1,39 @@
+class Person
+  def initialize(attrs)
+    @name = attrs[:name]
+  end
+end
+
+class Student < Person
+  def initialize(attrs)
+    super
+    @grad_class = attrs[:grad_class]
+  end
+
+  def to_s
+    "#{@name} (#{@grad_class})"
+  end
+end
+
+class Faculty < Person
+  def initialize(attrs)
+    super
+    @department = attrs[:department]
+  end
+
+  def to_s
+    "#{@name}, Professor of #{@department}"
+  end
+end
+
+class PersonFactory
+  def self.for(type, attrs)
+    Object.const_get(type).new attrs
+  end
+end
+
+student = PersonFactory.for 'Student', name: 'A Student', grad_class: 2020
+puts student
+
+prof = PersonFactory.for 'Faculty', name: 'Dr. Professor Science', department: 'Rocket Surgery'
+puts prof