[coding] Sandi Metz 2016-06-09

Picking on conditions

I come from smaltalk, with only 6 reserved words – and if was not one of them.

I don’t want a condition, I just want to send a message to an object (if is bad; it halts your OO thinking and leads you to write procedural code).

class Object
  def if_true() yield; end
  def if_false() self; end
end

class FalseClass
  def if_true()   self; end
  def if_false() yield; end
end

class NilClass
  def if_true()   self; end
  def if_false() yield; end
end

So we can do things like…

"anything".if_true { puts "evaluated block" }
nil.if_false       { puts "evaluated block" }

(1 == 1).
  if_true  { puts "it's true" }
  if_false { puts "it's false" }

Do not do it

Null Object Pattern

ids = ['pig', '', 'sheep']
animals = ids.map{ |id| Animal.find(id) }

=> [#<Animal: @name='pig'>,
    nil,
    #<Animal: @name='sheep'>]

animals.each {|animal|
  if animal.is_a?(NilClass)
    puts 'No animal'
  else
    puts animal.name
  end
}

Let’s use Null Object Pattern

class MissingAnimal
  def name() 'No animal'; end
end

Now we can do this:

ids = ['pig', '', 'sheep']
animals = ids.map{ |id| Animal.find(id) || MissingAnimal.new }

=> [#<Animal: @name='pig'>,
    #<MissingAnimal>,
    #<Animal: @name='sheep'>]

animals.each {|animal| puts animal.name }

And this can became better with

class GuaranteedAnimal
  def self.find(id)
    Animal.find(id) || MissingAnimal.new
  end
end

…because we replace

animals = ids.map{ |id| Animal.find(id) || MissingAnimal.new }

with

animals = ids.map{ |id| GuaranteedAnimal.find(id) }

If you can take this home and use it, it will improve your code.

Composition over inheritance

Here we need some code to show. Sorry folks ):