Ruby Mar 30th, 2015

Ruby Trick: Local Assignment over Method Sending

There is a weird situation I haven’t noticed before:

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class Counter
  attr_accessor :processed, :processed_names

  def initialize
    @processed = 0
    @processed_names = []
  end

  def foo
    processed += 1
  end

  def bar
    processed_names << 'a'
  end
end

w = Counter.new;
w.foo # => NoMethodError: undefined method `+' for nil:NilClass`
w.bar # => ['a']

Why the hell?

Local assignment always has precedence over method sending. Assignment happened in w.foo, which not in w.bar.

Check this one:

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class Person
  attr_accessor :name

  def foo
    name = 'John'
  end
end

t = Tao.new
t.name # => nil
t.foo  # => 'John'
t.name # => nil

name = 'John' only means a definition of a local variable, which won’t send = to name.