Implicit return value of Array#map in Ruby

Question

Given the following code:

class MagicList
  def items=(array)
    @items = array.map{|x| x*2}
  end

  def items
    @items
  end
end

list = MagicList.new
returns = list.items=([1, 2, 3])

puts returns.inspect    # => [1, 2, 3]
puts list.items.inspect # => [2, 4, 6]

I expected the value of returns to be [2, 4, 6], because @items as well as array.map{|x| x*2} both return this value. Why is it [1, 2, 3]?

Because Array#map doesn't change the original variable; Array#map! changes the original variable. — Robert K
– Robert K, Commented Jul 13, 2012 at 13:33
But I do not return the default variable. Even when I add return @items to def items= it still returns the original array. — iblue
– iblue, Commented Jul 13, 2012 at 13:35
@iblue Setters always return the right hand side of the assignment. — Michael Kohl
– Michael Kohl, Commented Jul 13, 2012 at 13:36
@RobertK It's still the last expression in the method and thus would be returned if the method weren't a setter. — Michael Kohl
– Michael Kohl, Commented Jul 13, 2012 at 13:36

Community · Accepted Answer · 2017-05-23 12:27:53Z

8

Because Ruby assignments always return the item they were passed, regardless of what the setter= method returns.

See also Is it possible to have class.property = x return something other than x?

edited May 23, 2017 at 12:27

CommunityBot

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answered Jul 13, 2012 at 13:35

Gareth

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6 Comments

iblue Over a year ago

Oh. I didn't know that. Where is this documented?

Stefan Over a year ago

@Gareth to be specific, the items= method does return [2,4,6], it's the assignment that is working differently when a = is involved.

Gareth Over a year ago

@Stefan I'm not sure where you get that impression from. returns = (list.items = [1, 2, 3]) would show that the return value of the setter is the same as the argument that's passed in. If [2, 4, 6] was being returned then how could the returns variable be set to anything else?

iblue Over a year ago

@Gareth: list.send(:items=, [1, 2, 3]) returns [2, 4, 6]

Stefan Over a year ago

@Gareth Ruby sets the variable to the right-most value because of the chained assignment. Try returns = list.send(:items=,[1,2,3])

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