Splat on a hash

Question

• A splat on a hash converts it into an array.

  [*{foo: :bar}] # => [[:foo, :bar]]

  Is there some hidden mechanism (such as implicit class cast) going on here, or is it a built-in primitive feature?

• Besides an array, are nil and hash the only things that disappear/change with the splat operator under Ruby 1.9?

Answer 1

A splat will attempt an explicit conversion of an object to an Array.

To do this, it will send to_a and expect an Array as a result.

class Foo
  def to_a
    [1,2,3]
  end
end

a, b, c = *Foo.new
a # => 1

If the object does not respond to to_a, then there is no effect, e.g. [*42] == [42]

Many builtin classes implement to_a. In particular:

• (because they include Enumerable)
  • Array
  • Hash
  • Range
  • IO and File
  • Enumerator
  • Enumerator::Lazy (Ruby 2.0)
  • Set and SortedSet
• (additional classes)
  • NilClass
  • MatchData
  • OpenStruct
  • Struct
  • Time
  • Matrix and Vector

All these can thus be splatted:

match, group, next_group = *"Hello, world".match(/(.*), (.*)/)
group # => "Hello"