gistfile1.rb

input = [ 1, 2, 3, 4, 5, 8, 9, 11, 12, 13, 15 ]

# Need to box the value to make it mutable
State = Struct.new(:last_value)

def returning(value) yield; value; end
                                          
# divide the input into runs of consecutive numbers
s = input.slice_before(State.new(input.first)) do |value, state|
  returning(value != state.last_value.succ) do
    state.last_value = value
  end
end

# replace runs of 3 or more with first-last
p s.map {|runs| runs.size < 3 ? runs : "#{runs.first}-#{runs.last}"}
   .flatten
   .join(', ')     # => 1-5, 8, 9, 11-13, 15  

I totally agree. I don't think this is a good example of how to use slice_before. But it's a great example of how to use returning

Thanks for posting this, I made use of this example in a refactoring of the scoring koan that I posted here: http://gist.github.com/580960

Totally loving playing with ruby's Enumerable.