I often need a variant of Enumerable#each method, with a “twist”: sometimes I want to have previous element available along with the current one, most often for comparison between the two. Here’s a trivial example:

array = [
  {name: 'foo', value: 15},
  {name: 'foo', value: 6},
  {name: 'bar', value: 2},
  {name: 'bar', value: 7},
  {name: 'bar', value: 14},
  {name: 'baz', value: 4},
  {name: 'baz', value: 1}
]

array.each_with_prev{|prev, curr|
  # Want to display name only before the first record in the group
  puts curr[:name] unless prev && prev[:name] == curr[:name]
  puts " #{curr[:value]}"
}
foo
    15
    6
bar
    2
    7
    14
baz
    4
    1

And here’s (very simple, inject based) implementation of each_with_prev:

module Enumerable
  def each_with_prev
    self.inject(nil){|prev, curr| yield prev, curr; curr}
    self
  end
end