transducers.rb

map = -> f {
  -> rf {
    -> acc, elem {
      rf[acc, f[elem]]
    }
  }
}

square = -> n { n**2 }
add = -> acc, n { acc + n }

[1,2,3].reduce(0, &map[square][add])
#=> 14

notify = -> n {puts "Working on: #{n}"; n}

[1,2,3].reduce(0, &map[notify][add])
# Working on: 1
# Working on: 2
# Working on: 3
#=> 6

# We can compose these nicely

notify_then_square = -> n { square[notify[n]] }

[1, 2, 3].reduce(0, &map[notify_then_square][add])
# Working on: 1
# Working on: 2
# Working on: 3
#=> 14

square_then_notify = -> n { notify[square[n]] }

[1, 2, 3].reduce(0, &map[square_then_notify][add])
# Working on: 1
# Working on: 4
# Working on: 9
#=> 14