Learn to Program, 2nd Edition

Methods That Return Procs

One of the cool things you can do with procs is create them in methods and return them. This allows all sorts of crazy programming power (things with impressive names, such as lazy evaluation, infinite data structures, and currying). I don’t actually do these things very often, but they are just about the sexiest programming techniques around.

In this example, compose takes two procs and returns a new proc that, when called, calls the first proc and passes its result into the second:

def compose proc1, proc2
  Proc.new do |x|
    proc2.call(proc1.call(x))
  end
end
square_it = Proc.new do |x|
  x * x
end
double_it = Proc.new do |x|
  x + x
end
double_then_square = compose double_it, square_it
square_then_double = compose ...