Why do the last two lines work?

return_procs.rb

def compose proc1, proc2
  Proc.new do |x|
    proc2.call(proc1.call(x))
  end
end

squareIt = Proc.new do |x|
  x**2
end

doubleIt = Proc.new do |x|
  x + x
end

squareThenDouble = compose squareIt, doubleIt
doubleThenSquare = compose doubleIt, squareIt

puts squareThenDouble.call(5)
puts doubleThenSquare.call(5)

puts squareThenDouble[5]
puts doubleThenSquare[5]

Taken from http://pine.fm/LearnToProgram/?Chapter=10 "Methods Which Return Procs"

Why do the last two lines work? Befuddled, I am.

Proc#[] is an alias of Proc#call docs

p = Proc.new { |n| n**2 }
p.call(5) # => 25
p[5]      # => 25

Yeah, what josh said ;)

Ah ha. Thanks dudes, that makes sense.