fun with continuations and a puzzler

1_continuations.rb

def square_with_continuation(x, continuation = ->(x) { x })
  continuation.call(x*x)
end

2_irb_sessions

1.9.3-p448 :004 > square_with_continuation(2)
 => 4
1.9.3-p448 :005 > square_with_continuation(2, ->(x){ x.to_s })
 => "4"
1.9.3-p448 :006 > square_with_continuation(2, :to_s.to_proc)
 => "4"
1.9.3-p448 :007 > square_with_continuation(2, &:to_s)
 => 4

3_compositions_with_continuations.rb

def apply_with_optional_continuation(function, continuation = ->(x) { x })
  Proc.new{ |p| continuation.call(function.call(p)) }
end

4_irb_sessions

1.9.3-p448 :004 > squarer = apply_with_optional_continuation(->(x) { x*x })
 => #<Proc:0x007fc1ba921340@(irb):2>
1.9.3-p448 :005 > squarer.call(2)
 => 4
1.9.3-p448 :006 > squarer_plus = apply_with_optional_continuation(->(x) { x*x }, :to_s.to_proc)
 => #<Proc:0x007fc1ba8ab6b8@(irb):2>
1.9.3-p448 :007 > squarer_plus.call(2)
 => "4"