when to use lambda, when to use proc.new

Another important but subtle difference between procs created with lambda and procs created with Proc.new is how they handle the return statement:

In Ruby 1.8, there are subtle differences between proc/lambda on the one hand, and Proc.new on the other.

  • What are those differences?
  • Can you give guidelines on how to decide which one to choose?
  • In Ruby 1.9, proc and lambda are different. What's the deal?

 

Another important but subtle difference between procs created with lambda and procs created with Proc.new is how they handle the return statement:

  • In a lambda-created proc, the return statement returns only from the proc itself
  • In a Proc.new-created proc, the return statement is a little more surprising: it returns control not just from the proc, but also from the method enclosing the proc!

Here's lambda-created proc's return in action. It behaves in a way that you probably expect:

def whowouldwin

  mylambda = lambda {return "Freddy"}
  mylambda.call

  # mylambda gets called and returns "Freddy", and execution
  # continues on the next line

  return "Jason"

end


whowouldwin
#=> "Jason" whowouldwin #=> "Jason"

Now here's a Proc.new-created proc's return doing the same thing. You're about to see one of those cases where Ruby breaks the much-vaunted Principle of Least Surprise:

def whowouldwin2

  myproc = Proc.new {return "Freddy"}
  myproc.call

  # myproc gets called and returns "Freddy", 
  # but also returns control from whowhouldwin2!
  # The line below *never* gets executed.

  return "Jason"

end


whowouldwin2         
#=> "Freddy"

Thanks to this surprising behavior (as well as less typing), I tend to favor using lambda over Proc.new when making procs.