When to use lambda, when to use Proc.new?

Question

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?

Answer 1

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"

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.