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 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.

To provide further clarification:

Joey says that the return behavior of Proc.new is surprising. However when you consider that Proc.new behaves like a block this is not surprising as that is exactly how blocks behave. lambas on the other hand behave more like methods.

This actually explains why Procs are flexible when it comes to arity (number of arguments) whereas lambdas are not. Blocks don't require all their arguments to be provided but methods do (unless a default is provided). While providing lambda argument default is not an option in Ruby 1.8, it is now supported in Ruby 1.9 with the alternative lambda syntax (as noted by webmat):

concat = ->(a, b=2){ "#{a}#{b}" }
concat.call(4,5) # => "45"
concat.call(1)   # => "12"

And Michiel de Mare (the OP) is incorrect about the Procs and lambda behaving the same with arity in Ruby 1.9. I have verified that they still maintain the behavior from 1.8 as specified above.

break statements don't actually make much sense in either Procs or lambdas. In Procs, the break would return you from Proc.new which has already been completed. And it doesn't make any sense to break from a lambda since it's essentially a method, and you would never break from the top level of a method.

next, redo, and raise behave the same in both Procs and lambdas. Whereas retry is not allowed in either and will raise an exception.

And finally, the proc method should never be used as it is inconsistent and has unexpected behavior. In Ruby 1.8 it actually returns a lambda! In Ruby 1.9 this has been fixed and it returns a Proc. If you want to create a Proc, stick with Proc.new.

For more information, I highly recommend O'Reilly's The Ruby Programming Language which is my source for most of this information.