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

(I know it had been asked several times but I couldn’t find satisfactory answer)Can somebody please explain Blocks, Procs and Lambdas and why one should be used over other, what are the situation when one should use proc, similar and/or lambda. Also there effect on computer memory. Practical examples please.

Answer 1

Try Robert Sosinski’s Tutorial or Learning to Program by Chris Pine.

For more foundation I suggest you read Why’s (poignant) Guide to Ruby. This guide is responsible for creating many of nowadays Ruby’s Pro! Make sure to take a look!

Explanation by Joey deVilla

Another important but subtle difference is in the way procs created with lambda and procs created with Proc.new 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 behaviour (as well as less typing), I tend to favour using lambda over Proc.new when making procs.