Proc.new vs Lambda in Ruby

March 19th, 2007

Posted by sam in Programming, Ruby

I found the following lines of code on Wikipedia today. It’s a very succinct description of one important difference between a lambda and a Proc. Try printing the return value of f.call for more insight.


def foo
  f = Proc.new { return "return from foo from inside proc" }
  f.call # control leaves foo here
  return "return from foo" 
end

def bar
  f = lambda { return "return from lambda" }
  f.call # control does not leave bar here
  return "return from bar" 
end

puts foo # prints "return from foo from inside proc" 
puts bar # prints "return from bar"

Whole article on Wikipedia about closures. . . And a bunch more on procs and blocks

10 Responses Follows

  1. Daniel says
    on April 26th, 2010 at 06:45 AM

    Thanks for this. Very helpful!

  2. David Rivers says
    on July 27th, 2010 at 11:37 AM

    Why didn’t you bring this to my attention 20 minutes ago in that phone interview!

  3. Troy Goode says
    on July 27th, 2010 at 05:14 PM

    Great tip – really helped clear some concepts up for me. Out of curiosity: given this, when would it be preferable to use proc instead of lambda?

  4. Nimesh Nikum says
    on August 10th, 2010 at 02:15 PM

    Really great findings and thanks for this :)

  5. Neelesh says
    on December 24th, 2010 at 08:16 AM

    Sam ,

    Good explanation, But when I’m using ############## def test(a) a.call(100) puts “You are in the method test” #yield 100 end a = lambda{|i| puts “You are in the block #{i}” return }

    test(a) ########### Output :

    You are in the block 100 You are in the method test

    Could u plz explain me…..

  6. Matt says
    on March 27th, 2011 at 08:11 PM

    @Neelesh – You are using puts statements. Those are always executed. Use return statements instead and you’ll see your code act as the example indicates.

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  9. John says
    on March 13th, 2012 at 03:49 AM

    Thanks for providing the clear insight on the difference between these two

  10. John says
    on May 10th, 2012 at 02:31 PM

    I prefer lambda, mainly because it’s more like a function if I called f() { return 1; } it wouldn’t be returned.


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