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Proc, Block, and Two Smoking Lambdas

· · Posted in Programming
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Ruby 1.9 has 4 different ways to deal with closures.

Cue music


Procs are the weird ones of the bunch. Technically, all of these things I'm going to describe are Procs. By that I mean, if you check the class, it's a Proc.

A Proc is made by using and passing a block, { |x| x }, or by using the proc keyword, proc { |x| x }.

A return from inside exits completely out of the method enclosing the Proc.

A Proc doesn't care about the arguments passed. If you define a Proc with two parameters, and you pass only 1, or possibly 3, it keeps on trucking. In the case of 1 argument, the second parameter will have the value nil. If you pass extra arguments, they will be ignored and lost.


Blocks are when you pass an anonymous closure to a method:

def my_method
  my_other_method(1) do |x, y|
    return x + y

They work exactly like a Proc. It wouldn't matter how many arguments my_other_method called yield with, the block would execute just fine.1 The return will also return out of my_method.


A lambda is probably what you deal with most of time. You make them with the lambda keyword: f = lambda { |x| x + 1 }. They are a bit different.

Unlike a Proc, using return in a lambda will simply return from the lambda, pretty much like you'd expect.

Also unlike a Proc, lambda likes to whine if you pass an incorrect number of arguments. It will blow up with an ArgumentError.


The stabby is new in Ruby 1.9, and is just syntactic sugar for lambda. These are equivalent:

f = lambda { |x| x + 1 }
f2 = ->(x) { x + 1 }

What's all this then?

So anyway I wrote some specs, and here they are (or rather their output). If you want to check out the actual specs, or run them for yourself, head on over to Github.

  1. The addition won't work too well, but hey.