Procs are easy to define! You just call Proc.new and pass in the block you want to save. Here’s how we’d create a proc called cube that cubes a number (raises it to the third power):

cube = Proc.new { |x| x ** 3 }

We can then pass the proc to a method that would otherwise take a block, and we don’t have to rewrite the block over and over!

[1, 2, 3].collect!(&cube)
# ==> [1, 8, 27]
[4, 5, 6].map!(&cube)
# ==> [64, 125, 216]

(The .collect! and .map! methods do the exact same thing.)

The & is used to convert the cube proc into a block (since .collect! and .map! normally take a block). We’ll do this any time we pass a proc to a method that expects a block.