The Elements of Style in Ruby #12: proc vs Proc.new

People are often confused about the fact that there are two ways to created procs in Ruby - via Kernel#proc and Proc.new. Let’s see them in action:

Proc.new { true }
# => #<Proc:0x007fe35440a058>

proc { true }
# => #<Proc:0x007fe35440a059>

Hmmm, it seems we get exactly the same results… While this is true on Ruby 1.9+, this was not always the case.

In Ruby 1.8, Kernel#proc is actually a synonym for Kernel#lambda which was extremely confusing, since as we all know lambdas an procs differ in subtle ways. Luckily sanity prevailed and Ruby 1.9 made Kernel#proc a synonym for Proc.new instead.

At this point, however, people couldn’t use Kernel#proc anymore if they wanted to write code that’s behaving in the same way on both Ruby 1.8 and Ruby 1.9 and the use of Kernel#proc was generally discouraged. Thankfully Ruby 1.8 is now dead and buried and there’s no reason to prefer Proc.new over Kernel#proc anymore. As a matter of fact - you should probably be using only Kernel#proc as it’s more concise and it’s symmetrical to Kernel#lambda.

lambda { true }
# => #<Proc:0x007fe35440a058 (lambda)>

proc { true }
# => #<Proc:0x007fe35440a059>

By the way, given proc’s fairly counter-intuitive behavior regarding return, you should probably use lambdas most of the time.