What is a Proc?

Essentially, Procs are anonymous methods (or nameless functions) containing code. They can be placed inside a variable and passed around like any other object or scalar value. They are created by Proc.new, lambda, and blocks (invoked by the yield keyword).

Note: Procs and lambdas do have subtle, but important, differences in ruby v1.8.6. However, I wouldn’t expect a candidate talk about these nitty-gritty details during an interview. (Kudos to Noah Thorp)

# wants a proc, a lambda, AND a block
def three_ways(proc, lambda, &block)
  proc.call
  lambda.call
  yield # like block.call
  puts "#{proc.inspect} #{lambda.inspect} #{block.inspect}"
end

anonymous = Proc.new { puts "I'm a Proc for sure." }
nameless  = lambda { puts "But what about me?" }

three_ways(anonymous, nameless) do
  puts "I'm a block, but could it be???"
end
 #=> I'm a Proc for sure.
 #=> But what about me?
 #=> I'm a block, but could it be???
 #=> #<Proc:0x00089d64> #<Proc:0x00089c74> #<Proc:0x00089b34>