Differ between Proc and lambda in Ruby!

First of all there is no difference between a Proc (read Do the Proc! ... a Ruby closure) and a lambda (read Have a lambda! ... a Ruby closure) in Ruby, technically spoken. Both are Proc objects:

Proc.new {}
=> #<Proc:0x000000029642c0@(irb):1>
lambda {}
=> #<Proc:0x000000029642c0@(irb):2 (lambda)>

but with a different flavor (please note the second object stringification).
Well, but that's not the only difference (otherwise having 2 equal closure types in Ruby would not make any sense).
The differences are:

1. lambda check the number of arguments and Procs do not.
2. Proc and lambda treat return differently

A lambda checks the number of arguments:

lamda = lambda { |language| puts language }
lamda.call 'Ruby'          # prints Ruby
lamda.call                 # ArgumentError: wrong number of arguments (0 for 1)
lamda.call 'Ruby', 'Java'  # ArgumentError: wrong number of arguments (2 for 1)

unlike the Proc:

proc = Proc.new { |language| puts language }
proc.call 'Ruby'          # prints Ruby
proc.call                 # prints nothing
proc.call 'Ruby', 'Java'  # prints Ruby, but ignores Java

In terms of the second difference, a lambda returning something explicitly, will not force the environment (of the closure) to return either:

def lambda_environment
  lamda = lambda { return }
  lamda.call
  puts 'Ruby'
end
lambda_environment # prints Ruby

The lambda returns into its environment and let it end its processing.
But the Proc does not:

def proc_environment
  proc = Proc.new { return }
  proc.call
  puts 'Ruby'
end
proc_environment # does not print anything

The Proc forces its environment to return immediately. Further articles of interest:

• Have a lambda! ... a Ruby closure
• Do the Proc! ... a Ruby closure

Supported by Ruby 2.1.1