martinciu's dev blog

It will be another not very innovative post about a something that probably most Ruby developers already know. Unfortunately I didn’t know about this before, and I think it will be a quite good form of this blog, that I will be posting things that I’ve recently learned. So here is what I’ve learned today.

Proc.new and lambda does almost the same job. They provide a dynamically created inline method. But they are slightly different. What are the differences? There are two (as far as I know at the moment)

1. Acceptance of parameters

Proc.new doesn’t care how many parameter do you provide, when lambda needs exact number of parameters. Example:

pr = Proc.new {|a,b,c| puts a,b,c}
pr.call(1, 2, 3, 4)
# 1
# 2
# 3

…and…

pr = lambda {|a,b,c| puts a,b,c}
pr.call(1, 2, 3, 4)
# ArgumentError: wrong number of arguments (4 for 3)

2. Returning from a proc
return called in Proc.new block returns from enclosing method when return called in lambda block returns from just a block.
Examples:

def my_method
  puts "Starting my_method"
  pr = Proc.new {return}
  pr.call
  puts "Finishing my_method"
end
 
puts "Before my_method"
my_method
puts "After my_method"
# Before my_method
# Starting my_method
# After my_method

but…

def my_method
  puts "Starting my_method"
  pr = lambda {return}
  pr.call
  puts "Finishing my_method"
end
 
puts "Before my_method"
my_method
puts "After my_method"
# Before my_method
# Starting my_method
# Finishing my_method
# After my_method

The other thing that is worth mentioning is that there exist an instruction called proc. What is wired, it works different in ruby 1.8 and in version 1.9. In Ruby 1.8 proc behaviors like lambda when in Ruby 1.9 it works like Proc.new. Because of this, and because of ruumors that proc instruction will be removed in ruby 2.0, I don’t recommend using it in your code.