Analyzing the different behaviours of Ruby's Proc

proc_types.rb

# My conclusion:
#   lambda     Use this if you want it to behave like a method.
#   proc       Avoid this, since its behaviour differs across commonly used versions.
#   Proc.new   Use this if you want it to behave like a block.
#   ->         This is just a 1.9 alternate syntax for lambda, so use it if you want method behaviour.



# use array, because 1.8.x doesn't have ordered hashes, which makes output confusing
proc_types = [
  ['lambda'   , method(:lambda)   ],
  ['proc'     , method(:proc)     ],
  ['Proc.new' , Proc.method(:new) ],
]

puts '', `ruby -v`

proc_types.each do |name, method|
  
  too_few_args = begin
    method.call { |a, b| }.call(1)
    "no error"
  rescue
    "error"
  end
  
  too_many_args = begin
    method.call { |a, b| }.call(1, 2, 3)
    "no error"
  rescue
    "error"
  end
  
  returns = begin
    def meth(method)
      method.call { return "from enclosing environment" }.call
      "from block itself"
    end
    meth method
  end

  format = "%-10s (too few args: %-8s) (too many args: %-8s) (returns: %-10s)\n"
  printf format , name, too_few_args, too_many_args, returns
end


# $ rvm proc_types.rb 
# 
# MacRuby 0.10 (ruby 1.9.2) [universal-darwin10.0, x86_64]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# 
# jruby 1.6.0 (ruby 1.8.7 patchlevel 330) (2011-03-15 f3b6154) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_24) [darwin-x86_64-java]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: error   ) (too many args: error   ) (returns: from block itself)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# 
# rubinius 1.1.0 (1.8.7 release 2010-09-23 JI) [x86_64-apple-darwin10.4.0]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: error   ) (too many args: error   ) (returns: from block itself)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# 
# ruby 1.8.6 (2010-02-05 patchlevel 399) [i686-darwin10.3.0]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: error   ) (too many args: error   ) (returns: from block itself)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# 
# ruby 1.8.7 (2010-08-16 patchlevel 302) [i686-darwin10.4.0]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: error   ) (too many args: error   ) (returns: from block itself)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# 
# ruby 1.9.1p378 (2010-01-10 revision 26273) [i386-darwin10.4.0]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# 
# ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.7.0]
# lambda     (too few args: error   ) (too many args: error   ) (returns: from block itself)
# proc       (too few args: no error) (too many args: no error) (returns: from enclosing environment)
# Proc.new   (too few args: no error) (too many args: no error) (returns: from enclosing environment)

My Conclusion

lambda: Use this if you want it to behave like a method.
proc: Avoid this, since its behaviour differs across commonly used versions.
Proc.new: Use this if you want it to behave like a block.
-> This is just a 1.9 alternate syntax for lambda, so use it if you want method behaviour.

Started a discussion about this on ruby-talk.

This is a helpful reference.