From: Paul Brannan <paul@...>
Date: 2001-12-14T01:53:39+09:00
Subject: [ruby-talk:28422] why is Proc's arity function so weird?

From the Pickaxe book (online, since the printed version is wrong, as is
the online description of Proc#arity):
  Proc.new {||}.arity      �   0
  Proc.new {|a|}.arity     �  -1
  Proc.new {|a,b|}.arity   �   2
  Proc.new {|a,b,c|}.arity �   3
  Proc.new {|*a|}.arity    �  -1
  Proc.new {|a,*b|}.arity  �  -2

Why does arity return -1 when there is 1 argument?  The problem with
this is the following:

  irb(main):003:0> p = Proc.new {|a|}
  #<Proc:0x402b0508>
  irb(main):004:0> p.call(1, 2, 3)
  nil
  irb(main):005:0> p = Proc.new {|a, b|}
  #<Proc:0x402aaf68>
  irb(main):006:0> p.call(1, 2, 3)
  ArgumentError: wrong # of arguments (3 for 2)
          from (irb):6
          from (irb):5:in `call'
          from (irb):6

If my Proc takes one argument, then I can pass as many arguments to it
as I want, but if it takes two arguments, then I get an ArgumentError if
I try to do that.

I understand that the reasoning here is to allow:

  h = { 1=>2, 2=>3 }
  h.each do |pair|
    p pair
  end

which produces:

  [1, 2]
  [2, 3]

but there really is no advantage to writing it that way versus:

  h = { 1=>2, 2=>3 }
  h.each do |*pair|
    p pair
  end

Is there another reason for this behavior that I am missing?

Paul