From: Derek Lewis <lewisd@...00f.net>
Date: 2003-12-16T01:26:34+09:00
Subject: Re: Arity?


I experienced this same confusion.

What I figured out (which I think is correct) is that the documentation
you quoted from "Programming Ruby" is correct for Ruby 1.6

The simpler results (0 for no-args, n for n args, -n for variable length,
minimum of n args) are for Ruby 1.8.

Here's two tests on different ruby versions.  Note that the only
difference is methods that take 1 argument:

lewisd@wedge:~/projects/rcoaster$ ruby -v
ruby 1.6.8 (2002-12-24) [i386-linux]
lewisd@wedge:~/projects/rcoaster$ ruby ./test/testArity.rb
||      = 0
|a|     = -1
|a,b|   = 2
|a,b,c| = 3
|*a|    = -1
|a,*b|  = -2
|a,b,*c|  = -3
lewisd@wedge:~/projects/rcoaster$


lewisd@derlewi:~/projects/rcoaster$ ruby -v
ruby 1.8.1 (2003-11-11) [i386-linux]
lewisd@derlewi:~/projects/rcoaster$ ruby ./test/testArity.rb
||      = 0
|a|     = 1
|a,b|   = 2
|a,b,c| = 3
|*a|    = -1
|a,*b|  = -2
|a,b,*c|  = -3
lewisd@derlewi:~/projects/rcoaster$


lewisd@wedge:~/projects/rcoaster$ cat ./test/testArity.rb
puts "||      = " + Proc.new {||}.arity.to_s
puts "|a|     = " + Proc.new {|a|}.arity.to_s
puts "|a,b|   = " + Proc.new {|a,b|}.arity.to_s
puts "|a,b,c| = " + Proc.new {|a,b,c|}.arity.to_s
puts "|*a|    = " + Proc.new {|*a|}.arity.to_s
puts "|a,*b|  = " + Proc.new {|a,*b|}.arity.to_s
puts "|a,b,*c|  = " + Proc.new {|a,b,*c|}.arity.to_s
lewisd@wedge:~/projects/rcoaster$



On Mon, 15 Dec 2003, Jon A. Lambert wrote:

> >From the Programming Ruby - The Pragmatic Programmer's Guide:
>
> --class Proc ---
> arity  prc.arity -> anInteger
>
> Returns the number of arguments required by the block. If the block
> takes no arguments, returns -1. If it takes one argument, returns -2.
> Otherwise, returns a positive argument count unless the last argument
> is prefixed with *, in which case the argument count is negated. The
> number of required arguments is anInteger for positive values, and
> ( anInteger +1).abs otherwise.
>
> 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
>
> --class Method --
>
> arity  meth.arity -> aFixnum
>
> Returns an indication of the number of arguments accepted by a method.
> Returns a nonnegative integer for methods that take a fixed number of
> arguments. For Ruby methods that take a variable number of arguments,
> returns -n-1, where n is the number of required arguments. For methods
> written in C, returns -1 if the call takes a variable number of arguments.
>
> ----
>
> I can make neither head nor tail of this as the values that are actually returned
> are not those in the example, and the text doesn't appear to make any sense at
> all to me.  Can someone explain what arity means and how it's determined?
>
> --
> J. Lambert
>
>

Derek Lewis

===================================================================
      Java Web-Application Developer

      Email    : email@lewisd.com
      Cellular : 604.312.2846
      Website  : http://www.lewisd.com

 "If you've got a 5000-line JSP page that has "all in one" support
 for three input forms and four follow-up screens, all controlled
 by "if" statements in scriptlets, well ... please don't show it
 to me :-).  Its almost dinner time, and I don't want to lose my
 appetite :-)."
      - Craig R. McClanahan