From: Pit Capitain <pit@...>
Date: 2002-09-13T17:51:48+09:00
Subject: Re: not grasping the method overloading/multi-dispatch thing

On 13 Sep 2002, at 7:22, Paul Brannan wrote:

> On Fri, Sep 13, 2002 at 05:41:20AM +0900, Rich Kilmer wrote:
> >    def fred(*args)
> >       case args.collect { |a| a.type}
> >          when [Float, Fixnum, String]
> >               f, i, s = args
> >               # ...
> 
> With this mechanism, how can I add a new signature to the list?  What
> if I want to extend fred() to work with GMP Integers?

Paul, in the former thread Robert Feldt showed another 
implementation in [ruby-talk:26792] that lets you do what you 
asked for. I came up with the following implementation based on 
Robert's code. Don't know why I didn't post it to the list then. 
Maybe because it still lacks some necessary features.

Regards,
Pit


class Module

  @@dispatch_db = Hash.new

  def overload( meth, *classes, &code )
    ( @@dispatch_db[ [ self, meth ] ] ||= {} )[ classes ] = code
    unless instance_methods.include?( meth )
      class_eval <<-EOS
        def #{meth}( *args )
          Module.dispatch( #{self}, :#{meth}, *args )
        end
      EOS
    end
  end

  def dispatch( cls, meth, *args )
    @@dispatch_db[ [ cls, meth ] ].each do | classes, code |
      catch( :nomatch ) do
        classes.size.times do | i |
          begin next if classes[ i ] === args[ i ]; rescue; end
          throw( :nomatch )
        end
        return code[ *args ]
      end
    end
    raise "dispatch error" 
  end
  
end


Test:


class C
  overload :m, String, Integer do | s, i |
    puts "m String, Integer called with #{s} and #{i}"
  end
end

c = C.new
c.m( "Test", 15 )                           # --> String, Integer
begin c.m( 2, "Test" ) rescue puts $! end   # --> dispatch error

class C
  overload :m, Integer, String do | i, s |
    puts "m Integer, String called with #{i} and #{s}"
  end
end

c.m( 2, "Test" )                            # --> Integer, String

class C
  overload :m, 0..9, String do | i, s |
    puts "m Digit, String called with #{i} and #{s}"
  end
end

c.m( 2, "Test" )                             # --> Digit, String
c.m( 20, "Test" )                            # --> Integer, String