From: Robert Feldt <feldt@...>
Date: 2001-11-28T23:29:04+09:00
Subject: [ruby-talk:26792] Re: overload possible?

On Wed, 28 Nov 2001, ts wrote:

> >>>>> "W" == Wyss Clemens <WYS@helbling.ch> writes:
> 
> W> No, UNLESS you ask Guy Decoux (ts) to give you his *extension* 
> 
>  No, no. He want the dispatch on the *number* of arguments which personnaly
>  I don't like. I prefer the dispatch on the *type* of arguments.
> 
>  Like said in [ruby-talk:26788] it's easy to have dispatch on the number of
>  arguments.
> 
And since this request is quite common below is some code for creating the
dispatcher automagically. I'm not very fond of the philosophy in doing
this, the code is not elegant and can probably be much shorter and its
probably buggy but since people keep asking stuff like this, why not?

It can dispatch both on type or number.

Regards,

Robert


class Module
  private

  @@dispatch_db = Hash.new

  def dispatch(targetMethodId, methodId, dispatchSpec)
    insert_db(targetMethodId, methodId, dispatchSpec)
    create_dispatch_method(methodId)
  end

  def insert_db(targetMethodId, methodId, dispatchSpec)
    key = [self, methodId]
    h = @@dispatch_db[key] || Hash.new
    h[dispatchSpec] = targetMethodId   # Will overwrite previous w. same
hash!
    @@dispatch_db[key] = h
  end

  def create_dispatch_method(methodId)
    body = ""
    d = @@dispatch_db[[self, methodId]].sort do |a,b|
      if Array === a[0]
	Array === b[0] ? (a[0].length <=> b[0].length) : -1
      else
	Array === b[0] ? 1 : a[0] <=> b[0]
      end
    end
    d.each_with_index do |(spec, target), i|
      body += <<-EOS
        #{i == 0 ? 'if' : 'elsif'} #{spec_to_code(spec)}
          #{target.to_s}(*args)
      EOS
    end
    class_eval <<-EOS
      def #{methodId.to_s}(*args)
        #{body}
        else
	  raise "Could not dispatch"
        end
      end
    EOS
  end

  def spec_to_code(spec)
    if Integer === spec
      "args.length == #{spec}"
    elsif Array === spec
      i = -1
      spec.map {|ts| "#{ts.inspect} === args[#{i+=1}]"}.join(" && ")
    end
  end
end

class Test
  def m1(a); puts "m1"; end
  def m2(a, b); puts "m2"; end
  def m_int(i); puts "m_int"; end

  dispatch :m1, :m, 1
  dispatch :m2, :m, 2
  dispatch :m_int, :m, [Integer]
end

t = Test.new
t.m("1")
t.m("1", "2")
t.m(1)