From: Robert Klemme <bob.news@...>
Date: 2004-06-08T16:13:44+09:00
Subject: Method Checks (was: Re: How to ducktype a Hash?)


First of all I'd like to make clear that this is still not Duck Typing!
Duck Typing simply assumes the instance at hand is the one expected.  If
it's not the code may run anyway or you'll be bitten by a NoMethodError.

"Sean O'Dell" <sean@celsoft.com> schrieb im Newsbeitrag
news:200406070944.10162.sean@celsoft.com...

> I'm not sure what this means.  I'm not looking for duck typing per se,
just a
> way to type check hash-like objects.  The only two things I care about
when
> checking for something in this way are that the object responds to a
group of
> methods which all take the same number of parameter, and the group of
methods
> represent a described interface; that is, a known functionality.  In
this
> particular case, the functionality should be that of a hash, or a
hash-like
> object.

You can do it like this, although I still dislike the strong focus on type
and type like checks:

#!/usr/bin/ruby

class MethodCriterium
  def initialize
    @methods = {}
  end

  def clear; @methods.clear; end

  def add(sym, arity)
    @methods[sym.to_sym] = arity
    self
  end

  def add_from_template(obj)
    obj.methods.each do |meth|
      add(meth, obj.method(meth).arity)
    end

    self
  end

  def satisfies(obj)
    begin
      @methods.each do |meth, arity|
        return false if arity != obj.method(meth).arity
      end

      true
    rescue NameError
      return false
    end
  end

  alias :=== :satisfies
end


a_hash = MethodCriterium.new
a_hash.add(:[], 1).add(:[]=, 2)

p a_hash === {}
p a_hash === []

a_hash = MethodCriterium.new.add_from_template( {} )

p a_hash === {}
p a_hash === []

an_array = MethodCriterium.new.add_from_template( [] )

[ [], {}, 5 ].each do |o|
  case o
    when a_hash
      puts "#{o.inspect} is a hash!"
    when an_array
      puts "#{o.inspect} is an array!"
    else
      puts "dunno bout #{o.inspect}"
  end
end


>>>>  Use at own risk. <<<<


Regards

    robert