From: Sam Roberts <sroberts@...>
Date: 2005-03-14T06:40:57+09:00
Subject: Re: how do you duck-type something to String, so String believes you?

Quoting ruby-ml@magical-cat.org, on Mon, Mar 14, 2005 at 04:12:24AM +0900:
> On 3/13/2005, "Sam Roberts" <sroberts@uniserve.com> wrote:
> 
> >I can give something a #to_str, which should be an indication that it is
> >duck-type compat to a String (as opposed to #to_s, which converts
> >something to a String).
> >
> >That's all fine, but that doesn't mean that String will allow itself to
> >be compared to my class:

...

> >I can add as many methods as I want to String, I can even proxy every
> >single method in it using #undefined_method (which is as duck-typed as
> >you can get) but I still don't think a String object will ever "== =>
> >true" to my class.
> >
> >Am I right about this?
> 
> If your class defines #to_str and #<==>, it should work.
> String#== checks for #to_str, if that's OK, transfers to
> String#<==> which checks for #to_str, if that's OK, transfers
> to OtherClass#<==>. Then your #<==> must take a String and
> compare it returning -1, 0 or 1.


Did some playing around with this. What I see is that if you want:

  "somestr" == your_obj

your_obj needs #to_str and #== (#<=> won't do it)


  "somestr" <=> your_obj

your_obj needs #to_str and #<=>


  "somestr".eql?(your_obj)

you can't do do this.


So String will 'invert" == and <=> if the argument has a #to_str, but no
other methods will be inverted.


Do I understand correctly?

Sam

The code I played with:

class Str
  def to_str; 'str'; end
# def eql?(s); true; end
# def ==(s); true; end
  def eql?(s); true; end
# def <=>(s); 0; end
end

p Str.new == 'a'
p 'a' == Str.new

p Str.new.eql?('a')
p 'a'.eql?(Str.new)

p Str.new <=> 'a'
p 'a' <=> Str.new