From: Xavier Noria <fxn@...>
Date: 2012-08-06T03:32:06+09:00
Subject: Re: Redefining constants for a given instance only

--f46d042de42364319304c688fa10
Content-Type: text/plain; charset=UTF-8

On Sun, Aug 5, 2012 at 7:19 PM, Andrea Dallera <andrea@andreadallera.com>wrote:

 Hello,
>
>     let's say we have two empty classes:
>
> class ClassA
>
> end
>
> class ClassB
>
> end
>
> then we have a third one, which has a method that references the first
> class:
>
> class Evaluator
>
>     def evaluate
>        return ClassA
>    end
>
> end
>
> What I'd like to do is to be able to redefine what ClassA is for a given
> instance of Evaluator, like so:
>
> ev = Evaluator.new
> <redefine ClassA to ClassB in some way, for ev only>
> ev.evaluate #ClassB
>
> I found out how to do this, but only class-wide:
>
> ev = Evaluator.new
> ev.evaluate #ClassA
> class Evaluator
>   ClassA = ClassB
> end
> ev.evaluate #ClassB
>
> Ideas?
>
> Andrea
>

Constants are only storage, that method is returning a class object. The
normal implementation would be

    def evaluate
      if some condition
        ClassA
      else
        ClassB
      end
    end

because the caller gets a class object, the constant (or variable, or
method call, or whatever) #evaluate happened to take the class object from
is irrelevant to the caller.

Now, I guess from your question that approach is not good for your problem,
but in that case I'd like to understand why to suggest another one.

--f46d042de42364319304c688fa10
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

On Sun, Aug 5, 2012 at 7:19 PM, Andrea Dallera <span dir=3D"ltr">&lt;<a hre=
f=3D"mailto:andrea@andreadallera.com" target=3D"_blank">andrea@andreadaller=
a.com</a>&gt;</span> wrote:<div><br><div class=3D"gmail_quote"><blockquote =
class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid=
;padding-left:1ex">





<div>
<div style=3D"font:normal undefined undefined;color:undefined"><div>Hello,<=
/div><div><br></div><div>=C2=A0 =C2=A0 let&#39;s say we have two empty clas=
ses:</div><div><br></div><div>class ClassA</div><div><br></div><div>end</di=
v><div>




<br></div><div>class ClassB</div><div><br></div><div>end</div><div><br></di=
v><div>then we have a third one, which has a method that references the fir=
st class:</div><div><br></div><div>class Evaluator</div><div>=C2=A0 =C2=A0<=
/div>



<div>
=C2=A0 =C2=A0def evaluate</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0return Class=
A</div><div>=C2=A0 =C2=A0end</div><div><br></div><div>end</div><div><br></d=
iv><div>What I&#39;d like to do is to be able to redefine what ClassA is fo=
r a given instance of Evaluator, like so:</div>




<div><br></div><div>ev =3D Evaluator.new</div><div>&lt;redefine ClassA to C=
lassB in some way, for ev only&gt;</div><div>ev.evaluate #ClassB</div><div>=
<br></div><div>I found out how to do this, but only class-wide:</div><div>




<br></div><div>ev =3D Evaluator.new</div><div>ev.evaluate #ClassA</div><div=
>class Evaluator</div><div>=C2=A0 ClassA =3D ClassB</div><div>end</div><div=
>ev.evaluate #ClassB</div><div><br></div><div>Ideas?</div><div><br></div><d=
iv>Andrea</div>




</div></div></blockquote><div><br></div><div>Constants are only storage, th=
at method is returning a class object. The normal implementation would be</=
div><div><br></div><div>=C2=A0 =C2=A0 def evaluate</div><div>=C2=A0 =C2=A0 =
=C2=A0 if some condition</div>




<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 ClassA</div><div>=C2=A0 =C2=A0 =C2=A0 else=
</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 ClassB</div><div>=C2=A0 =C2=A0 =C2=
=A0 end</div><div>=C2=A0 =C2=A0 end</div><div><br></div><div>because the ca=
ller gets a class object, the constant (or variable, or method call, or wha=
tever) #evaluate happened to take the class object from is irrelevant to th=
e caller.</div>




<div><br></div><div>Now, I guess from your question that approach is not go=
od for your problem, but in that case I&#39;d like to understand why to sug=
gest another one.</div><div><br></div></div></div>

--f46d042de42364319304c688fa10--