From: Josh Cheek <josh.cheek@...>
Date: 2012-05-10T13:27:31+09:00
Subject: Re: How to make a given klass to include a module

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On Wed, May 9, 2012 at 6:36 AM, I=F1aki Baz Castillo <ibc@aliax.net> wrote:

> Hi, simple code:
>
> ----------------------------
>  module M
>    def hello
>      "HELLO"
>    end
>  end
>
>  class A
>  end
>
>
>  klass =3D A
> ----------------------------
>
> Now I want to make an instance of klass (so an instance of A) but also
> need that klass includes module M.
> Note however that I cannot change class A definition since klass is
> given in runtime by the user.
>
> NOTE: It must be efficient, I don't want to use eval and so.
>
> Thanks a lot.
>
>
Since we don't want to change A, use a subclass. It will have all the
behaviour of A, but you can include modules and so forth while leaving M in
pristine condition.

klass =3D Class.new(A) { include M }
klass.new.hello # =3D> "HELLO"
A.new.respond_to? :hello # =3D> false


Alternatively, extend your instances:

klass =3D A
instance =3D klass.new
instance.extend M
instance.hello # =3D> "HELLO"
A.new.respond_to? :hello # =3D> false

Extending works by opening up the singleton class and including within it.
This means the instance behaves as if A had M included, even though it
doesn't.

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<div class=3D"gmail_quote">On Wed, May 9, 2012 at 6:36 AM, I=F1aki Baz Cast=
illo <span dir=3D"ltr">&lt;<a href=3D"mailto:ibc@aliax.net" target=3D"_blan=
k">ibc@aliax.net</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote"=
 style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi, simple code:<br>
<br>
----------------------------<br>
 =A0module M<br>
 =A0 =A0def hello<br>
 =A0 =A0 =A0&quot;HELLO&quot;<br>
 =A0 =A0end<br>
 =A0end<br>
<br>
 =A0class A<br>
 =A0end<br>
<br>
<br>
 =A0klass =3D A<br>
----------------------------<br>
<br>
Now I want to make an instance of klass (so an instance of A) but also<br>
need that klass includes module M.<br>
Note however that I cannot change class A definition since klass is<br>
given in runtime by the user.<br>
<br>
NOTE: It must be efficient, I don&#39;t want to use eval and so.<br>
<br>
Thanks a lot.<br><br></blockquote><div><br></div><div>Since we don&#39;t wa=
nt to change A, use a subclass. It will have all the behaviour of A, but yo=
u can include modules and so forth while leaving M in pristine condition.</=
div>
<div><br></div><div><div>klass =3D Class.new(A) { include M }=A0</div><div>=
klass.new.hello # =3D&gt; &quot;HELLO&quot;</div><div>A.new.respond_to? :he=
llo # =3D&gt; false</div></div><div><br></div><div><br></div><div>Alternati=
vely, extend your instances:</div>
<div><div><br></div><div>klass =3D A</div><div>instance =3D klass.new</div>=
<div>instance.extend M</div><div>instance.hello # =3D&gt; &quot;HELLO&quot;=
</div><div>A.new.respond_to? :hello # =3D&gt; false</div></div><div><br></d=
iv><div>
Extending works by opening up the singleton class and including within it. =
This means the instance behaves as if A had M included, even though it does=
n&#39;t.</div></div>

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