From: Ehsanul Hoque <ehsanul_g3@...>
Date: 2009-11-20T14:25:20+09:00
Subject: Re: eval behaves differently inside a method definition...

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Hi Tim.

> #lets give strings the ability to evaluate theirselves as if in irb.
> >> class String
> >> def irb
> >> eval self
> >> end
> >> end
> =3D> nil
> >> "puts 'look at me'"
> =3D> "puts 'look at me'"
> >> "puts 'look at me'".irb
> look at me
> =3D> nil
>=20
> #good looks like strings can interpret theirselves. But how about this
> case?
>=20
> >> "class Pumpkin=3Bdef pie=3Bputs 'yum'=3Bend=3Bend".irb
> =3D> nil
> >> Pumpkin.new
> NameError: uninitialized constant Pumpkin
> 	from (irb):287
> 	from /usr/local/bin/irb:12:in `<main>'
>=20
> #not flying. Is it a problem with the string? Try with eval of the
> same string:
>=20
> >> eval "class Pumpkin=3Bdef pie=3Bputs 'yum'=3Bend=3Bend"
> =3D> nil
> >> Pumpkin.new
> =3D> #<Pumpkin:0x109896c>
>=20
> No problem evaluating the class definition in a string. So eval
> behaves differently inside a method definition.

It's not that eval behaves differently inside a method definition. It's jus=
t that it takes into account the scope in which it was called.

When inside the String class=2C or any other class for that matter=2C if yo=
u create another class=2C you can't call it from outside the parent class w=
ithout referring to the parent. So for example:

class String
  class Pumpkin
    def pie
       puts 'yum'
    end
  end
end
Pumpkin.new  #=3D> NameError: uninitialized constant Pumpkin
String::Pumpkin.new #=3D> #<Pumpkin:0x109896c>

The parent class effectively acts as a namespace for the subclass. BTW=2C I=
 really don't think I should call it a "subclass"=2C as that seems to imply=
 inheritence - none of that here. Maybe someone else can tell us the proper=
 name for this structure/pattern.

So you probably understand the issue by now=2C eval within the String class=
 (whether or not inside a method definition) creates a "subclass" of sorts=
=2C and hence you can't call it directly outside. If you tried to call "Str=
ing::Pumpkin.new" instead after using your "irb" method=2C it would have wo=
rked (try it out). Alternately=2C you could have done it this way:

class String
def irb
eval self
end
end
"class ::Pumpkin=3Bdef pie=3Bputs 'yum'=3Bend=3Bend".irb
Pumpkin.new.pie

Hope that helps.
- Ehsan
 		 	   		 =20
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