Re: What’s the best way to check if a feature/class has been loaded?

On Wed, Mar 21, 2012 at 22:11, Xavier Noria <[e-mail]> wrote:
> On Wed, Mar 21, 2012 at 9:51 PM, Nikolai Weibull <[e-mail]> wrote:
>
>> There=E2=80=99s a timing issue with using const_defined? followed by con=
st_get
>> instead of calling const_get directly and catching NameError.
>
>
> What do you mean?

I mean that if const_defined? returns true, then between that code
being executed and the call to const_get, some other piece of code may
remove the constant, resulting in an uncaught NameError.

> The problem with const_get is that it follows the ancestors, of course, a=
nd
> you have no way around that in 1.8.

Yes, we=E2=80=99ve already come to that conclusion.  You don=E2=80=99t need=
 to keep
repeating it.

I see that you completely cut out the part about const_defined? not
calling const_missing, which is a rather big part of the problem with
using const_defined? in the first place.

>> I guess
>>
>> def feature?(path)
>> =C2=A0 path.split('::').reduce(Object){ |o, e|
>> =C2=A0 =C2=A0 c =3D begin o.const_get(e); rescue NameError; return nil e=
nd
>> =C2=A0 =C2=A0 return nil if c =3D=3D (o !=3D Object and begin Object.con=
st_get(e);
>> rescue NameError; nil end)
>> =C2=A0 =C2=A0 c
>> =C2=A0 }
>> end

> Nikolai, I don't really understand what you need.

Xavier, please don=E2=80=99t begin sentences with the person you=E2=80=99re=
 responding
to=E2=80=99s name like that.  It=E2=80=99s like your speaking to me as if I=
 was a
child and it feels very condescending.

> You are askind for
> defined? X::Y that goes step by step and your original code does raise an
> error for X::Y integer.

I don=E2=80=99t follow.

> Then this second version cannot say that X::Y is defined in
>
> =C2=A0 =C2=A0 module X
> =C2=A0 =C2=A0 =C2=A0 Y =3D nil
> =C2=A0 =C2=A0end

True.

> And you refuse to reuse much more simple and proven existing solutions.

You=E2=80=99re making it sound like I=E2=80=99m being stubborn.  I just don=
=E2=80=99t think
your proposed solution is better.

> You want something with so special semantics that definitely is NOT
> built-in. That answers you original question.

Thank you for building me a straw man.  I=E2=80=99ve always wanted one.

I want a way to tell if a class or module has been defined/loaded.
The check would then be to see if X was loaded in your example above,
not X::Y.

An alternative is to check $LOADED_FEATURES.  This isn=E2=80=99t
straightforward either, as it doesn=E2=80=99t contain the exact argument gi=
ven
to require.  There are internal functions like rb_provided that could
have been exposed to make it easy to check if a feature had been
loaded/is available.

This part of Ruby is surely inspired by Emacs=E2=80=99 way of doing this, s=
ee

http://www.gnu.org/software/emacs/manual/html_node/elisp/Named-Features.htm=
l

but doesn=E2=80=99t provide a way to check if a feature has been loaded.
(There=E2=80=99s a difference here, of course, as Ruby implicitly uses the
(expanded) path, with possible suffixes/extensions, of the argument to
require as the feature, not a symbol as Emacs forces you to explicitly
use as an argument to provide.)

To sum up, ruby should provide a method =E2=80=9Cprovided?=E2=80=9D that ch=
ecks if its
argument would return true or false when given as an argument to
require.  Provided? would return this results negation.

Checking the presence of constants is a workaround that I=E2=80=99ve been
using in the past, but was recently burned by, as, even in the case of
using defined?, this may travel unwanted paths in the resolution of
such constants.