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

On Thu, Mar 22, 2012 at 06:56, Xavier Noria <[e-mail]> wrote:
> On Thu, Mar 22, 2012 at 6:15 AM, Nikolai Weibull <[e-mail]> wrote:

>> =C2=A0I see that you completely cut out the part about const_defined? no=
t
>> calling const_missing, which is a rather big part of the problem with
>> using const_defined? in the first place.

> We are supposedly emulating defined? somehow but for constant paths and
> without going up the ancestor chain in each step doesn't it? defined? doe=
s
> not call const_missing.

True, I was seeing the wrong test output for that one.

>> An alternative is to check $LOADED_FEATURES. =C2=A0This isn=E2=80=99t
>> straightforward either, as it doesn=E2=80=99t contain the exact argument=
 given
>> to require. =C2=A0There are internal functions like rb_provided that cou=
ld
>> have been exposed to make it easy to check if a feature had been
>> loaded/is available.

> File names and class objects, and module objects, and constants... you
> cannot derive one from the other. They are decoupled in Ruby except for t=
he
> fact that the class/module keywords assign, and that if you assign an
> anonymus class/module to a constant, then its name is set after the
> constant.
>
> But in Ruby file foo.rb can define the constant Bar, which may hold a mod=
ule
> whose name is "Wadus". They are quite orthogonal features.

Yes, I realize that, but let=E2=80=99s forget the constant bit (as I tried =
to
do in the part that you cut out from the rest of this discussion on
$LOADED_FEATURES, require, and provided?) and focus on the original
problem of determining if a feature is available or not.  In my first
e-mail I explained that I=E2=80=99d been using defined? to perform such tes=
ts,
but that it doesn=E2=80=99t work as intended.  I proposed an alternative to
defined? that tried to walk a constant path without ever returning to
the top level, but I wasn=E2=80=99t happy with the solution and, as we=E2=
=80=99ve
seen, there are semantic issues with such a solution (should
const_missing be called or not?).  I was, however, originally looking
for a better alternative to the constant lookup altogether.  That=E2=80=99s
why I mentioned =E2=80=9Cfeature=E2=80=9D in my first e-mail.

As I said, Ruby uses the (expanded) path of the argument to require as
the =E2=80=9Cfeature=E2=80=9D.  If Ruby provided a convenient way to check =
if a path
was in $LOADED_FEATURES that=E2=80=99d solve my use case.  This can of cour=
se
be emulated, and I=E2=80=99m surely making too big a deal about this, but I=
=E2=80=99d
rather have Kernel.provided? that wraps the extant rb_provided than
having to define

def provided?(path)
  $LOADED_FEATURES.any?{ |e| e.end_with? path + File.extname(e) }
end

for each project that needs this functionality.  Finally, such a
definition can never truly emulate rb_provided (or what the return
value would be from require), as Ruby doesn=E2=80=99t expose the =E2=80=9Cl=
oading=E2=80=9D
table.  (This solution won=E2=80=99t take autoloads into account either, if
one wants that to be done, but they=E2=80=99re going away in 3.0, so let=E2=
=80=99s
ignore them ;-)