Feature #6376: Feature lookup and checking if feature is loaded

• Status: Assigned
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[trans (Thomas Sawyer)]

$LOADED_FEATURES is useful to know what "files" have been loaded. But it doesn't really tell us what "features" have been loaded. If there where were a way to look-up a load path, without actually loading it then it would be possible to compare that to $LOADED_FEATURES and thus know. e.g.

require 'ostruct'
$LOADED_FEATURES #=> [..., "/home/trans/.local/lib/ry/rubies/1.9.3-p125/lib/ruby/1.9.1/ostruct.rb"]

path = require_path('ostruct') #=> "/home/trans/.local/lib/ry/rubies/1.9.3-p125/lib/ruby/1.9.1/ostruct.rb"
$LOADED_FEATURES.include?(path)

Of course, it would be nice to also have:

required?('ostruct')  #=> true

These methods could be class methods of special module, if it's important to keep the Kernel more tidy, e.g. Ruby.required?('ostruct').

I am currently working on a project where I need this (and have a couple of other projects that could use it too) and I've had to implement the whole thing from scratch, which isn't simple, nor fast, nor am I 100% confident that it specs exactly to Ruby's own lookup procedure. So it would be much better if Ruby would expose its lookup functionality.

[trans (Thomas Sawyer)]

Oh, I forget to mention that there seems to be no way to see what the "current loading feature" is either, as it appears that it is not added to $LOADED_FEATURES until after loading is completed, which kind of surprised me. Is that right?

[now (Nikolai Weibull)]

On Sun, Apr 29, 2012 at 00:03, trans (Thomas Sawyer)
[e-mail] wrote:

These methods could be class methods of special module, if it's important to keep the Kernel more tidy, e.g. Ruby.required?('ostruct').

Why not add it to $LOADED_FEATURES?

[mame (Yusuke Endoh)]

• Status changed from Open to Feedback

Of course you know what is defined by the feature you loaded, don't you?
(If not, you must not load such a file; it is very dangerous)

So you can use: defined?(OpenStruct)

--
Yusuke Endoh [e-mail]

[now (Nikolai Weibull)]

On Thu, May 3, 2012 at 6:02 AM, mame (Yusuke Endoh) [e-mail] wrote:

Issue #6376 has been updated by mame (Yusuke Endoh).

Status changed from Open to Feedback

Of course you know what is defined by the feature you loaded, don't you?
(If not, you must not load such a file; it is very dangerous)

So you can use: defined?(OpenStruct)

Except that defined? won’t work for paths:

class A
end
class B
end
p defined?(A::B)

so if you have a file that provides A::B and B has already been
provided by another library, then defined? won’t work.

[mame (Yusuke Endoh)]

2012/5/3 Nikolai Weibull [e-mail]:

so if you have a file that provides A::B and B has already been
provided by another library, then defined? won’t work.

Then, defined?(A::B.some_class_method) or else.

Anyway, I don't think it is a good idea to depend whether a feature is
"loaded" by require or not.
What we really need to know is, whether a feature that you need is
"defined", doesn't it?

--
Yusuke Endoh [e-mail]

[trans (Thomas Sawyer)]

I think it depends. For on thing, a library's api can change over time. So that might not be the best fit, if what your asking is if library xyz is being used?. That's a more general question. Two different libraries might share some of the same module namespaces.

[now (Nikolai Weibull)]

On Thu, May 3, 2012 at 6:53 AM, Yusuke Endoh [e-mail] wrote:

2012/5/3 Nikolai Weibull [e-mail]:

so if you have a file that provides A::B and B has already been
provided by another library, then defined? won’t work.

Then, defined?(A::B.some_class_method) or else.

That’s far from good enough.

Anyway, I don't think it is a good idea to depend whether a feature is
"loaded" by require or not.
What we really need to know is, whether a feature that you need is
"defined", doesn't it?

Yes, certainly. Add loaded?/feature? that works like defined? except
that it doesn’t leak its lookup into parent namespaces.

[ko1 (Koichi Sasada)]

• Assignee set to mame (Yusuke Endoh)

mame-san, could you judge this ticket?

[mame (Yusuke Endoh)]

• Status changed from Feedback to Assigned
• Target version changed from 2.0.0 to 2.6

If I had a right to judge a feature request, I would reject this.
But actually I have no right. Assigning to matz. (virtual rejection?)

I don't see what OP really want to do. Doesn't Rescue'ing a LoadError from require help?

begin
require "foo"
FooExist = true
rescue LoadError
FooExist = false
end

A dirty operation should be performed by a dirty code.

--
Yusuke Endoh [e-mail]

[trans (Thomas Sawyer)]

=begin
@mame (Yusuke Endoh) Your example would load the library. The request is to know where a library comes from ((({require_path('ostruct')}))). As an additional benefit it would allow us to know if a library has been loaded or not. It's nothing to do with catching an load error.
=end

[mame (Yusuke Endoh)]

• Assignee changed from mame (Yusuke Endoh) to matz (Yukihiro Matsumoto)

Yes I know what you want. But I don't know why you want it. In general, I don't think that it is a good idea to depend on whether a feature is loaded or not. Rather, you should make sure to require what feature you need.

So, please elaborate your use case. I guessed one use case: you want to use one of many alternative libraries, for example, either eventmachine or Celluloid::IO. In such a case, you may want to try to require one, and if a LoadError is raised, then require the other.

--
Yusuke Endoh [e-mail]

[naruse (Yui NARUSE)]

• Target version deleted (2.6)