use: distinguishing compile-time vs run-time dependencies

perl5ever has asked for the wisdom of the Perl Monks concerning the following question:

I was wonder if anyone has found it valuable to distinguish between compile-time and run-time module dependencies.

Example - in this case the module Fcntl is a compile-time dependency of module Bar:

  package Bar;
  use Smart::Comments;
  sub blather {
    ### $foo
  }
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However, in this example, LWP::UserAgent is just a run-time dependency of module Foo:

  package Foo;
  use LWP::UserAgent;

  sub make_agent { LWP::UserAgent->new }
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It could also be written like this:

  package Foo;
  # NOTE: this module needs LWP::UserAgent loaded in order to work
  sub make_agent { LWP::UserAgent->new }
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It would then be some other code's responsibility to ensure that LWP::UserAgent was loaded before any subs in package Foo were called.

Specifically, a compile-time dependency is a module which needs to be loaded in order for a compile-check to work. A run-time dependency is one that only required to be loaded at run time.

So the question again - has anyone found it useful to formally declare a module's run-time only dependencies - perhaps with a pragma like:

use runtime LWP::UserAgent;
[download]

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Re: use: distinguishing compile-time vs run-time dependencies by chromatic (Archbishop) on Aug 08, 2011 at 00:59 UTC
That sounds like dependency injection (part of the larger system of inversion of control). I hadn't thought of a formal specification for such things, but that's an interesting concept. See perhaps Bread::Board for practical uses. A `injection` framework might even export a constant of the appropriate name (`LWP::UserAgent`) such that the IoC registry could get the appropriate class object. Interesting.	[reply] [d/l] [select]
Re: use: distinguishing compile-time vs run-time dependencies by moritz (Cardinal) on Aug 08, 2011 at 01:06 UTC
require `LWP::UserAgent` There you go, declares a run-time dependency, and even loads the module (without importing) if it hasn't been done yet. Perl 6 - second systems done right	[reply] [d/l]
Re^2: use: distinguishing compile-time vs run-time dependencies by perl5ever (Pilgrim) on Aug 08, 2011 at 02:33 UTC
`require` basically does the job, but it could be useful to be able to gather the use information more formally.	[reply] [d/l]
Re^2: use: distinguishing compile-time vs run-time dependencies by perl5ever (Pilgrim) on Aug 08, 2011 at 16:31 UTC
One advantage of a 'use'-pragma to declare the run-time dependency is that the module name can be checked at compile time, e.g.: `use required Some::Module;` [download] can verify that `Some::Module` is in the `@INC` path at compile time. Simply using `require Some::Module` won't do that.	[reply] [d/l] [select]
Re: use: distinguishing compile-time vs run-time dependencies by Khen1950fx (Canon) on Aug 08, 2011 at 01:10 UTC
I used Module::Runtime. For example: `#!/usr/bin/perl use strict; use warnings; use Test::More tests => 3; BEGIN { use_ok "Module::Runtime", qw(use_module); } my $run = use_module("LWP::UserAgent", 6.02); my ($result, $err); sub test_use_module($;$;) { my($name, $version) = @_; $result = eval { use_module($name, $version ) }; $err = $@; } test_use_module("LWP::UserAgent", 6.02); is( $err, "" ); is( $result, "LWP::UserAgent");` [download]	[reply] [d/l]
Re: use: distinguishing compile-time vs run-time dependencies by JavaFan (Canon) on Aug 08, 2011 at 10:11 UTC
Specifically, a compile-time dependency is a module which needs to be loaded in order for a compile-check to work. A run-time dependency is one that only required to be loaded at run time. That isn't very different than solving the halting problem, isn't it? Suppose a module "Dependency::Checker" exists, which exports a subroutine `runtime_dependency` that does what it says: `package Foo; use Dependency::Checker 'runtime_dependency'; use LWP::UserAgent; sub make_agent {LWP::UserAgent->new} BEGIN { make_agent() if runtime_dependency("LWP::UserAgent"); } 1;` [download] What will happen?	[reply] [d/l] [select]
Re^2: use: distinguishing compile-time vs run-time dependencies by perl5ever (Pilgrim) on Aug 08, 2011 at 16:29 UTC
I'm not asking for a why to figure out what kind of dependency a module is. I'm just asking whether it is useful to _declare_ what kind of dependency a module is.	[reply]
Re^3: use: distinguishing compile-time vs run-time dependencies by JavaFan (Canon) on Aug 08, 2011 at 16:44 UTC
Well, unless the declaration isn't enforced (that is, it's only informative), it's the same problem. But if it's only informative, you can already do this; it's called "comments".	[reply]