Re^4: Is 'use vars' really obsolete?

Did you catch the last line of my post: FWIW, the module being used, exported variable names with values corresponding to found paths of their lower-case names as executables, so any EXPORT list would be variable per usage (though fixed per program invocation).

I didn't think Exporter would work in this case because, 1) I'm not exporting anything, and 2) the programs using the module get differently named IMPORTS based on what they pass (not based on anything in the used module -- i.e. I'm not exporting any package variables.

Example: I had a program that created file-system snapshots. In main I needed to call ~10 external programs, so I had:

  use Cmds qw(sudo dd rsync dmsetup lvremove touch cp rm mount umount 
+);
[download]

In places in the program where I wanted to call a program, I'd construct a command. Since the program ran part-time as root, I wanted to make sure that common commands resolved to the real commands in the right places. Additionally, I didn't want it to fail unpredictably -- but instead fail at startup there would be no changes to the system state. In the above usage, Cmds uses a safe internal path and then puts variables containing the absolute paths of the commands into the caller's package name space. The variables never exist in Cmds, so I don't see that Exporter would even work in this case.

The calling program would reference the external command paths by using the up-cased name of the command as a variable. Example:

open ($fh, "|$Sudo $Dd conv=fsync of=$fn 2>/dev/null");
[download]

So Cmds allows me to call external commands, safely, using builtin paths, allows me to use those paths via variables that can be used in strings, and ensures presence of needed external commands before running the program.

As for the use case being complicated, it was easier to make work 7 years ago than other methods. I just ran across it when checking something out in the module and it seemed like an interesting case where 'our' wasn't an easy replacement for use vars.

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Re^5: Is 'use vars' really obsolete? by LanX (Saint) on Sep 23, 2017 at 00:39 UTC
> Did you catch the last line of my post: No I have problems to parse the phrase structure, sorry. FWIW: I can think of three ways of a "dynamic" import without eval Exporter has an `export_to_level()` method see http://search.cpan.org/~toddr/Exporter-5.72/lib/Exporter.pm#Exporting_Without_Using_Exporter's_import_Method you could call `vars->import(List)` inside your own `import()` you could just directly set the stash just like `vars` does, see my other post Re^4: Is 'use vars' really obsolete? (use vars mechanics) None tested. Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply] [d/l] [select]
Re^6: Is 'use vars' really obsolete? by perl-diddler (Chaplain) on Sep 23, 2017 at 01:58 UTC
Did you catch the last line of my post: No I have problems to parse the phrase structure, sorry. No, I'm sorry! Thank you for your patience. Anyway, ignoring that -- I tried to "re-explain it" -- why using Exporter would be a problem. I probably could do something similar with the "export_to_level" ... but why? It seems like overkill to pull in Exporter to do what took 3-4 lines of code in my module -- just to get "away" from using "`use vars`". On calling `vars->import(List)`, do you mean in the program that's using the module? Where would var->import import things from? I.e. do you mean I'd do something like: `use Cmds qw(sudo dd); vars->import(Sudo Dd);` [download] It seems cleaner to only specify the external cmd list once, but I'm not sure this is what you meant. As for setting the stash -- probably could but that'd involve getting more into the internals of perl and I'd prefer to not do that, since the perl internals could easily change in a new release and then I'd have to fix my usage of it. More work! ARG! :-) Why would I want to steer away from the eval? This is code that is usually only executed once at `BEGIN` time. While I may not want to go back and change this module, I probably do want to go change some other modules I care about more, I'm still not clear why I'd prefer to get deeper into perl internals to get away from a "generic" eval? Most of my modules work back to 5.8 and some back to 5.6. I usually try to spend sometime to use less advanced methods so I can be compatible with older perls. Not always successful, but I still try. -linda	[reply] [d/l] [select]
Re^7: Is 'use vars' really obsolete? by LanX (Saint) on Sep 23, 2017 at 11:21 UTC
> On calling vars->import(List), do you mean in the program that's using the module? Where would var->import import things from? No you call it inside your import(). at least that's what works with `strict` and `warnings` See Re: Code Reuse > since the perl internals could easily change in a new release That's extremely unlikely in this case, since it goes to the very foundations of Perl 4, symbol tables and strict. And if there was any kind of such change of internals, it would most likely imply killing `use vars` off, and you'd have more serious problems then. It's like the chance of the planet spontaneously swapping magnetic poles. ;) Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply] [d/l]
Re^8: Is 'use vars' really obsolete? by Anonymous Monk on Sep 23, 2017 at 16:58 UTC
Re^9: Is 'use vars' really obsolete? by perl-diddler (Chaplain) on Sep 24, 2017 at 21:14 UTC
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Re^9: Is 'use vars' really obsolete? by LanX (Saint) on Sep 23, 2017 at 19:15 UTC


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