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Code style advice: where to put "use" statements?

by eyepopslikeamosquito (Canon)
on Feb 27, 2008 at 04:12 UTC ( #670475=perlquestion: print w/ replies, xml ) Need Help??
eyepopslikeamosquito has asked for the wisdom of the Perl Monks concerning the following question:

This came up during a code review today. I was reviewing code like this:

sub some_function { use SomeModule; # ... }
Now, I tend to put all use statements at the top of the file. Occasionally, I might put a require statement inside a sub if the module is big and slow to load or unavailable on some platforms, because require is performed at run time. However, given that use statements are performed at compile time, it seems confusing to me to put a use statement inside a sub.

I couldn't find any style advice on this issue in either Perl Best Practices or Perl::Critic. What advice would you give?

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Re: Code style advice: where to put "use" statements?
by kyle (Abbot) on Feb 27, 2008 at 04:44 UTC

    I've seen that practice also. I think it can make sense if some_function is the only place SomeModule is used. That way if the sub changes so that it doesn't use that module anymore, you can get rid of the use line right there.

    That said, I've never done it myself. The top of any module tends to look like this:

    package MyApp::Foo; use strict; use warnings; use Core::Module qw( import these ); use CPAN::Module; use MyApp::Module; use MyApp::Other::Module;

    I put the pragmas at the very top. Local modules are grouped together, and external modules are grouped together.

    I'd rather have everything at the top like that. The practice you show is liable to get cluttered, using the same module in multiple places. That doesn't cost anything in terms of resources, but I think it's a small speedbump to comprehension.

      This is exactly the same layout that I use (the same module blocks, the same blocks order and even the same newlines between the blocks ;-)

      Ciao,
      Emanuele.
Re: Code style advice: where to put "use" statements?
by BrowserUk (Pope) on Feb 27, 2008 at 04:46 UTC

    It does have the notional affect of stating that the module's use is limited, by convention rather than scoping, to the subroutine.

    However, there is nothing to enforce that, so the usage could 'leak', which could be construed as misleading. And what would you (they) do if a module is subsequently needed in another subroutine in the same file?


    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
      And what would you (they) do if a module is subsequently needed in another subroutine in the same file?

      Unless the other subroutine is in a different package it doesn't matter whether it also "use"s the module or not, it's loaded and will be available (I think you are probably aware of this, just putting it in as clarification). See this example

      #!/usr/bin/perl use strict; use warnings; mainsub(); secsub(); thirdsub(); Frob::fourthsub(); sub mainsub { use Data::Dumper; } sub secsub { my $r = "secsub"; print Dumper $r; } sub thirdsub { use Data::Dumper; my $r = "thirdsub"; print Dumper $r; } package Frob; sub fourthsub { my $r = "thirdsub"; print Dumper $r; }
      Output:
      Name "Frob::Dumper" used only once: possible typo at useuse.pl line 31 +. $VAR1 = 'secsub'; $VAR1 = 'thirdsub'; print() on unopened filehandle Dumper at useuse.pl line 31.

      IMO there are several very good reasons not to put "use" into a subroutine (or other block) but rather at the top of a package:

      • It obfuscates which modules you are using, one has to hunt through the entire file to find out which modules are loaded.
      • To make this more confusing, if the file contains multiple packages one needs to limit ones search to just that package, which requires thought instead of just editor-fu
      • Programmers unfamiliar with the compile-time loading property of "use" might be surprised/confused about symbols from that module being available in other subroutines
      • Conversely, programmers who come to rely on just sticking "use" anywhere in a file might be surprised that it's not available in a different package (this goes for putting "use" underneath the package declaration as well, but at least here the ordering gives a hint that the two are relevant to each other).

      All dogma is stupid.
        there are several very good reasons not to put "use" into a subroutine (or other block)

        I was pretty convinced already, but your list settles the matter for me.

        That said, for those not convinced that every abstraction is best served by being OOified, the notion of block-scoped imports is tantalising. Makes me wonder if a new pragma--say: uselocal Some::Module qw[ :stuff ]; wouldn't be possible.

        Unlike require, the module would be loaded at compile time, but the import list would not be processed until runtime.

        At runtime, the import list would be processed per use, but the callers package would be localised (and self initialised) first so that imports would disappear at close of block.

        The problem is how to localise a hash entry within the scope of the caller?

        BTW: I love your tagline. It's duality reminds of the old standby "this statement is false". Comtemplating it brings to mind the nightmares I had as a kid when I realised the implications of the infinity of space.


        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
Re: Code style advice: where to put "use" statements?
by Fletch (Chancellor) on Feb 27, 2008 at 04:50 UTC

    Since imports happen at a package level any symbols will still be available outside the sub in the same package, so that's not a reason to do it (just as an example why someone possibly might think they want to do it (in a misguided attempt to limit the scope of imports)).

    I think I might have done it once or twice when it's a module that's only used in a single sub rather than throughout the code (so you stash it away in that sub rather than in the "global" list of includes at the start of your code), but other than that I'd be biased against it.

    Update: Added extra parenthetical in parenthetical. And it looks like I'm not the only one that's done it for a "just used here" notation. Heh.

    The cake is a lie.
    The cake is a lie.
    The cake is a lie.

Re: Code style advice: where to put "use" statements?
by apl (Monsignor) on Feb 27, 2008 at 05:49 UTC
    The original implementor may have been thinking about moving some_function to a module. In any event, I'd move the statement to the top of the source as

    use SomeModule;  # employed solely by sub some_function as of YY/MM/DD

    YMMV.

Re: Code style advice: where to put "use" statements?
by rhesa (Vicar) on Feb 27, 2008 at 15:26 UTC
    I was going to suggest that maybe the following pattern might be stylistically sound:
    sub something { use Foo; # ... no Foo; }
    I was reminded of this by both lexical pragmata like strict, and Moose which advocates that in its code samples.

    The module in question would need to have a functioning unimport implementation, and a quick test with Exporter showed me that Exporter doesn't give you that for free, so it's probably not usually the case that no Foo would work at all.

    However, even if Foo had implemented unimport, I doubt it'd work inside subs. And I'm afraid I'm not fascinated enough to investigate further (because I don't see much practical value). I'll just agree with the general sentiment that use clauses preferably go near the top of the file.

Re: Code style advice: where to put "use" statements?
by Argel (Prior) on Feb 27, 2008 at 23:15 UTC
    I have a feeling that the reason it is not covered in PBP or elsewhere is it's taken for granted that they should be up at the top.

    As for putting it inside of a sub to indicate it is only used by that sub -- that could be confusing to less experienced Perl programmers and might cause it to get missed if someone is eyeballing the script for dependancies. It would be better to put it at the top and add a comment explaining where it is used.

Re: Code style advice: where to put "use" statements?
by sundialsvc4 (Monsignor) on Feb 28, 2008 at 03:18 UTC

    Devise a company standard and stick with it.

Re: Code style advice: where to put "use" statements?
by talexb (Canon) on Feb 28, 2008 at 04:11 UTC

    Unless a module really only applies to a very small scope, I put all of my use statements up at the top of the file.

    But if there's a piece of Perl code that needs to run as quickly as possible, I'd probably re-consider that approach. :)

    Alex / talexb / Toronto

    "Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds

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