phpman > perldoc > common::sense(3pm)

NAME
    common::sense - save a tree AND a kitten, use common::sense!

SYNOPSIS
       use common::sense;

       # Supposed to be mostly the same, with much lower memory usage, as:

       # use utf8;
       # use strict qw(vars subs);
       # use feature qw(say state switch);
       # use feature qw(unicode_strings unicode_eval current_sub fc evalbytes);
       # no feature qw(array_base);
       # no warnings;
       # use warnings qw(FATAL closed threads internal debugging pack
       #                 prototype inplace io pipe unpack malloc glob
       #                 digit printf layer reserved taint closure semicolon);
       # no warnings qw(exec newline unopened);

DESCRIPTION
       “Nothing is more fairly distributed than common sense: no one thinks
       he needs more of it than he already has.”

       – René Descartes

    This module implements some sane defaults for Perl programs, as defined by two typical (or not
    so typical - use your common sense) specimens of Perl coders. In fact, after working out details
    on which warnings and strict modes to enable and make fatal, we found that we (and our code
    written so far, and others) fully agree on every option, even though we never used warnings
    before, so it seems this module indeed reflects a "common" sense among some long-time Perl
    coders.

    The basic philosophy behind the choices made in common::sense can be summarised as: "enforcing
    strict policies to catch as many bugs as possible, while at the same time, not limiting the
    expressive power available to the programmer".

    Two typical examples of how this philosophy is applied in practise is the handling of
    uninitialised and malloc warnings:

    *uninitialised*
        "undef" is a well-defined feature of perl, and enabling warnings for using it rarely catches
        any bugs, but considerably limits you in what you can do, so uninitialised warnings are
        disabled.

    *malloc*
        Freeing something twice on the C level is a serious bug, usually causing memory corruption.
        It often leads to side effects much later in the program and there are no advantages to not
        reporting this, so malloc warnings are fatal by default.

    Unfortunately, there is no fine-grained warning control in perl, so often whole groups of useful
    warnings had to be excluded because of a single useless warning (for example, perl puts an
    arbitrary limit on the length of text you can match with some regexes before emitting a warning,
    making the whole "regexp" category useless).

    What follows is a more thorough discussion of what this module does, and why it does it, and
    what the advantages (and disadvantages) of this approach are.

RATIONALE
    use utf8
        While it's not common sense to write your programs in UTF-8, it's quickly becoming the most
        common encoding, is the designated future default encoding for perl sources, and the most
        convenient encoding available (you can do really nice quoting tricks...). Experience has
        shown that our programs were either all pure ascii or utf-8, both of which will stay the
        same.

        There are few drawbacks to enabling UTF-8 source code by default (mainly some speed hits due
        to bugs in older versions of perl), so this module enables UTF-8 source code encoding by
        default.

    use strict qw(subs vars)
        Using "use strict" is definitely common sense, but "use strict 'refs'" definitely overshoots
        its usefulness. After almost two decades of Perl hacking, we decided that it does more harm
        than being useful. Specifically, constructs like these:

           @{ $var->[0] }

        Must be written like this (or similarly), when "use strict 'refs'" is in scope, and $var can
        legally be "undef":

           @{ $var->[0] || [] }

        This is annoying, and doesn't shield against obvious mistakes such as using "", so one would
        even have to write (at least for the time being):

           @{ defined $var->[0] ? $var->[0] : [] }

        ... which nobody with a bit of common sense would consider writing: clear code is clearly
        something else.

        Curiously enough, sometimes perl is not so strict, as this works even with "use strict" in
        scope:

           for (@{ $var->[0] }) { ...

        If that isn't hypocrisy! And all that from a mere program!

    use feature qw(say state given ...)
        We found it annoying that we always have to enable extra features. If something breaks
        because it didn't anticipate future changes, so be it. 5.10 broke almost all our XS modules
        and nobody cared either (or at least I know of nobody who really complained about gratuitous
        changes - as opposed to bugs).

        Few modules that are not actively maintained work with newer versions of Perl, regardless of
        use feature or not, so a new major perl release means changes to many modules - new keywords
        are just the tip of the iceberg.

        If your code isn't alive, it's dead, Jim - be an active maintainer.

        But nobody forces you to use those extra features in modules meant for older versions of
        perl - common::sense of course works there as well. There is also an important other mode
        where having additional features by default is useful: commandline hacks and internal use
        scripts: See "much reduced typing", below.

        There is one notable exception: "unicode_eval" is not enabled by default. In our opinion,
        "use feature" had one main effect - newer perl versions don't value backwards compatibility
        and the ability to write modules for multiple perl versions much, after all, you can use
        feature.

        "unicode_eval" doesn't add a new feature, it breaks an existing function.

    no warnings, but a lot of new errors
        Ah, the dreaded warnings. Even worse, the horribly dreaded "-w" switch: Even though we don't
        care if other people use warnings (and certainly there are useful ones), a lot of warnings
        simply go against the spirit of Perl.

        Most prominently, the warnings related to "undef". There is nothing wrong with "undef": it
        has well-defined semantics, it is useful, and spitting out warnings you never asked for is
        just evil.

        The result was that every one of our modules did "no warnings" in the past, to avoid
        somebody accidentally using and forcing his bad standards on our code. Of course, this
        switched off all warnings, even the useful ones. Not a good situation. Really, the "-w"
        switch should only enable warnings for the main program only.

        Funnily enough, perllexwarn explicitly mentions "-w" (and not in a favourable way, calling
        it outright "wrong"), but standard utilities, such as prove, or MakeMaker when running "make
        test", still enable them blindly.

        For version 2 of common::sense, we finally sat down a few hours and went through *every
        single warning message*, identifying - according to common sense - all the useful ones.

        This resulted in the rather impressive list in the SYNOPSIS. When we weren't sure, we didn't
        include the warning, so the list might grow in the future (we might have made a mistake,
        too, so the list might shrink as well).

        Note the presence of "FATAL" in the list: we do not think that the conditions caught by
        these warnings are worthy of a warning, we *insist* that they are worthy of *stopping* your
        program, *instantly*. They are *bugs*!

        Therefore we consider "common::sense" to be much stricter than "use warnings", which is good
        if you are into strict things (we are not, actually, but these things tend to be
        subjective).

        After deciding on the list, we ran the module against all of our code that uses
        "common::sense" (that is almost all of our code), and found only one occurrence where one of
        them caused a problem: one of elmex's (unreleased) modules contained:

           $fmt =~ s/([^\s\[]*)\[( [^\]]* )\]/\x0$1\x1$2\x0/xgo;

        We quickly agreed that indeed the code should be changed, even though it happened to do the
        right thing when the warning was switched off.

    much reduced typing
        Especially with version 2.0 of common::sense, the amount of boilerplate code you need to add
        to get *this* policy is daunting. Nobody would write this out in throwaway scripts,
        commandline hacks or in quick internal-use scripts.

        By using common::sense you get a defined set of policies (ours, but maybe yours, too, if you
        accept them), and they are easy to apply to your scripts: typing "use common::sense;" is
        even shorter than "use warnings; use strict; use feature ...".

        And you can immediately use the features of your installed perl, which is more difficult in
        code you release, but not usually an issue for internal-use code (downgrades of your
        production perl should be rare, right?).

    mucho reduced memory usage
        Just using all those pragmas mentioned in the SYNOPSIS together wastes <blink>*776
        kilobytes*</blink> of precious memory in my perl, for *every single perl process using our
        code*, which on our machines, is a lot. In comparison, this module only uses *four*
        kilobytes (I even had to write it out so it looks like more) of memory on the same platform.

        The money/time/effort/electricity invested in these gigabytes (probably petabytes globally!)
        of wasted memory could easily save 42 trees, and a kitten!

        Unfortunately, until everybody applies more common sense, there will still often be modules
        that pull in the monster pragmas. But one can hope...

THERE IS NO 'no common::sense'!!!! !!!! !!
    This module doesn't offer an unimport. First of all, it wastes even more memory, second, and
    more importantly, who with even a bit of common sense would want no common sense?

STABILITY AND FUTURE VERSIONS
    Future versions might change just about everything in this module. We might test our modules and
    upload new ones working with newer versions of this module, and leave you standing in the rain
    because we didn't tell you. In fact, we did so when switching from 1.0 to 2.0, which enabled
    gobs of warnings, and made them FATAL on top.

    Maybe we will load some nifty modules that try to emulate "say" or so with perls older than 5.10
    (this module, of course, should work with older perl versions - supporting 5.8 for example is
    just common sense at this time. Maybe not in the future, but of course you can trust our common
    sense to be consistent with, uhm, our opinion).

WHAT OTHER PEOPLE HAD TO SAY ABOUT THIS MODULE
    apeiron

       "... wow"
       "I hope common::sense is a joke."

    crab

       "i wonder how it would be if joerg schilling wrote perl modules."

    Adam Kennedy

       "Very interesting, efficient, and potentially something I'd use all the time."
       [...]
       "So no common::sense for me, alas."

    H.Merijn Brand

       "Just one more reason to drop JSON::XS from my distribution list"

    Pista Palo

       "Something in short supply these days..."

    Steffen Schwigon

       "This module is quite for sure *not* just a repetition of all the other
       'use strict, use warnings'-approaches, and it's also not the opposite.
       [...] And for its chosen middle-way it's also not the worst name ever.
       And everything is documented."

    BKB

       "[Deleted - thanks to Steffen Schwigon for pointing out this review was
       in error.]"

    Somni

       "the arrogance of the guy"
       "I swear he tacked somenoe else's name onto the module
       just so he could use the royal 'we' in the documentation"

    Anonymous Monk

       "You just gotta love this thing, its got META.json!!!"

    dngor

       "Heh.  '"<elmex at ta-sa.org>"'  The quotes are semantic
       distancing from that e-mail address."

    Jerad Pierce

       "Awful name (not a proper pragma), and the SYNOPSIS doesn't tell you
       anything either. Nor is it clear what features have to do with "common
       sense" or discipline."

    acme

       "THERE IS NO 'no common::sense'!!!! !!!! !!"

    apeiron (meta-comment about us commenting^Wquoting his comment)

       "How about quoting this: get a clue, you fucktarded amoeba."

    quanth

       "common sense is beautiful, json::xs is fast, Anyevent, EV are fast and
       furious. I love mlehmannware ;)"

    apeiron

       "... it's mlehmann's view of what common sense is. His view of common
       sense is certainly uncommon, insofar as anyone with a clue disagrees
       with him."

    apeiron (another meta-comment)

       "apeiron wonders if his little informant is here to steal more quotes"

    ew73

       "... I never got past the SYNOPSIS before calling it shit."
       [...]
       How come no one ever quotes me. :("

    chip (not willing to explain his cryptic questions about links in Changes files)

       "I'm willing to ask the question I've asked. I'm not willing to go
       through the whole dance you apparently have choreographed. Either
       answer the completely obvious question, or tell me to fuck off again."

FREQUENTLY ASKED QUESTIONS
    Or frequently-come-up confusions.

    Is this module meant to be serious?
        Yes, we would have put it under the "Acme::" namespace otherwise.

    But the manpage is written in a funny/stupid/... way?
        This was meant to make it clear that our common sense is a subjective thing and other people
        can use their own notions, taking the steam out of anybody who might be offended (as some
        people are always offended no matter what you do).

        This was a failure.

        But we hope the manpage still is somewhat entertaining even though it explains boring
        rationale.

    Why do you impose your conventions on my code?
        For some reason people keep thinking that "common::sense" imposes process-wide limits, even
        though the SYNOPSIS makes it clear that it works like other similar modules - i.e. only
        within the scope that "use"s them.

        So, no, we don't - nobody is forced to use this module, and using a module that relies on
        common::sense does not impose anything on you.

    Why do you think only your notion of common::sense is valid?
        Well, we don't, and have clearly written this in the documentation to every single release.
        We were just faster than anybody else w.r.t. to grabbing the namespace.

    But everybody knows that you have to use strict and use warnings, why do you disable them?
        Well, we don't do this either - we selectively disagree with the usefulness of some warnings
        over others. This module is aimed at experienced Perl programmers, not people migrating from
        other languages who might be surprised about stuff such as "undef". On the other hand, this
        does not exclude the usefulness of this module for total newbies, due to its strictness in
        enforcing policy, while at the same time not limiting the expressive power of perl.

        This module is considerably *more* strict than the canonical "use strict; use warnings", as
        it makes all its warnings fatal in nature, so you can not get away with as many things as
        with the canonical approach.

        This was not implemented in version 1.0 because of the daunting number of warning categories
        and the difficulty in getting exactly the set of warnings you wish (i.e. look at the
        SYNOPSIS in how complicated it is to get a specific set of warnings - it is not reasonable
        to put this into every module, the maintenance effort would be enormous).

    But many modules "use strict" or "use warnings", so the memory savings do not apply?
        I suddenly feel sad...

        But yes, that's true. Fortunately "common::sense" still uses only a miniscule amount of RAM.

    But it adds another dependency to your modules!
        It's a fact, yeah. But it's trivial to install, most popular modules have many more
        dependencies. And we consider dependencies a good thing - it leads to better APIs, more
        thought about interworking of modules and so on.

    Why do you use JSON and not YAML for your META.yml?
        This is not true - YAML supports a large subset of JSON, and this subset is what META.yml is
        written in, so it would be correct to say "the META.yml is written in a common subset of
        YAML and JSON".

        The META.yml follows the YAML, JSON and META.yml specifications, and is correctly parsed by
        CPAN, so if you have trouble with it, the problem is likely on your side.

    But! But!
        Yeah, we know.

AUTHOR
     Marc Lehmann <schmorp AT schmorp.de>
     http://home.schmorp.de/

     Robin Redeker, "<elmex at ta-sa.org>".