1SENSE(1) User Contributed Perl Documentation SENSE(1)
2
6 common::sense - save a tree AND a kitten, use common::sense!
7
9 use common::sense;
10 # Supposed to be mostly the same, with much lower memory usage, as:
12 # use utf8;
14 # use strict qw(vars subs);
15 # use feature qw(say state switch);
16 # use feature qw(unicode_strings unicode_eval current_sub fc evalbytes);
17 # no feature qw(array_base);
18 # no warnings;
19 # use warnings qw(FATAL closed threads internal debugging pack
20 # prototype inplace io pipe unpack malloc glob
21 # digit printf layer reserved taint closure semicolon);
22 # no warnings qw(exec newline unopened);
23
25 XNothing is more fairly distributed than common sense: no one thinks
26 he needs more of it than he already has.X
27 X Rene Descartes
29 This module implements some sane defaults for Perl programs, as defined
31 by two typical (or not so typical - use your common sense) specimens of
32 Perl coders. In fact, after working out details on which warnings and
33 strict modes to enable and make fatal, we found that we (and our code
34 written so far, and others) fully agree on every option, even though we
35 never used warnings before, so it seems this module indeed reflects a
36 "common" sense among some long-time Perl coders.
37 The basic philosophy behind the choices made in common::sense can be
39 summarised as: "enforcing strict policies to catch as many bugs as
40 possible, while at the same time, not limiting the expressive power
41 available to the programmer".
42 Two typical examples of how this philosophy is applied in practise is
44 the handling of uninitialised and malloc warnings:
45 uninitialised
47 "undef" is a well-defined feature of perl, and enabling warnings
48 for using it rarely catches any bugs, but considerably limits you
49 in what you can do, so uninitialised warnings are disabled.
50 malloc
52 Freeing something twice on the C level is a serious bug, usually
53 causing memory corruption. It often leads to side effects much
54 later in the program and there are no advantages to not reporting
55 this, so malloc warnings are fatal by default.
56 Unfortunately, there is no fine-grained warning control in perl, so
58 often whole groups of useful warnings had to be excluded because of a
59 single useless warning (for example, perl puts an arbitrary limit on
60 the length of text you can match with some regexes before emitting a
61 warning, making the whole "regexp" category useless).
62 What follows is a more thorough discussion of what this module does,
64 and why it does it, and what the advantages (and disadvantages) of this
65 approach are.
66
68 use utf8
69 While it's not common sense to write your programs in UTF-8, it's
70 quickly becoming the most common encoding, is the designated future
71 default encoding for perl sources, and the most convenient encoding
72 available (you can do really nice quoting tricks...). Experience
73 has shown that our programs were either all pure ascii or utf-8,
74 both of which will stay the same.
75 There are few drawbacks to enabling UTF-8 source code by default
77 (mainly some speed hits due to bugs in older versions of perl), so
78 this module enables UTF-8 source code encoding by default.
79 use strict qw(subs vars)
81 Using "use strict" is definitely common sense, but "use strict
82 'refs'" definitely overshoots its usefulness. After almost two
83 decades of Perl hacking, we decided that it does more harm than
84 being useful. Specifically, constructs like these:
85 @{ $var->[0] }
87 Must be written like this (or similarly), when "use strict 'refs'"
89 is in scope, and $var can legally be "undef":
90 @{ $var->[0] || [] }
92 This is annoying, and doesn't shield against obvious mistakes such
94 as using "", so one would even have to write (at least for the time
95 being):
96 @{ defined $var->[0] ? $var->[0] : [] }
98 ... which nobody with a bit of common sense would consider writing:
100 clear code is clearly something else.
101 Curiously enough, sometimes perl is not so strict, as this works
103 even with "use strict" in scope:
104 for (@{ $var->[0] }) { ...
106 If that isn't hypocrisy! And all that from a mere program!
108 use feature qw(say state given ...)
110 We found it annoying that we always have to enable extra features.
111 If something breaks because it didn't anticipate future changes, so
112 be it. 5.10 broke almost all our XS modules and nobody cared either
113 (or at least I know of nobody who really complained about
114 gratuitous changes - as opposed to bugs).
115 Few modules that are not actively maintained work with newer
117 versions of Perl, regardless of use feature or not, so a new major
118 perl release means changes to many modules - new keywords are just
119 the tip of the iceberg.
120 If your code isn't alive, it's dead, Jim - be an active maintainer.
122 But nobody forces you to use those extra features in modules meant
124 for older versions of perl - common::sense of course works there as
125 well. There is also an important other mode where having
126 additional features by default is useful: commandline hacks and
127 internal use scripts: See "much reduced typing", below.
128 There is one notable exception: "unicode_eval" is not enabled by
130 default. In our opinion, "use feature" had one main effect - newer
131 perl versions don't value backwards compatibility and the ability
132 to write modules for multiple perl versions much, after all, you
133 can use feature.
134 "unicode_eval" doesn't add a new feature, it breaks an existing
136 function.
137 no warnings, but a lot of new errors
139 Ah, the dreaded warnings. Even worse, the horribly dreaded "-w"
140 switch: Even though we don't care if other people use warnings (and
141 certainly there are useful ones), a lot of warnings simply go
142 against the spirit of Perl.
143 Most prominently, the warnings related to "undef". There is nothing
145 wrong with "undef": it has well-defined semantics, it is useful,
146 and spitting out warnings you never asked for is just evil.
147 The result was that every one of our modules did "no warnings" in
149 the past, to avoid somebody accidentally using and forcing his bad
150 standards on our code. Of course, this switched off all warnings,
151 even the useful ones. Not a good situation. Really, the "-w" switch
152 should only enable warnings for the main program only.
153 Funnily enough, perllexwarn explicitly mentions "-w" (and not in a
155 favourable way, calling it outright "wrong"), but standard
156 utilities, such as prove, or MakeMaker when running "make test",
157 still enable them blindly.
158 For version 2 of common::sense, we finally sat down a few hours and
160 went through every single warning message, identifying - according
161 to common sense - all the useful ones.
162 This resulted in the rather impressive list in the SYNOPSIS. When
164 we weren't sure, we didn't include the warning, so the list might
165 grow in the future (we might have made a mistake, too, so the list
166 might shrink as well).
167 Note the presence of "FATAL" in the list: we do not think that the
169 conditions caught by these warnings are worthy of a warning, we
170 insist that they are worthy of stopping your program, instantly.
171 They are bugs!
172 Therefore we consider "common::sense" to be much stricter than "use
174 warnings", which is good if you are into strict things (we are not,
175 actually, but these things tend to be subjective).
176 After deciding on the list, we ran the module against all of our
178 code that uses "common::sense" (that is almost all of our code),
179 and found only one occurrence where one of them caused a problem:
180 one of elmex's (unreleased) modules contained:
181 $fmt =~ s/([^\s\[]*)\[( [^\]]* )\]/\x0$1\x1$2\x0/xgo;
183 We quickly agreed that indeed the code should be changed, even
185 though it happened to do the right thing when the warning was
186 switched off.
187 much reduced typing
189 Especially with version 2.0 of common::sense, the amount of
190 boilerplate code you need to add to get this policy is daunting.
191 Nobody would write this out in throwaway scripts, commandline hacks
192 or in quick internal-use scripts.
193 By using common::sense you get a defined set of policies (ours, but
195 maybe yours, too, if you accept them), and they are easy to apply
196 to your scripts: typing "use common::sense;" is even shorter than
197 "use warnings; use strict; use feature ...".
198 And you can immediately use the features of your installed perl,
200 which is more difficult in code you release, but not usually an
201 issue for internal-use code (downgrades of your production perl
202 should be rare, right?).
203 mucho reduced memory usage
205 Just using all those pragmas mentioned in the SYNOPSIS together
206 wastes <blink>776 kilobytes</blink> of precious memory in my perl,
207 for every single perl process using our code, which on our
208 machines, is a lot. In comparison, this module only uses four
209 kilobytes (I even had to write it out so it looks like more) of
210 memory on the same platform.
211 The money/time/effort/electricity invested in these gigabytes
213 (probably petabytes globally!) of wasted memory could easily save
214 42 trees, and a kitten!
215 Unfortunately, until everybody applies more common sense, there
217 will still often be modules that pull in the monster pragmas. But
218 one can hope...
219
221 This module doesn't offer an unimport. First of all, it wastes even
222 more memory, second, and more importantly, who with even a bit of
223 common sense would want no common sense?
224
226 Future versions might change just about everything in this module. We
227 might test our modules and upload new ones working with newer versions
228 of this module, and leave you standing in the rain because we didn't
229 tell you. In fact, we did so when switching from 1.0 to 2.0, which
230 enabled gobs of warnings, and made them FATAL on top.
231 Maybe we will load some nifty modules that try to emulate "say" or so
233 with perls older than 5.10 (this module, of course, should work with
234 older perl versions - supporting 5.8 for example is just common sense
235 at this time. Maybe not in the future, but of course you can trust our
236 common sense to be consistent with, uhm, our opinion).
237
239 apeiron
240 "... wow"
242 "I hope common::sense is a joke."
243 crab
245 "i wonder how it would be if joerg schilling wrote perl modules."
247 Adam Kennedy
249 "Very interesting, efficient, and potentially something I'd use all the time."
251 [...]
252 "So no common::sense for me, alas."
253 H.Merijn Brand
255 "Just one more reason to drop JSON::XS from my distribution list"
257 Pista Palo
259 "Something in short supply these days..."
261 Steffen Schwigon
263 "This module is quite for sure *not* just a repetition of all the other
265 'use strict, use warnings'-approaches, and it's also not the opposite.
266 [...] And for its chosen middle-way it's also not the worst name ever.
267 And everything is documented."
268 BKB
270 "[Deleted - thanks to Steffen Schwigon for pointing out this review was
272 in error.]"
273 Somni
275 "the arrogance of the guy"
277 "I swear he tacked somenoe else's name onto the module
278 just so he could use the royal 'we' in the documentation"
279 Anonymous Monk
281 "You just gotta love this thing, its got META.json!!!"
283 dngor
285 "Heh. '"<elmex at ta-sa.org>"' The quotes are semantic
287 distancing from that e-mail address."
288 Jerad Pierce
290 "Awful name (not a proper pragma), and the SYNOPSIS doesn't tell you
292 anything either. Nor is it clear what features have to do with "common
293 sense" or discipline."
294 acme
296 "THERE IS NO 'no common::sense'!!!! !!!! !!"
298 apeiron (meta-comment about us commenting^Wquoting his comment)
300 "How about quoting this: get a clue, you fucktarded amoeba."
302 quanth
304 "common sense is beautiful, json::xs is fast, Anyevent, EV are fast and
306 furious. I love mlehmannware ;)"
307 apeiron
309 "... it's mlehmann's view of what common sense is. His view of common
311 sense is certainly uncommon, insofar as anyone with a clue disagrees
312 with him."
313 apeiron (another meta-comment)
315 "apeiron wonders if his little informant is here to steal more quotes"
317 ew73
319 "... I never got past the SYNOPSIS before calling it shit."
321 [...]
322 How come no one ever quotes me. :("
323 chip (not willing to explain his cryptic questions about links in
325 Changes files)
326 "I'm willing to ask the question I've asked. I'm not willing to go
328 through the whole dance you apparently have choreographed. Either
329 answer the completely obvious question, or tell me to fuck off again."
330
332 Or frequently-come-up confusions.
333 Is this module meant to be serious?
335 Yes, we would have put it under the "Acme::" namespace otherwise.
336 But the manpage is written in a funny/stupid/... way?
338 This was meant to make it clear that our common sense is a
339 subjective thing and other people can use their own notions, taking
340 the steam out of anybody who might be offended (as some people are
341 always offended no matter what you do).
342 This was a failure.
344 But we hope the manpage still is somewhat entertaining even though
346 it explains boring rationale.
347 Why do you impose your conventions on my code?
349 For some reason people keep thinking that "common::sense" imposes
350 process-wide limits, even though the SYNOPSIS makes it clear that
351 it works like other similar modules - i.e. only within the scope
352 that "use"s them.
353 So, no, we don't - nobody is forced to use this module, and using a
355 module that relies on common::sense does not impose anything on
356 you.
357 Why do you think only your notion of common::sense is valid?
359 Well, we don't, and have clearly written this in the documentation
360 to every single release. We were just faster than anybody else
361 w.r.t. to grabbing the namespace.
362 But everybody knows that you have to use strict and use warnings, why
364 do you disable them?
365 Well, we don't do this either - we selectively disagree with the
366 usefulness of some warnings over others. This module is aimed at
367 experienced Perl programmers, not people migrating from other
368 languages who might be surprised about stuff such as "undef". On
369 the other hand, this does not exclude the usefulness of this module
370 for total newbies, due to its strictness in enforcing policy, while
371 at the same time not limiting the expressive power of perl.
372 This module is considerably more strict than the canonical "use
374 strict; use warnings", as it makes all its warnings fatal in
375 nature, so you can not get away with as many things as with the
376 canonical approach.
377 This was not implemented in version 1.0 because of the daunting
379 number of warning categories and the difficulty in getting exactly
380 the set of warnings you wish (i.e. look at the SYNOPSIS in how
381 complicated it is to get a specific set of warnings - it is not
382 reasonable to put this into every module, the maintenance effort
383 would be enormous).
384 But many modules "use strict" or "use warnings", so the memory savings
386 do not apply?
387 I suddenly feel sad...
388 But yes, that's true. Fortunately "common::sense" still uses only a
390 miniscule amount of RAM.
391 But it adds another dependency to your modules!
393 It's a fact, yeah. But it's trivial to install, most popular
394 modules have many more dependencies. And we consider dependencies a
395 good thing - it leads to better APIs, more thought about
396 interworking of modules and so on.
397 Why do you use JSON and not YAML for your META.yml?
399 This is not true - YAML supports a large subset of JSON, and this
400 subset is what META.yml is written in, so it would be correct to
401 say "the META.yml is written in a common subset of YAML and JSON".
402 The META.yml follows the YAML, JSON and META.yml specifications,
404 and is correctly parsed by CPAN, so if you have trouble with it,
405 the problem is likely on your side.
406 But! But!
408 Yeah, we know.
409
411 Marc Lehmann <schmorp@schmorp.de>
412 http://home.schmorp.de/
413 Robin Redeker, "<elmex at ta-sa.org>".
415perl v5.34.0 2022-01-21 SENSE(1)