CodingGuidelines: on splitting a long line
[git/git.git] / Documentation / CodingGuidelines
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6d0618a8 1Like other projects, we also have some guidelines to keep to the
2de9b711 2code. For Git in general, three rough rules are:
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3
4 - Most importantly, we never say "It's in POSIX; we'll happily
5 ignore your needs should your system not conform to it."
6 We live in the real world.
7
8 - However, we often say "Let's stay away from that construct,
9 it's not even in POSIX".
10
11 - In spite of the above two rules, we sometimes say "Although
12 this is not in POSIX, it (is so convenient | makes the code
13 much more readable | has other good characteristics) and
14 practically all the platforms we care about support it, so
15 let's use it".
16
17 Again, we live in the real world, and it is sometimes a
18 judgement call, the decision based more on real world
19 constraints people face than what the paper standard says.
20
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21 - Fixing style violations while working on a real change as a
22 preparatory clean-up step is good, but otherwise avoid useless code
23 churn for the sake of conforming to the style.
24
25 "Once it _is_ in the tree, it's not really worth the patch noise to
26 go and fix it up."
27 Cf. http://article.gmane.org/gmane.linux.kernel/943020
28
c5e366b1 29Make your code readable and sensible, and don't try to be clever.
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30
31As for more concrete guidelines, just imitate the existing code
32(this is a good guideline, no matter which project you are
dfb047b9 33contributing to). It is always preferable to match the _local_
2de9b711 34convention. New code added to Git suite is expected to match
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35the overall style of existing code. Modifications to existing
36code is expected to match the style the surrounding code already
37uses (even if it doesn't match the overall style of existing code).
38
39But if you must have a list of rules, here they are.
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40
41For shell scripts specifically (not exhaustive):
42
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43 - We use tabs for indentation.
44
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45 - Case arms are indented at the same depth as case and esac lines,
46 like this:
47
48 case "$variable" in
49 pattern1)
50 do this
51 ;;
52 pattern2)
53 do that
54 ;;
55 esac
f36a4fa8 56
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57 - Redirection operators should be written with space before, but no
58 space after them. In other words, write 'echo test >"$file"'
59 instead of 'echo test> $file' or 'echo test > $file'. Note that
60 even though it is not required by POSIX to double-quote the
61 redirection target in a variable (as shown above), our code does so
62 because some versions of bash issue a warning without the quotes.
63
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64 (incorrect)
65 cat hello > world < universe
66 echo hello >$world
67
68 (correct)
69 cat hello >world <universe
70 echo hello >"$world"
71
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72 - We prefer $( ... ) for command substitution; unlike ``, it
73 properly nests. It should have been the way Bourne spelled
74 it from day one, but unfortunately isn't.
75
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76 - If you want to find out if a command is available on the user's
77 $PATH, you should use 'type <command>', instead of 'which <command>'.
78 The output of 'which' is not machine parseable and its exit code
79 is not reliable across platforms.
80
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81 - We use POSIX compliant parameter substitutions and avoid bashisms;
82 namely:
6d0618a8 83
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84 - We use ${parameter-word} and its [-=?+] siblings, and their
85 colon'ed "unset or null" form.
6d0618a8 86
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87 - We use ${parameter#word} and its [#%] siblings, and their
88 doubled "longest matching" form.
6d0618a8 89
bc979945 90 - No "Substring Expansion" ${parameter:offset:length}.
055467dd 91
bc979945 92 - No shell arrays.
6d0618a8 93
bc979945 94 - No strlen ${#parameter}.
6d0618a8 95
bc979945 96 - No pattern replacement ${parameter/pattern/string}.
6d0618a8 97
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98 - We use Arithmetic Expansion $(( ... )).
99
100 - Inside Arithmetic Expansion, spell shell variables with $ in front
101 of them, as some shells do not grok $((x)) while accepting $(($x))
102 just fine (e.g. dash older than 0.5.4).
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103
104 - We do not use Process Substitution <(list) or >(list).
105
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106 - Do not write control structures on a single line with semicolon.
107 "then" should be on the next line for if statements, and "do"
108 should be on the next line for "while" and "for".
109
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110 (incorrect)
111 if test -f hello; then
112 do this
113 fi
114
115 (correct)
116 if test -f hello
117 then
118 do this
119 fi
120
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121 - We prefer "test" over "[ ... ]".
122
123 - We do not write the noiseword "function" in front of shell
124 functions.
125
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126 - We prefer a space between the function name and the parentheses,
127 and no space inside the parentheses. The opening "{" should also
128 be on the same line.
129
130 (incorrect)
131 my_function(){
132 ...
133
134 (correct)
135 my_function () {
136 ...
03b05c7d 137
009c98ee 138 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 139 [::], [==], or [..]) for portability.
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140
141 - We do not use \{m,n\};
142
143 - We do not use -E;
144
a58088ab 145 - We do not use ? or + (which are \{0,1\} and \{1,\}
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146 respectively in BRE) but that goes without saying as these
147 are ERE elements not BRE (note that \? and \+ are not even part
148 of BRE -- making them accessible from BRE is a GNU extension).
149
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150 - Use Git's gettext wrappers in git-sh-i18n to make the user
151 interface translatable. See "Marking strings for translation" in
152 po/README.
153
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154For C programs:
155
156 - We use tabs to indent, and interpret tabs as taking up to
157 8 spaces.
158
159 - We try to keep to at most 80 characters per line.
160
2de9b711 161 - We try to support a wide range of C compilers to compile Git with,
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162 including old ones. That means that you should not use C99
163 initializers, even if a lot of compilers grok it.
164
165 - Variables have to be declared at the beginning of the block.
166
167 - NULL pointers shall be written as NULL, not as 0.
168
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169 - When declaring pointers, the star sides with the variable
170 name, i.e. "char *string", not "char* string" or
171 "char * string". This makes it easier to understand code
172 like "char *string, c;".
173
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174 - Use whitespace around operators and keywords, but not inside
175 parentheses and not around functions. So:
176
177 while (condition)
178 func(bar + 1);
179
180 and not:
181
182 while( condition )
183 func (bar+1);
184
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185 - We avoid using braces unnecessarily. I.e.
186
187 if (bla) {
188 x = 1;
189 }
190
191 is frowned upon. A gray area is when the statement extends
192 over a few lines, and/or you have a lengthy comment atop of
193 it. Also, like in the Linux kernel, if there is a long list
194 of "else if" statements, it can make sense to add braces to
195 single line blocks.
196
691d0dd0 197 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 198
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199 - Try to make your code understandable. You may put comments
200 in, but comments invariably tend to stale out when the code
201 they were describing changes. Often splitting a function
202 into two makes the intention of the code much clearer.
203
b75a6ca7 204 - Multi-line comments include their delimiters on separate lines from
205 the text. E.g.
206
207 /*
208 * A very long
209 * multi-line comment.
210 */
211
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212 Note however that a comment that explains a translatable string to
213 translators uses a convention of starting with a magic token
214 "TRANSLATORS: " immediately after the opening delimiter, even when
215 it spans multiple lines. We do not add an asterisk at the beginning
216 of each line, either. E.g.
217
218 /* TRANSLATORS: here is a comment that explains the string
219 to be translated, that follows immediately after it */
220 _("Here is a translatable string explained by the above.");
221
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222 - Double negation is often harder to understand than no negation
223 at all.
224
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225 - There are two schools of thought when it comes to comparison,
226 especially inside a loop. Some people prefer to have the less stable
227 value on the left hand side and the more stable value on the right hand
228 side, e.g. if you have a loop that counts variable i down to the
229 lower bound,
230
231 while (i > lower_bound) {
232 do something;
233 i--;
234 }
235
236 Other people prefer to have the textual order of values match the
237 actual order of values in their comparison, so that they can
238 mentally draw a number line from left to right and place these
239 values in order, i.e.
240
241 while (lower_bound < i) {
242 do something;
243 i--;
244 }
245
246 Both are valid, and we use both. However, the more "stable" the
247 stable side becomes, the more we tend to prefer the former
248 (comparison with a constant, "i > 0", is an extreme example).
249 Just do not mix styles in the same part of the code and mimic
250 existing styles in the neighbourhood.
251
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252 - There are two schools of thought when it comes to splitting a long
253 logical line into multiple lines. Some people push the second and
254 subsequent lines far enough to the right with tabs and align them:
255
256 if (the_beginning_of_a_very_long_expression_that_has_to ||
257 span_more_than_a_single_line_of ||
258 the_source_text) {
259 ...
260
261 while other people prefer to align the second and the subsequent
262 lines with the column immediately inside the opening parenthesis,
263 with tabs and spaces, following our "tabstop is always a multiple
264 of 8" convention:
265
266 if (the_beginning_of_a_very_long_expression_that_has_to ||
267 span_more_than_a_single_line_of ||
268 the_source_text) {
269 ...
270
271 Both are valid, and we use both. Again, just do not mix styles in
272 the same part of the code and mimic existing styles in the
273 neighbourhood.
274
275 - When splitting a long logical line, some people change line before
276 a binary operator, so that the result looks like a parse tree when
277 you turn your head 90-degrees counterclockwise:
278
279 if (the_beginning_of_a_very_long_expression_that_has_to
280 || span_more_than_a_single_line_of_the_source_text) {
281
282 while other people prefer to leave the operator at the end of the
283 line:
284
285 if (the_beginning_of_a_very_long_expression_that_has_to ||
286 span_more_than_a_single_line_of_the_source_text) {
287
288 Both are valid, but we tend to use the latter more, unless the
289 expression gets fairly complex, in which case the former tends to
290 be easier to read. Again, just do not mix styles in the same part
291 of the code and mimic existing styles in the neighbourhood.
292
293 - When splitting a long logical line, with everything else being
294 equal, it is preferable to split after the operator at higher
295 level in the parse tree. That is, this is more preferable:
296
297 if (a_very_long_variable * that_is_used_in +
298 a_very_long_expression) {
299 ...
300
301 than
302
303 if (a_very_long_variable *
304 that_is_used_in + a_very_long_expression) {
305 ...
306
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307 - Some clever tricks, like using the !! operator with arithmetic
308 constructs, can be extremely confusing to others. Avoid them,
309 unless there is a compelling reason to use them.
310
311 - Use the API. No, really. We have a strbuf (variable length
312 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 313 string_list for sorted string lists, a hash map (mapping struct
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314 objects) named "struct decorate", amongst other things.
315
316 - When you come up with an API, document it.
317
318 - The first #include in C files, except in platform specific
319 compat/ implementations, should be git-compat-util.h or another
320 header file that includes it, such as cache.h or builtin.h.
321
322 - If you are planning a new command, consider writing it in shell
323 or perl first, so that changes in semantics can be easily
2de9b711 324 changed and discussed. Many Git commands started out like
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325 that, and a few are still scripts.
326
2de9b711 327 - Avoid introducing a new dependency into Git. This means you
6d0618a8 328 usually should stay away from scripting languages not already
2de9b711 329 used in the Git core command set (unless your command is clearly
6d0618a8 330 separate from it, such as an importer to convert random-scm-X
2de9b711 331 repositories to Git).
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332
333 - When we pass <string, length> pair to functions, we should try to
334 pass them in that order.
c455bd89 335
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336 - Use Git's gettext wrappers to make the user interface
337 translatable. See "Marking strings for translation" in po/README.
338
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339For Perl programs:
340
341 - Most of the C guidelines above apply.
342
343 - We try to support Perl 5.8 and later ("use Perl 5.008").
344
345 - use strict and use warnings are strongly preferred.
346
347 - Don't overuse statement modifiers unless using them makes the
348 result easier to follow.
349
350 ... do something ...
351 do_this() unless (condition);
352 ... do something else ...
353
354 is more readable than:
355
356 ... do something ...
357 unless (condition) {
358 do_this();
359 }
360 ... do something else ...
361
362 *only* when the condition is so rare that do_this() will be almost
363 always called.
364
365 - We try to avoid assignments inside "if ()" conditions.
366
367 - Learn and use Git.pm if you need that functionality.
368
369 - For Emacs, it's useful to put the following in
370 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
371
372 ;; note the first part is useful for C editing, too
373 ((nil . ((indent-tabs-mode . t)
374 (tab-width . 8)
375 (fill-column . 80)))
376 (cperl-mode . ((cperl-indent-level . 8)
377 (cperl-extra-newline-before-brace . nil)
378 (cperl-merge-trailing-else . t))))
379
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380For Python scripts:
381
382 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
383
384 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
385
386 - Where required libraries do not restrict us to Python 2, we try to
387 also be compatible with Python 3.1 and later.
388
389 - When you must differentiate between Unicode literals and byte string
390 literals, it is OK to use the 'b' prefix. Even though the Python
391 documentation for version 2.6 does not mention this prefix, it has
392 been supported since version 2.6.0.
393
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394Writing Documentation:
395
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396 Most (if not all) of the documentation pages are written in the
397 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
398 processed into HTML and manpages (e.g. git.html and git.1 in the
399 same directory).
bb9f2aec 400
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401 The documentation liberally mixes US and UK English (en_US/UK)
402 norms for spelling and grammar, which is somewhat unfortunate.
403 In an ideal world, it would have been better if it consistently
404 used only one and not the other, and we would have picked en_US
405 (if you wish to correct the English of some of the existing
406 documentation, please see the documentation-related advice in the
407 Documentation/SubmittingPatches file).
408
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409 Every user-visible change should be reflected in the documentation.
410 The same general rule as for code applies -- imitate the existing
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411 conventions.
412
413 A few commented examples follow to provide reference when writing or
414 modifying command usage strings and synopsis sections in the manual
415 pages:
c455bd89 416
b1afe49d 417 Placeholders are spelled in lowercase and enclosed in angle brackets:
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418 <file>
419 --sort=<key>
420 --abbrev[=<n>]
421
469bfc96 422 Possibility of multiple occurrences is indicated by three dots:
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423 <file>...
424 (One or more of <file>.)
425
426 Optional parts are enclosed in square brackets:
427 [<extra>]
428 (Zero or one <extra>.)
429
430 --exec-path[=<path>]
431 (Option with an optional argument. Note that the "=" is inside the
432 brackets.)
433
434 [<patch>...]
435 (Zero or more of <patch>. Note that the dots are inside, not
436 outside the brackets.)
437
438 Multiple alternatives are indicated with vertical bar:
439 [-q | --quiet]
440 [--utf8 | --no-utf8]
441
442 Parentheses are used for grouping:
443 [(<rev>|<range>)...]
444 (Any number of either <rev> or <range>. Parens are needed to make
445 it clear that "..." pertains to both <rev> and <range>.)
446
447 [(-p <parent>)...]
448 (Any number of option -p, each with one <parent> argument.)
449
450 git remote set-head <name> (-a | -d | <branch>)
451 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
452 brackets) be provided.)
453
454 And a somewhat more contrived example:
455 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
456 Here "=" is outside the brackets, because "--diff-filter=" is a
457 valid usage. "*" has its own pair of brackets, because it can
458 (optionally) be specified only when one or more of the letters is
459 also provided.
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460
461 A note on notation:
462 Use 'git' (all lowercase) when talking about commands i.e. something
463 the user would type into a shell and use 'Git' (uppercase first letter)
464 when talking about the version control system and its properties.
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465
466 A few commented examples follow to provide reference when writing or
467 modifying paragraphs or option/command explanations that contain options
468 or commands:
469
470 Literal examples (e.g. use of command-line options, command names, and
471 configuration variables) are typeset in monospace, and if you can use
472 `backticks around word phrases`, do so.
473 `--pretty=oneline`
474 `git rev-list`
475 `remote.pushdefault`
476
477 Word phrases enclosed in `backtick characters` are rendered literally
478 and will not be further expanded. The use of `backticks` to achieve the
479 previous rule means that literal examples should not use AsciiDoc
480 escapes.
481 Correct:
482 `--pretty=oneline`
483 Incorrect:
484 `\--pretty=oneline`
485
486 If some place in the documentation needs to typeset a command usage
487 example with inline substitutions, it is fine to use +monospaced and
488 inline substituted text+ instead of `monospaced literal text`, and with
489 the former, the part that should not get substituted must be
490 quoted/escaped.