general_coding_standards.diviner

@title General Coding Standards
@group standards

This document is a general coding standard for contributing to Phabricator,
Arcanist, and Diviner.

= Overview =

This document contains practices and guidelines which apply across languages.
Contributors should follow these guidelines. These guidelines are not
hard-and-fast but should be followed unless there is a compelling reason to
deviate from them.

= Code Complexity =

  - Prefer to write simple code which is easy to understand. The simplest code
    is not necessarily the smallest, and some changes which make code larger
    (such as decomposing complex expressions and choosing more descriptive
    names) may also make it simpler. Be willing to make size tradeoffs in favor
    of simplicity.
  - Prefer simple methods and functions which take a small number of parameters.
    Avoid methods and functions which are long and complex, or take an
    innumerable host of parameters. When possible, decompose monolithic, complex
    methods into several focused, simpler ones.
  - Avoid putting many ideas on a single line of code.

For example, avoid this kind of code:

  COUNTEREXAMPLE
  $category_map = array_combine(
    $dates,
    array_map(create_function('$z', 'return date("F Y", $z);'), $dates));

Expressing this complex transformation more simply produces more readable code:

  $category_map = array();
  foreach ($dates as $date) {
    $category_map[$date] = date('F Y', $date);
  }

And, obviously, don't do this sort of thing:

  COUNTEREXAMPLE
  if ($val = $some->complicatedConstruct() && !!~blarg_blarg_blarg() & $flags
        ? HOPE_YOU_MEMORIZED == $all_the_lexical_binding_powers : <<<'Q'
  ${hahaha}
  Q
  );


= Performance =

  - Prefer to write efficient code.
  - Strongly prefer to drive optimization decisions with hard data. Avoid
    optimizing based on intuition or rumor if you can not support it with
    concrete measurements.
  - Prefer to optimize code which is slow and runs often. Optimizing code which
    is fast and runs rarely is usually a waste of time, and can even be harmful
    if it makes that code more difficult to understand or maintain. You can
    determine if code is fast or slow by measuring it.
  - Reject performance discussions that aren't rooted in concrete data.

In Phabricator, you can usually use the builtin XHProf profiling to quickly
gather concrete performance data.


= Naming Things =

  - Follow language-specific conventions.
  - Name things unambiguously.
  - Choose descriptive names.
  - Avoid nonstandard abbreviations (common abbreviations like ID, URI and HTTP
    are fine).
  - Spell words correctly.
  - Use correct grammar.

For example, avoid these sorts of naming choices:

  COUNTEREXAMPLE
  $PIE->GET_FLAVOR();       //  Unconventional.
  $thing->doStuff();        //  Ambiguous.
  $list->empty();           //  Ambiguous -- is it isEmpty() or makeEmpty()?
  $e = 3;                   //  Not descriptive.
  $this->updtHndlr();       //  Nonstandard abbreviation.
  $this->chackSpulls();     //  Misspelling, ungrammatical.

Prefer these:

  $pie->getFlavor();        //  Conventional.
  $pie->bake();             //  Unambiguous.
  $list->isEmpty();         //  Unambiguous.
  $list->makeEmpty();       //  Unambiguous.
  $edge_count = 3;          //  Descriptive.
  $this->updateHandler();   //  No nonstandard abbreviations.
  $this->getID();           //  Standard abbreviation.
  $this->checkSpelling();   //  Correct spelling and grammar.


= Error Handling =

  - Strongly prefer to detect errors.
  - Strongly prefer to fail fast and loudly. The maximum cost of script
    termination is known, bounded, and fairly small. The maximum cost of
    continuing script execution when errors have occurred is unknown and
    unbounded. This also makes APIs much easier to use and problems far easier
    to debug.

When you ignore errors, defer error handling, or degrade the severity of errors
by treating them as warnings and then dismissing them, you risk dangerous
behavior which may be difficult to troubleshoot:

  COUNTEREXAMPLE
  exec('echo '.$data.' > file.bak');                //  Bad!
  do_something_dangerous();

  exec('echo '.$data.' > file.bak', $out, $err);    //  Also bad!
  if ($err) {
    debug_rlog("Unable to copy file!");
  }
  do_something_dangerous();

Instead, fail loudly:

  exec('echo '.$data.' > file.bak', $out, $err);    //  Better
  if ($err) {
    throw new Exception("Unable to copy file!");
  }
  do_something_dangerous();

But the best approach is to use or write an API which simplifies condition
handling and makes it easier to get right than wrong:

  execx('echo %s > file.bak', $data);               //  Good
  do_something_dangerous();

  Filesystem::writeFile('file.bak', $data);         //  Best
  do_something_dangerous();

See @{article@arcanist:Command Execution} for details on the APIs used in this
example.

= Documentation, Comments and Formatting =

  - Prefer to remove code by deleting it over removing it by commenting it out.
    It shall live forever in source control, and can be retrieved therefrom if
    it is ever again called upon.
  - In source code, use only ASCII printable characters plus space and linefeed.
    Do not use UTF-8 or other multibyte encodings.