mirror of
https://github.com/golang/go
synced 2024-11-12 06:40:22 -07:00
d1a3b98a8d
R=rsc DELTA=73 (27 added, 25 deleted, 21 changed) OCL=32587 CL=32587
674 lines
19 KiB
HTML
674 lines
19 KiB
HTML
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<h2 id="introduction">Introduction</h2>
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<p>
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This document gives tips for writing clear, idiomatic Go code
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and points out common mistakes.
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It augments the <a href="go_spec.html">language specification</a>
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and the <a href="go_tutorial.html">tutorial</a>, both of which you
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should read first.
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</p>
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<h3 id="read">Read good code</h3>
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<p>
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The first step in learning to write good code is to read good code.
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The <a href="/src/pkg/">Go package sources</a>
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are intended to serve not
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only as the core library but also as examples of how to
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use the language. Read them and follow their example.
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</p>
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<h3 id="be-consistent">Be consistent</h3>
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<p>
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Programmers often want their style to be distinctive,
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writing loops backwards or using custom spacing and
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naming conventions. Such idiosyncracies come at a
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price, however: by making the code look different,
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they make it harder to understand.
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Consistency trumps personal
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expression in programming.
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</p>
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<p>
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If a program does the same thing twice,
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it should do it the same way both times.
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Conversely, if two different sections of a
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program look different, the reader will
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expect them to do different things.
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</p>
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<p>
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Consider <code>for</code> loops.
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Traditionally, a loop over <code>n</code>
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elements begins:
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</p>
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<pre>
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for i := 0; i < n; i++ {
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</pre>
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<p>
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Much of the time, the loop could run in the opposite order
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and still be correct:
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</p>
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<pre>
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for i := n-1; i >= 0; i-- {
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</pre>
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<p>
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The convention
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is to count up unless to do so would be incorrect.
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A loop that counts down implicitly says “something
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special is happening here.”
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A reader who finds a program in which some
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loops count up and the rest count down
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will spend time trying to understand why.
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</p>
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<p>
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Loop direction is just one
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programming decision that must be made
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consistently; others include
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formatting, naming variables and methods,
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whether a type
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has a constructor, what tests look like, and so on.
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Why is this variable called <code>n</code> here and <code>cnt</code> there?
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Why is the <code>Log</code> constructor <code>CreateLog</code> when
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the <code>List</code> constructor is <code>NewList</code>?
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Why is this data structure initialized using
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a structure literal when that one
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is initialized using individual assignments?
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These questions distract from the important one:
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what does the code do?
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Moreover, internal consistency is important not only within a single file,
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but also within the the surrounding source files.
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When editing code, read the surrounding context
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and try to mimic it as much as possible, even if it
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disagrees with the rules here.
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It should not be possible to tell which lines
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you wrote or edited based on style alone.
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Consistency about little things
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lets readers concentrate on big ones.
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</p>
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<h2 id="formatting">Formatting</h2>
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<p>
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Formatting issues are the most contentious
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but the least consequential.
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People adapt to different formatting styles,
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but they shouldn't be asked to.
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Everyone
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should use the same formatting; as in English,
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consistent punctuation and spacing make the
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text easier to read.
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Most of the local formatting style can be
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picked up by reading existing Go programs,
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but to make them explicit here are some common points.
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</p>
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<h3 id="tabs">Use tabs</h3>
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<p>
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Use tabs, not spaces, for indentation.
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</p>
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<h3 id="columns">Don't worry about columnation</h3>
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<p>
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Let tools such as <code>gofmt</code> take care of lining things up.
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</p>
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<h3 id="white-space">Trim trailing white space</h3>
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<p>
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There should be no trailing white space at the end of lines.
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</p>
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<h3 id="line-wrapping">Don't wrap lines</h3>
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<p>
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Go has no 80-character limit. Don't bother with fancy line
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wrapping just because a line is wider than a punched card.
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If a line is too long, indent with an extra tab.
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</p>
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<h3 id="parens">Omit parentheses in control structures</h3>
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<p>Go does not require parentheses around the expression
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following the <code>for</code>, <code>if</code>, <code>range</code>,
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<code>switch</code>, and <code>return</code> keywords.
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</p>
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<h3 id="line-comments">Use line comments</h3>
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<p>
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Go provides C-style <code>/* */</code> block comments
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and C++-style <code>//</code> line comments.
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Use line comments by default,
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reserving block comments for top-level package comments
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and commenting out large swaths of code.
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</p>
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<h3 id="doc-comments">Write doc comments</h3>
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<p>
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If a comment immediately precedes a top-level declaration,
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the <a href="/">Go documentation server</a>
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<font color=red>(TODO: that's not a public URL.)</font>
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uses that comment as the documentation
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for the constant, function, method, package, type or variable being declared.
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These are called <i>doc comments</i>.
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To detach a comment from a declaration, insert a blank
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line between them.
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</p>
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<p>
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Every exported (capitalized) name in a program should
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have a doc comment, as should the package declaration itself.
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If a name appears multiple times due to forward declarations
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or appearance in multiple source files within a package, only
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one instance requires a doc comment, and any one will do.
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</p>
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<p>
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Doc comments consist of complete English sentences.
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The first sentence should be a one-sentence summary that
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starts with the name being declared:
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</p>
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<pre>
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// Quote returns a double-quoted Go string literal
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// representing s. The returned string s uses Go escape
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// sequences (\t, \n, \xFF, \u0100) for control characters
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// and non-ASCII characters.
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func Quote(s string) string {
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</pre>
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<p>
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Use of complete English sentences admits
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a wider variety of automated presentations.
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</p>
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<h3 id="ascii-art">Avoid ASCII Art</h3>
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<p>
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Go programs are meant to read equally well using
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fixed-width and variable-width fonts.
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Don't use fancy formattings that depend on fixed-width fonts.
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In particular, don't assume that a single space is the same
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width as every other character.
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If you need to make a columnated table, use tabs to separate
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the columns and the pretty printer will make
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sure the columns are lined up properly in the output.
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</p>
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<p>
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If you must use comments to separate
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sections in a file, use a simple block comment:
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</p>
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<pre>
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/*
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* Helper routines for simplifying the fetching of optional fields of basic type.
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* If the field is missing, they return the zero for the type.
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*/
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</pre>
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or
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<pre>
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/*
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Helper routines for simplifying the fetching of optional fields of basic type.
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If the field is missing, they return the zero for the type.
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*/
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</pre>
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<p>
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Comments are text, not HTML; they contain no markup.
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Refrain from ASCII embellishment like *this* or /this/.
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</p>
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<h2 id="names">Names</h2>
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<h3 id="mixed-caps">Use MixedCaps</h3>
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<p>
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Go uses the case of the first letter in a name to decide
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whether the name is visible in other packages.
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Multiword names use MixedCaps or mixedCaps
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rather than underscores.
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</p>
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<h3 id="package-names">Use short package names</h3>
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<p>
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Package names are lower case single-word names:
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there should be no need for underscore or mixedCaps.
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The package name is conventionally the base name of
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the source directory: the package in <code>src/pkg/container/vector</code>
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is installed as <code>"container/vector"</code> but has name <code>vector</code>,
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not <code>container_vector</code> and not <code>containerVector</code>.
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The package name is only the default name used
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when importing the package; it need not be unique
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across all source code.
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</p>
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<h3 id="name-length">Avoid long names</h3>
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<p>
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A name's length should not exceed its information content.
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For a function-local variable
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in scope only for a few lines, the name <code>i</code> conveys just
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as much information as <code>index</code> or <code>idx</code> and is easier to read.
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Letters are easier to distinguish than numbers; use <code>i</code> and <code>j</code>
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not <code>i1</code> and <code>i2</code>.
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</p>
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<p>
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Exported names must convey more information
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because they appear far from their origin.
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Even so, longer names are not always better,
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and the package name can help convey information:
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the buffered <code>Reader</code> is <code>bufio.Reader</code>, not <code>bufio.BufReader</code>.
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Similarly, <code>once.Do</code> is as precise and evocative as
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<code>once.DoOrWaitUntilDone</code>, and <code>once.Do(f)</code> reads
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better than <code>once.DoOrWaitUntilDone(f)</code>.
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Contrary to popular belief, encoding small essays into
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function names does not make it possible
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to use them without documentation.
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</p>
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<h3 id="interfacers">Use the -er convention for interface names</h3>
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<p>
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One-method interfaces are conventionally named by
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the method name plus the -er suffix: <code>Reader</code>,
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<code>Writer</code>, <code>Formatter</code>.
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</p>
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<h3 id="common-names">Use canonical names</h3>
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<p>
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A few method names—<code>Read</code>, <code>Write</code>, <code>Close</code>, <code>Flush</code>, <code>String</code>—have
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canonical signatures and meanings. To avoid confusion,
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don't give your method one of those names unless it
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has the same signature and meaning.
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Conversely, if your type implements a method with the
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same meaning as a method on a well-known type,
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give it the same name and signature.
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</p>
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<p>
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Some function-local variables have canonical names too.
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Just as <code>i</code> is idiomatic in Go for an
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index variable, <code>n</code> is idiomatic for a count, <code>b</code> for a <code>[]byte</code>,
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<code>s</code> for a <code>string</code>, <code>r</code> for a <code>Reader</code>,
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<code>err</code> for an <code>os.Error</code>
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and so on.
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Don't mix shorthands: it is especially confusing to
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have two different variables <code>i</code> and <code>idx</code>,
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or <code>n</code> and <code>cnt</code>.
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</p>
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<h2 id="idioms">Idioms</h2>
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TODO: Add links to code once godoc can handle it.
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<h3 id="address-literals">Address literals to allocate and initialize</h3>
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<p>
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Taking the address of a struct or array literal evaluates to a
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new instance each time it is evaluated.
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Use these expressions to avoid the repetition of filling
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out a data structure.
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</p>
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<h3 id="buffer-slice">Use parallel assignment to slice a buffer</h3>
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<pre>
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hdr, body, checksum := buf[0:20], buf[20:len(buf)-4], buf[len(buf)-4:len(buf)];
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</pre>
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<h2 id="control-flow">Control Flow</h2>
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<h3 id="else">Omit needless else bodies</h3>
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<p>
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When an <code>if</code> statement doesn't flow into the next statement—that is,
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the body ends in <code>break</code>, <code>continue</code>,
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<code>goto</code>, or <code>return</code>—omit the <code>else</code>.
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</p>
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<pre>
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f, err := os.Open(name, os.O_RDONLY, 0);
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if err != nil {
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return err;
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}
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codeUsing(f);
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</pre>
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<h3 id="switch">Switch</h3>
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<p>
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Go's <code>switch</code> is more general than C's.
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When an <code>if</code>-<code>else if</code>-<code>else</code> chain has three or more bodies,
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or an <code>if</code> condition has a long list of alternatives,
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it will be clearer if rewritten as a <code>switch</code>.
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</p>
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<a href="/src/pkg/http/url.go">go/src/pkg/http/url.go</a>:
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<pre>
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func unhex(c byte) byte {
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switch {
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case '0' <= c && c <= '9':
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return c - '0'
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case 'a' <= c && c <= 'f':
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return c - 'a' + 10
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case 'A' <= c && c <= 'F':
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return c - 'A' + 10
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}
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return 0
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}
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</pre>
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<a href="/src/pkg/http/url.go">go/src/pkg/http/url.go</a>:
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<pre>
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func shouldEscape(c byte) bool {
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switch c {
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case ' ', '?', '&', '=', '#', '+', '%':
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return true
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}
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return false
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}
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</pre>
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<a href="/src/pkg/bytes/bytes.go">go/src/pkg/bytes/bytes.go</a>:
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<pre>
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// Compare returns an integer comparing the two byte arrays lexicographically.
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// The result will be 0 if a==b, -1 if a < b, and +1 if a > b
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func Compare(a, b []byte) int {
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for i := 0; i < len(a) && i < len(b); i++ {
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switch {
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case a[i] > b[i]:
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return 1
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case a[i] < b[i]:
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return -1
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}
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}
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switch {
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case len(a) < len(b):
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return -1
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case len(a) > len(b):
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return 1
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}
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return 0
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}
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</pre>
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<h2 id="functions">Functions</h2>
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<h3 id="omit-wrappers">Omit needless wrappers</h3>
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<p>
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Functions are great for factoring out common code, but
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if a function is only called once,
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ask whether it is necessary,
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especially if it is just a short wrapper around another function.
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This style is rampant in C++ code: wrappers
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call wrappers that call wrappers that call wrappers.
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This style hinders people trying to understand the program,
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not to mention computers trying to execute it.
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</p>
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<h3 id="multiple-returns">Return multiple values</h3>
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<p>
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If a function must return multiple values, it can
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do so directly.
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There is no need to pass a pointer to a return value.
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</p>
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<h2 id="errors">Errors</h2>
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<h3 id="error-returns">Return <code>os.Error</code>, not <code>bool</code></h3>
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<p>
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Especially in libraries, functions tend to have multiple error modes.
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Instead of returning a boolean to signal success,
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return an <code>os.Error</code> that describes the failure.
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Even if there is only one failure mode now,
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there may be more later.
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</p>
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<h3 id="handle-errors-first">Handle errors first</h3>
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<p>
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Error cases tend to be simpler than non-error cases,
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and it helps readability when the non-error flow
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of control is always down the page.
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Also, error cases tend to end in jumps,
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so that there is <a href="#else">no need for an explicit else</a>.
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</p>
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<pre>
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if len(name) == 0 {
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return os.EINVAL;
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}
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if IsDir(name) {
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return os.EISDIR;
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}
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f, err := os.Open(name, os.O_RDONLY, 0);
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if err != nil {
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return err;
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}
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codeUsing(f);
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</pre>
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<h3 id="error-context">Return structured errors</h3>
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Implementations of <code>os.Error</code>s should
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describe the error but also include context.
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For example, <code>os.Open</code> returns an <code>os.PathError</code>:
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<a href="/src/pkg/os/file.go">/src/pkg/os/file.go</a>:
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<pre>
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XXX definition of PathError and .String
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</pre>
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<code>PathError</code>'s <code>String</code> formats
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the error nicely and is the usual way the error gets used.
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Callers that care about the precise error details can
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use a type switch or a type guard to look for specific
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errors and then extract details.
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<pre>
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XXX example here - MkdirAll
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</pre>
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<h2 id="types">Programmer-defined types</h2>
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<h3 id="constructors">Use <code>NewTypeName</code> for constructors</h3>
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<p>
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The constructor for the type <code>pkg.MyType</code> should
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be named <code>pkg.NewMyType</code> and should return <code>*pkg.MyType</code>.
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The implementation of <code>NewTypeName</code> often uses the
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<a href="#allocating-a-struct">struct allocation idiom</a>.
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</p>
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<a href="xxx">go/src/pkg/os/file.go</a>:
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<pre>
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func NewFile(fd int, name string) *File {
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if file < 0 {
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return nil
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}
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return &File{fd, name, nil, 0}
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}
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</pre>
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<p>Packages that export only a single type sometimes
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shorten <code>NewTypeName</code> to <code>New</code>;
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the vector constructor is
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<code>vector.New</code>, not <code>vector.NewVector</code>.
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</p>
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<p>
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A type that is intended to be allocated
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as part of a larger struct may have an <code>Init</code> method
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that must be called explicitly.
|
|
Conventionally, the <code>Init</code> method returns
|
|
the object being initialized, to make the constructor trivial:
|
|
</p>
|
|
|
|
<a href="xxx">go/src/pkg/container/vector/vector.go</a>:
|
|
<pre>
|
|
func New(len int) *Vector {
|
|
return new(Vector).Init(len)
|
|
}
|
|
</pre>
|
|
|
|
<h3 id="zero-value">Make the zero value meaningful</h3>
|
|
|
|
<p>
|
|
In Go, newly allocated memory and newly declared variables are zeroed.
|
|
If a type is intended to be allocated without using a constructor
|
|
(for example, as part of a larger struct or declared as a local variable),
|
|
define the meaning of the zero value and arrange for that meaning
|
|
to be useful.
|
|
</p>
|
|
|
|
<p>
|
|
For example, <code>sync.Mutex</code> does not
|
|
have an explicit constructor or <code>Init</code> method.
|
|
Instead, the zero value for a <code>sync.Mutex</code>
|
|
is defined to be an unlocked mutex.
|
|
</p>
|
|
|
|
<h2 id="interfaces">Interfaces</h2>
|
|
|
|
<h3 id="accept-interface-values">Accept interface values</h3>
|
|
|
|
buffered i/o takes a Reader, not an os.File. XXX
|
|
|
|
<h3 id="return-interface-values">Return interface values</h3>
|
|
|
|
<p>
|
|
If a type exists only to implement an interface
|
|
and has no exported methods beyond that interface,
|
|
there is no need to publish the type itself.
|
|
Instead, write a constructor that returns an interface value.
|
|
</p>
|
|
|
|
<p>
|
|
For example, both <code>crc32.NewIEEE()</code> and <code>adler32.New()</code>
|
|
return type <code>hash.Hash32</code>.
|
|
Substituting the CRC-32 algorithm for Adler-32 in a Go program
|
|
requires only changing the constructor call:
|
|
the rest of the code is unaffected by the change of algorithm.
|
|
</p>
|
|
|
|
<h3 id="asdf">Use interface adapters to expand an implementation</h3>
|
|
|
|
XXX
|
|
|
|
<h3 id="fdsa">Use anonymous fields to incorporate an implementation</h3>
|
|
|
|
XXX
|
|
|
|
<h2>Data-Driven Programming</h2>
|
|
|
|
<p>
|
|
tables
|
|
</p>
|
|
|
|
<p>
|
|
XXX struct tags for marshalling.
|
|
template
|
|
eventually datafmt
|
|
</p>
|
|
|
|
<h2>Concurrency</h2>
|
|
|
|
<h3 id="share-memory">Share memory by communicating</h3>
|
|
|
|
<p>
|
|
Do not communicate by sharing memory;
|
|
instead, share memory by communicating.
|
|
</p>
|
|
|
|
<p>
|
|
XXX, more here.
|
|
</p>
|
|
|
|
|
|
<h2>Testing</h2>
|
|
|
|
<h3 id="no-abort">Run tests to completion</h3>
|
|
|
|
<p>
|
|
Tests should not stop early just because one case has misbehaved.
|
|
If at all possible, let tests continue, in order to characterize the
|
|
problem in more detail.
|
|
For example, it is more useful for a test to report that <code>isPrime</code>
|
|
gives the wrong answer for 2, 3, 5, and 7 (or for 2, 4, 8, and 16) than to report
|
|
that <code>isPrime</code> gives the wrong answer for 2 and therefore
|
|
no more tests were run.
|
|
</p>
|
|
|
|
<h3 id="good-errors">Print useful errors when tests fail</h3>
|
|
|
|
<p>
|
|
If a test fails, print a concise message explaining the context,
|
|
what happened, and what was expected.
|
|
Many testing environments encourage causing the
|
|
program to crash, but stack traces and core dumps
|
|
have low signal to noise ratios and require reconstructing
|
|
the situation from scratch.
|
|
The programmer who triggers the test failure may be someone
|
|
editing the code months later or even someone editing a different
|
|
package on which the code depends.
|
|
Time invested writing a good error message now pays off when
|
|
the test breaks later.
|
|
</p>
|
|
|
|
<h3 id="data-driven-tests">Use data-driven tests</h3>
|
|
|
|
<p>
|
|
Many tests reduce to running the same code multiple times,
|
|
with different input and expected output.
|
|
Instead of using cut and paste to write this code,
|
|
create a table of test cases and write a single test that
|
|
iterates over the table.
|
|
Once the table is written, you might find that it
|
|
serves well as input to multiple tests. For example,
|
|
a single table of encoded/decoded pairs can be
|
|
used by both <code>TestEncoder</code> and <code>TestDecoder</code>.
|
|
</p>
|
|
|
|
<p>
|
|
This data-driven style dominates in the Go package tests.
|
|
<br>
|
|
<!-- search for for.*range here -->
|
|
</p>
|
|
|
|
<h3 id="reflect.DeepEqual">Use reflect.DeepEqual to compare complex values</h3>
|
|
|
|
<p>
|
|
The <code>reflect.DeepEqual</code> function tests
|
|
whether two complex data structures have equal values.
|
|
If a function returns a complex data structure,
|
|
<code>reflect.DeepEqual</code> combined with table-driven testing
|
|
makes it easy to check that the return value is
|
|
exactly as expected.
|
|
</p>
|
|
|
|
</div>
|
|
</body>
|
|
</html>
|