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Effective Go; draft
TBR=r OCL=30741 CL=30741
This commit is contained in:
Russ Cox
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doc/effective_go.html
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doc/effective_go.html
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<h2 id="introduction">Introduction</h2>
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<p>
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Go is a new programming language.
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It eliminates some of the pitfalls in languages
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like C++ and Java but introduces other ones.
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Many remain the same.
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This page gives tips for writing clear, idiomatic Go code
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and points out common mistakes to avoid.
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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 be familiar with.
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</p>
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<h3 id="read">Read</h3>
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<p>
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The first step to improving as a writer is to read.
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This step is as necessary for programming as it is for prose,
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and it is skipped as often by programmers as by writers.
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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.
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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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Consistency makes programs easy to read.
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If a program says the same thing twice,
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it should say it the same way both times,
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so that the parallel structure is clear.
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Conversely, if two different sections of
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program look different, the reader will
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expect them to be doing 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 in most languages (including Go)
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is to count up unless doing so would be incorrect.
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A loop that counts down implicitly says “there's
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a specific reason to count down here; this situation is special.”
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A reader who finds a program in which half the
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loops count up and the other half count down
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will spend time trying to understand why.
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A programmer who inserts counting-down
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loops for variety wastes the reader's time.
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</p>
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<p>
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Loop direction is hardly the only
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programming decision which a programmer
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might want to use to be distinctive:
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tabs or spaces, choice of variable names,
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choice of method names, whether a type
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has a constructor, what tests look like, and on and on.
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As in the loop example, inconsistency
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sows confusion, and wastes time.
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Why is this variable called n here and cnt here?
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Why is the <code>Log</code> constructor <code>CreateLog</code> when
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the <code>Vector</code> constructor is <code>NewVector</code>?
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Why is this data structure initialized using
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a structure literal when this other one
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is initialized using individual assignments?
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Why is this idiom used here but not there?
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And on and on.
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These questions distract from the important one:
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what does the code do?
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Being consistent about little things
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lets readers concentrate on big ones.
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</p>
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<p>
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This document describes how to use Go effectively.
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Equally important, it describes how to use Go idiomatically,
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so that a Go programmer seeing your code for
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the first time can focus on what it does
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and not why it is inconsistent with typical Go practices.
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Consistency trumps every item listed below.
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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.
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</p>
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<p>
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Internal consistency is important in a single file,
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but source files do not exist in isolation.
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Code must be consistent with the surrounding source file as well,
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or the same problems arise.
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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 quite well to different formatting styles,
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even if at first the styles “look weird.”
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The most important consideration is that if everyone
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uses the same formatting, then a reader picking up an
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unfamiliar piece of code can focus on what
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the code does instead of what it looks like.
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Most of the local formatting style can and should be
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picked up by reading existing Go programs (see above).
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The following are the most common issues
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that Google programmers run into.
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</p>
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<h3 id="tabs">Use tabs</h3>
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<p>
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The local style is to use tabs, not spaces, for indentation.
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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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Files should not contain trailing white space at the end of lines.
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The script <code>/home/rsc/bin/g4ws</code> removes trailing
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whitespace from all open files in the current g4 client.
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</p>
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<h3 id="line-wrapping">Don't wrap lines mechanically</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 you must wrap a line, indent with a single tab.
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</p>
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<h3 id="parens">Omit parentheses</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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and <code>switch</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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The local style is to 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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uses that comment as the documentation
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for the constant, function, package, type or variable being declared.
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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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</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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instead of:
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</p>
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<pre class="bad">
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/* not Go style */
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// Return a double-quoted Go string literal representing s....
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func Quote(s string) string {
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</pre>
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<p>
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The complete English sentence form 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 (in progress) 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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<p>
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instead of:
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</p>
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<pre class="bad">
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/* not Go style */
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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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</pre>
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<p>
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Comments are text, not HTML, and not any kind of markup.
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Refrain from ASCII embellishment like *this* or /this/.
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As usual, read the Go sources for examples.
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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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In Go, 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 lowercase 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 vector,
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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 a unique
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identifier.
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</p>
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<h3 id="name-length">Avoid lengthy 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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On the same note, <code>i</code> and <code>j</code> are better pair of names for
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index variables than <code>i1</code> and <code>i2</code> (or, worse, <code>index1</code> and <code>index2</code>),
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because they are easier to tell apart when reading
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the program quickly.
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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 in a larger variety of contexts.
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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>. Using an interface name distinct
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from the method name keeps an anonymous struct
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field of type <code>Reader</code> from conflicting with its own
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<code>Read</code> method.
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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, equally important, the same 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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||||
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||||
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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)], 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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||||
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||||
<h3 id="else">Omit needless else bodies</h3>
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||||
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||||
<p>
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||||
If an <code>if</code> body doesn't flow off the end of the
|
||||
body—that is, the body ends in <code>break</code>, <code>continue</code>,
|
||||
<code>goto</code>, or <code>return</code>—it is preferable to omit the <code>else</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
For example:
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
f, err := os.Open(name, os.O_RDONLY, 0);
|
||||
if err != nil {
|
||||
return err;
|
||||
}
|
||||
codeUsing(f);
|
||||
f.Close();
|
||||
moreCode();
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
is preferable to:
|
||||
|
||||
<pre class="bad">
|
||||
/* not Go style */
|
||||
if f, err := os.Open(name, os.O_RDONLY, 0); err != nil {
|
||||
return err;
|
||||
} else {
|
||||
codeUsing(f);
|
||||
f.Close();
|
||||
}
|
||||
moreCode();
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
The first form
|
||||
avoids unnecessary indentation
|
||||
and makes it clear that <code>moreCode()</code>
|
||||
only runs when <code>f.Close()</code> does.
|
||||
</p>
|
||||
|
||||
<h3 id="switch">Switch</h3>
|
||||
|
||||
<p>
|
||||
Go's <code>switch</code> is more powerful than C's.
|
||||
When an <code>if</code>-<code>else if</code>-<code>else</code> chain has three or more bodies,
|
||||
or an <code>if</code> condition has a long list of alternatives,
|
||||
consider rewriting it using <code>switch</code>.
|
||||
</p>
|
||||
|
||||
<a href="/src/pkg/bytes/bytes.go">go/src/pkg/bytes/bytes.go</a>:
|
||||
<pre>
|
||||
// Compare returns an integer comparing the two byte arrays lexicographically.
|
||||
// The result will be 0 if a==b, -1 if a < b, and +1 if a > b
|
||||
func Compare(a, b []byte) int {
|
||||
for i := 0; i < len(a) && i < len(b); i++ {
|
||||
switch {
|
||||
case a[i] > b[i]:
|
||||
return 1
|
||||
case a[i] < b[i]:
|
||||
return -1
|
||||
}
|
||||
}
|
||||
switch {
|
||||
case len(a) < len(b):
|
||||
return -1
|
||||
case len(a) > len(b):
|
||||
return 1
|
||||
}
|
||||
return 0
|
||||
}
|
||||
</pre>
|
||||
|
||||
<a href="/src/pkg/http/url.go">go/src/pkg/http/url.go</a>:
|
||||
<pre>
|
||||
func unhex(c byte) byte {
|
||||
switch {
|
||||
case '0' <= c && c <= '9':
|
||||
return c - '0'
|
||||
case 'a' <= c && c <= 'f':
|
||||
return c - 'a' + 10
|
||||
case 'A' <= c && c <= 'F':
|
||||
return c - 'A' + 10
|
||||
}
|
||||
return 0
|
||||
}
|
||||
</pre>
|
||||
|
||||
<a href="/src/pkg/http/url.go">go/src/pkg/http/url.go</a>:
|
||||
<pre>
|
||||
func shouldEscape(c byte) bool {
|
||||
switch c {
|
||||
case ' ', '?', '&', '=', '#', '+', '%':
|
||||
return true
|
||||
}
|
||||
return false
|
||||
}
|
||||
</pre>
|
||||
|
||||
<h2 id="functions">Functions</h2>
|
||||
|
||||
<h3 id="omit-wrappers">Omit needless wrappers</h3>
|
||||
|
||||
<p>
|
||||
Functions are great for factoring out common functionality.
|
||||
If a function is only called once,
|
||||
ask whether the function is really necessary,
|
||||
especially if it is just a short wrapper around another function.
|
||||
This style runs rampant in C++ code: wrappers
|
||||
call wrappers that call wrappers that call wrappers.
|
||||
Doing this hinders people trying to understand the program,
|
||||
not to mention computers trying to execute it.
|
||||
</p>
|
||||
|
||||
<h3 id="multiple-returns">Return multiple values</h3>
|
||||
|
||||
<p>
|
||||
If a function must return multiple values, it can
|
||||
do so directly.
|
||||
The C “pass in a pointer to a return value”
|
||||
idiom is dead.
|
||||
</p>
|
||||
|
||||
|
||||
<h2 id="errors">Errors</h2>
|
||||
|
||||
<h3 id="handle-errors-first">Handle errors first</h3>
|
||||
|
||||
<p>
|
||||
Errors tend to be simpler than non-error cases,
|
||||
and it helps readability when the non-error flow
|
||||
of control is always down the page.
|
||||
Also, error cases tend to end in jumps,
|
||||
so that there is <a href="#else">no need for an explicit else</a>.
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
f, err := os.Open(name, os.O_RDONLY, 0);
|
||||
if err != nil {
|
||||
return err;
|
||||
}
|
||||
codeUsing(f);
|
||||
f.Close();
|
||||
moreCode();
|
||||
</pre>
|
||||
|
||||
is preferable to:
|
||||
|
||||
<pre class="bad">
|
||||
/* not Go style */
|
||||
f, err := os.Open(name, os.O_RDONLY, 0);
|
||||
if err == nil {
|
||||
codeUsing(f);
|
||||
f.Close();
|
||||
} else {
|
||||
return err;
|
||||
}
|
||||
moreCode();
|
||||
</pre>
|
||||
|
||||
<h3 id="error-returns">Return <code>os.Error</code>, not <code>bool</code></h3>
|
||||
|
||||
<p>
|
||||
Few functions have just one failure mode.
|
||||
Instead of returning a boolean to signal success,
|
||||
return an <code>os.Error</code> that describes the failure.
|
||||
Even if there is only one failure mode now,
|
||||
there may be more later.
|
||||
</p>
|
||||
|
||||
<h3 id="error-context">Return structured errors</h3>
|
||||
|
||||
Implementations of <code>os.Error</code>s should
|
||||
describe the error but also include context.
|
||||
For example, <code>os.Open</code> returns an <code>os.PathError</code>:
|
||||
|
||||
<a href="/src/pkg/os/file.go">/src/pkg/os/file.go</a>:
|
||||
<pre>
|
||||
XXX definition of PathError and .String
|
||||
</pre>
|
||||
|
||||
<code>PathError</code>'s <code>String</code> formats
|
||||
the error nicely and is the usual way the error gets used.
|
||||
Callers that care about the precise error details can
|
||||
use a type switch or a type guard to look for specific
|
||||
errors and then extract details.
|
||||
|
||||
<pre>
|
||||
XXX example here - MkdirAll
|
||||
</pre>
|
||||
|
||||
<h2 id="types">Programmer-defined types</h2>
|
||||
|
||||
<h3 id="constructors">Use <code>NewTypeName</code> for constructors</h3>
|
||||
|
||||
<p>
|
||||
The constructor for the type <code>pkg.MyType</code> should
|
||||
be named <code>pkg.NewMyType</code> and should return <code>*pkg.MyType</code>.
|
||||
The implementation of <code>NewTypeName</code> often uses the
|
||||
<a href="#allocating-a-struct">struct allocation idiom</a>.
|
||||
</p>
|
||||
|
||||
<a href="xxx">go/src/pkg/os/file.go</a>:
|
||||
<pre>
|
||||
func NewFile(fd int, name string) *File {
|
||||
if file < 0 {
|
||||
return nil
|
||||
}
|
||||
return &File{fd, name, nil, 0}
|
||||
}
|
||||
</pre>
|
||||
|
||||
<p>Packages that export only a single type sometimes
|
||||
shorten <code>NewTypeName</code> to <code>New</code>;
|
||||
for example, the vector constructor is
|
||||
<code>vector.New</code>, not <code>vector.NewVector</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
A type that is intended to be allocated
|
||||
as part of a larger struct may have an <code>Init</code> method
|
||||
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 cannot distinguish the two algorithms.
|
||||
</p>
|
||||
|
||||
<h3 id="">Use interface adapters to expand an implementation</h3>
|
||||
|
||||
XXX
|
||||
|
||||
<h3 id="">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>
|
||||
Loading…
Reference in New Issue
Block a user