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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Go errors Package — Ultimate Guide</title>
<style>
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
:root {
--bg: #f6f8fa;
--surface: #ffffff;
--surface2: #f0f2f5;
--border: #d0d7de;
--accent: #0a8060;
--accent2: #0969da;
--accent3: #9a6700;
--accent4: #cf222e;
--text: #1f2328;
--muted: #656d76;
--tag-bg: #dafbe1;
--tag-text: #1a7f37;
--font-mono: 'JetBrains Mono', 'Fira Code', 'Cascadia Code', monospace;
--font-sans: 'Inter', 'Segoe UI', system-ui, sans-serif;
--radius: 12px;
}
html { scroll-behavior: smooth; }
body {
background: var(--bg);
color: var(--text);
font-family: var(--font-sans);
line-height: 1.6;
padding: 0 0 80px;
}
a { color: var(--accent2); text-decoration: none; }
a:hover { text-decoration: underline; }
.hero {
background: linear-gradient(135deg, #eaf2ff 0%, #e6f6f1 50%, #f0f8ff 100%);
border-bottom: 1px solid var(--border);
padding: 72px 24px 60px;
text-align: center;
position: relative;
overflow: hidden;
}
.hero::before {
content: '';
position: absolute;
inset: 0;
background:
radial-gradient(ellipse 60% 40% at 30% 60%, rgba(10,128,96,.06) 0%, transparent 70%),
radial-gradient(ellipse 50% 40% at 70% 40%, rgba(9,105,218,.05) 0%, transparent 70%);
pointer-events: none;
}
.hero-badge {
display: inline-block;
font-family: var(--font-mono);
font-size: .72rem;
letter-spacing: .1em;
text-transform: uppercase;
background: var(--tag-bg);
color: var(--tag-text);
border: 1px solid #82cfaa;
border-radius: 100px;
padding: 4px 14px;
margin-bottom: 20px;
}
.hero h1 {
font-size: clamp(2rem, 5vw, 3.4rem);
font-weight: 800;
letter-spacing: -.03em;
line-height: 1.1;
margin-bottom: 16px;
}
.hero h1 span { color: var(--accent2); }
.hero p {
font-size: 1.1rem;
color: var(--muted);
max-width: 540px;
margin: 0 auto 32px;
}
.hero-pills {
display: flex;
flex-wrap: wrap;
gap: 10px;
justify-content: center;
}
.hero-pills span {
background: var(--surface2);
border: 1px solid var(--border);
border-radius: 100px;
font-family: var(--font-mono);
font-size: .78rem;
padding: 5px 14px;
color: var(--muted);
}
.layout {
display: grid;
grid-template-columns: 200px 1fr 220px;
max-width: 1300px;
margin: 0 auto;
gap: 0;
}
.pkg-nav {
position: sticky;
top: 0;
height: 100vh;
overflow-y: auto;
padding: 32px 0 32px 24px;
}
.pkg-list {
list-style: none;
display: flex;
flex-direction: column;
gap: 4px;
}
.pkg-list a {
display: flex;
align-items: center;
gap: 8px;
font-size: .86rem;
font-family: var(--font-mono);
color: var(--muted);
padding: 7px 10px;
border-radius: 7px;
border: 1px solid transparent;
transition: all .15s;
text-decoration: none;
}
.pkg-list a:hover {
color: var(--text);
background: var(--surface);
border-color: var(--border);
}
.pkg-list a.active {
color: var(--accent2);
background: rgba(9,105,218,.07);
border-color: rgba(9,105,218,.2);
font-weight: 600;
}
.pkg-list a .pkg-icon { font-size: .85rem; width: 18px; text-align: center; }
.home-link {
display: block;
font-weight: 700;
font-size: .88rem;
color: var(--text);
padding: 4px 10px 18px;
letter-spacing: -.01em;
text-decoration: none;
}
.home-link:hover { color: var(--accent2); text-decoration: none; }
.nav-group {
font-size: .62rem;
font-weight: 700;
letter-spacing: .12em;
text-transform: uppercase;
color: var(--muted);
padding: 4px 10px 6px;
margin-top: 8px;
}
.toc-nav {
position: sticky;
top: 0;
height: 100vh;
overflow-y: auto;
padding: 32px 24px 32px 0;
}
.toc-title {
font-size: .68rem;
font-weight: 700;
letter-spacing: .12em;
text-transform: uppercase;
color: var(--muted);
margin-bottom: 12px;
padding-left: 8px;
}
.toc {
list-style: none;
display: flex;
flex-direction: column;
gap: 2px;
}
.toc a {
display: block;
font-size: .8rem;
color: var(--muted);
padding: 5px 8px;
border-radius: 6px;
border-right: 2px solid transparent;
transition: all .15s;
}
.toc a:hover {
color: var(--text);
background: var(--surface);
border-right-color: var(--accent);
text-decoration: none;
}
.content {
padding: 40px 32px;
min-width: 0;
border-left: 1px solid var(--border);
border-right: 1px solid var(--border);
}
.section {
margin-bottom: 60px;
scroll-margin-top: 24px;
}
.section-header {
display: flex;
align-items: center;
gap: 12px;
margin-bottom: 24px;
padding-bottom: 14px;
border-bottom: 1px solid var(--border);
}
.section-icon {
width: 36px;
height: 36px;
border-radius: 8px;
display: grid;
place-items: center;
font-size: 1.1rem;
flex-shrink: 0;
}
.icon-blue { background: rgba(9,105,218,.1); color: var(--accent2); }
.icon-green { background: rgba(10,128,96,.1); color: var(--accent); }
.icon-gold { background: rgba(154,103,0,.1); color: var(--accent3); }
.icon-red { background: rgba(207,34,46,.1); color: var(--accent4); }
.icon-purple { background: rgba(130,80,223,.1); color: #5a32a3; }
.section-header h2 {
font-size: 1.3rem;
font-weight: 700;
letter-spacing: -.02em;
}
.section-header .section-tag {
margin-left: auto;
font-family: var(--font-mono);
font-size: .68rem;
background: var(--surface2);
border: 1px solid var(--border);
border-radius: 100px;
padding: 2px 10px;
color: var(--muted);
}
.card {
background: var(--surface);
border: 1px solid var(--border);
border-radius: var(--radius);
overflow: hidden;
margin-bottom: 20px;
transition: border-color .2s;
}
.card:hover { border-color: #adbac7; }
.card-header {
display: flex;
align-items: center;
justify-content: space-between;
padding: 14px 18px 12px;
border-bottom: 1px solid var(--border);
background: var(--surface2);
}
.card-title {
font-size: .9rem;
font-weight: 600;
color: var(--text);
}
.card-badge {
font-family: var(--font-mono);
font-size: .65rem;
border-radius: 6px;
padding: 2px 8px;
font-weight: 600;
}
.badge-blue { background: rgba(9,105,218,.08); color: var(--accent2); border: 1px solid rgba(9,105,218,.2); }
.badge-green { background: rgba(10,128,96,.08); color: var(--accent); border: 1px solid rgba(10,128,96,.2); }
.badge-gold { background: rgba(154,103,0,.08); color: var(--accent3); border: 1px solid rgba(154,103,0,.2); }
.badge-red { background: rgba(207,34,46,.08); color: var(--accent4); border: 1px solid rgba(207,34,46,.2); }
.badge-purple { background: rgba(130,80,223,.08); color: #5a32a3; border: 1px solid rgba(130,80,223,.2); }
.code-wrap {
position: relative;
padding: 20px 20px 18px;
}
pre {
font-family: var(--font-mono);
font-size: .82rem;
line-height: 1.75;
overflow-x: auto;
tab-size: 4;
}
.kw { color: #cf222e; }
.fn { color: #8250df; }
.str { color: #0a3069; }
.cmt { color: #6e7781; font-style: italic; }
.num { color: #0550ae; }
.pkg { color: #0969da; }
.typ { color: #953800; }
.out { color: #1a7f37; }
.callout {
display: flex;
gap: 12px;
background: #ddf4ff;
border: 1px solid #54aeff80;
border-radius: 8px;
padding: 14px 16px;
margin-bottom: 20px;
font-size: .86rem;
color: #1f2328;
}
.callout.warn {
background: #fff8c5;
border-color: #d4a72c80;
}
.callout.tip {
background: #dafbe1;
border-color: #4ac26b80;
}
.callout-icon { flex-shrink: 0; font-size: 1rem; margin-top: 1px; }
.two-col { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }
.ref-table {
width: 100%;
border-collapse: collapse;
font-size: .83rem;
border-radius: var(--radius);
overflow: hidden;
border: 1px solid var(--border);
}
.ref-table thead th {
background: var(--surface2);
padding: 10px 14px;
text-align: left;
font-weight: 600;
font-size: .75rem;
letter-spacing: .05em;
text-transform: uppercase;
color: var(--muted);
border-bottom: 1px solid var(--border);
}
.ref-table tbody tr { border-bottom: 1px solid var(--border); }
.ref-table tbody tr:last-child { border-bottom: none; }
.ref-table tbody tr:hover { background: var(--surface2); }
.ref-table td { padding: 9px 14px; vertical-align: top; }
.ref-table td:first-child {
font-family: var(--font-mono);
font-size: .78rem;
color: var(--accent3);
white-space: nowrap;
}
.ref-table td:nth-child(2) {
font-family: var(--font-mono);
font-size: .75rem;
color: var(--accent2);
}
.ref-table td:last-child { color: var(--muted); }
@media (max-width: 900px) {
.layout { grid-template-columns: 1fr; }
.pkg-nav, .toc-nav { display: none; }
.content { padding: 24px 16px; border: none; }
.two-col { grid-template-columns: 1fr; }
}
</style>
</head>
<body>
<div class="hero">
<h1>Go <span>errors</span> Package</h1>
<p>Create errors, wrap them with context, and inspect chains with errors.Is, errors.As, and custom error types.</p>
<div class="hero-pills">
<span>errors.New</span>
<span>fmt.Errorf %w</span>
<span>errors.Is</span>
<span>errors.As</span>
<span>errors.Unwrap</span>
<span>errors.Join</span>
<span>Custom types</span>
</div>
</div>
<div class="layout">
<nav class="pkg-nav">
<a href="index.html" class="home-link">Go Handbook</a>
<div class="nav-group">Language</div>
<ul class="pkg-list">
<li><a href="basics.html"><span class="pkg-icon">📖</span>Basics</a></li>
<li><a href="data-structures.html"><span class="pkg-icon">📦</span>Data Structures</a></li>
<li><a href="concurrency.html"><span class="pkg-icon">🔀</span>Concurrency</a></li>
<li><a href="testing.html"><span class="pkg-icon">🧪</span>Testing</a></li>
<li><a href="reflection.html"><span class="pkg-icon">🔬</span>Reflection</a></li>
<li><a href="file-data.html"><span class="pkg-icon">📁</span>File & Data</a></li>
<li><a href="web.html"><span class="pkg-icon">🌐</span>Web</a></li>
<li><a href="packages-modules.html"><span class="pkg-icon">🗃</span>Packages & Modules</a></li>
<li><a href="performance.html"><span class="pkg-icon">⚡</span>Performance</a></li>
<li><a href="gotchas.html"><span class="pkg-icon">⚠️</span>Gotchas</a></li>
</ul>
<div class="nav-group">Packages</div>
<ul class="pkg-list">
<li><a href="strings-package-guide.html"><span class="pkg-icon">🔤</span>strings</a></li>
<li><a href="fmt-package-guide.html"><span class="pkg-icon">🖨</span>fmt</a></li>
<li><a href="strconv-package-guide.html"><span class="pkg-icon">🔢</span>strconv</a></li>
<li><a href="sort-package-guide.html"><span class="pkg-icon">📊</span>sort</a></li>
<li><a href="slices-package-guide.html"><span class="pkg-icon">🗂</span>slices</a></li>
<li><a href="io-package-guide.html"><span class="pkg-icon">🔌</span>io</a></li>
<li><a href="os-package-guide.html"><span class="pkg-icon">💻</span>os</a></li>
<li><a href="errors-package-guide.html" class="active"><span class="pkg-icon">⚠️</span>errors</a></li>
<li><a href="context-package-guide.html"><span class="pkg-icon">🎯</span>context</a></li>
<li><a href="time-package-guide.html"><span class="pkg-icon">⏱</span>time</a></li>
<li><a href="math-package-guide.html"><span class="pkg-icon">🧮</span>math</a></li>
</ul>
</nav>
<main class="content">
<!-- ── 1. CREATING ERRORS ────────────────────────────── -->
<section class="section" id="creating">
<div class="section-header">
<div class="section-icon icon-red">🆕</div>
<h2>Creating Errors</h2>
<span class="section-tag">errors.New · fmt.Errorf</span>
</div>
<div class="callout">
<span class="callout-icon">ℹ️</span>
<div>Go errors are just values that implement the <code>error</code> interface: <code>Error() string</code>. Use <code>errors.New</code> for static messages and <code>fmt.Errorf</code> when you need to embed dynamic values.</div>
</div>
<div class="two-col">
<div class="card">
<div class="card-header">
<span class="card-title">errors.New — simple static errors</span>
<span class="card-badge badge-red">errors.New</span>
</div>
<div class="code-wrap">
<pre><span class="kw">import</span> <span class="str">"errors"</span>
<span class="cmt">// Define sentinel errors at package level</span>
<span class="kw">var</span> (
ErrNotFound = <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"not found"</span>)
ErrPermission = <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"permission denied"</span>)
ErrTimeout = <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"operation timed out"</span>)
)
<span class="kw">func</span> <span class="fn">findUser</span>(id <span class="typ">int</span>) (*<span class="typ">User</span>, <span class="typ">error</span>) {
<span class="kw">if</span> id <= <span class="num">0</span> {
<span class="kw">return</span> <span class="kw">nil</span>, ErrNotFound
}
<span class="cmt">// ...</span>
}</pre>
</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">fmt.Errorf — errors with context</span>
<span class="card-badge badge-gold">fmt.Errorf</span>
</div>
<div class="code-wrap">
<pre><span class="kw">import</span> <span class="str">"fmt"</span>
<span class="kw">func</span> <span class="fn">loadConfig</span>(path <span class="typ">string</span>) <span class="typ">error</span> {
_, err := <span class="pkg">os</span>.<span class="fn">Open</span>(path)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="cmt">// %w wraps the original error</span>
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"loadConfig %q: %w"</span>, path, err)
}
<span class="kw">return</span> <span class="kw">nil</span>
}
<span class="cmt">// Without %w — context only, no unwrapping</span>
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"invalid port %d"</span>, port)</pre>
</div>
</div>
</div>
</section>
<!-- ── 2. WRAPPING ───────────────────────────────────── -->
<section class="section" id="wrapping">
<div class="section-header">
<div class="section-icon icon-blue">🎁</div>
<h2>Wrapping & Unwrapping</h2>
<span class="section-tag">%w · errors.Unwrap</span>
</div>
<div class="callout tip">
<span class="callout-icon">💡</span>
<div>Wrap errors as they travel up the call stack to add context at each layer. The convention is <code>"operationName context: %w"</code> — a colon-separated chain of what went wrong and where.</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Building an error chain with %w</span>
<span class="card-badge badge-blue">Wrapping</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Low-level: returns a raw OS error</span>
<span class="kw">func</span> <span class="fn">readFile</span>(path <span class="typ">string</span>) ([]<span class="typ">byte</span>, <span class="typ">error</span>) {
data, err := <span class="pkg">os</span>.<span class="fn">ReadFile</span>(path)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="kw">return</span> <span class="kw">nil</span>, <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"readFile: %w"</span>, err)
}
<span class="kw">return</span> data, <span class="kw">nil</span>
}
<span class="cmt">// Mid-level: adds more context</span>
<span class="kw">func</span> <span class="fn">loadConfig</span>(env <span class="typ">string</span>) (*<span class="typ">Config</span>, <span class="typ">error</span>) {
data, err := <span class="fn">readFile</span>(<span class="str">"/etc/app/"</span> + env + <span class="str">".json"</span>)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="kw">return</span> <span class="kw">nil</span>, <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"loadConfig %q: %w"</span>, env, err)
}
<span class="cmt">// ...</span>
}
<span class="cmt">// Resulting error message:</span>
<span class="cmt">// loadConfig "prod": readFile: open /etc/app/prod.json: no such file or directory</span></pre>
</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">errors.Unwrap — peel one layer off</span>
<span class="card-badge badge-blue">Unwrap</span>
</div>
<div class="code-wrap">
<pre><span class="kw">var</span> ErrBase = <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"base error"</span>)
wrapped := <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"layer 1: %w"</span>, ErrBase)
wrapped2 := <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"layer 2: %w"</span>, wrapped)
<span class="pkg">errors</span>.<span class="fn">Unwrap</span>(wrapped2) <span class="cmt">// layer 1: base error</span>
<span class="pkg">errors</span>.<span class="fn">Unwrap</span>(wrapped) <span class="cmt">// base error (ErrBase)</span>
<span class="pkg">errors</span>.<span class="fn">Unwrap</span>(ErrBase) <span class="cmt">// nil — nothing left to unwrap</span>
<span class="cmt">// In practice you rarely call Unwrap directly.</span>
<span class="cmt">// Use errors.Is / errors.As instead — they traverse the chain for you.</span></pre>
</div>
</div>
</section>
<!-- ── 3. ERRORS.IS ──────────────────────────────────── -->
<section class="section" id="is">
<div class="section-header">
<div class="section-icon icon-green">🔍</div>
<h2>errors.Is</h2>
<span class="section-tag">Sentinel value matching</span>
</div>
<div class="callout">
<span class="callout-icon">ℹ️</span>
<div><code>errors.Is(err, target)</code> walks the entire error chain (unwrapping at each step) and returns <code>true</code> if any error in the chain equals <code>target</code>. Never use <code>==</code> for wrapped errors.</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">errors.Is — check for a specific sentinel error</span>
<span class="card-badge badge-green">errors.Is</span>
</div>
<div class="code-wrap">
<pre><span class="kw">import</span> (
<span class="str">"database/sql"</span>
<span class="str">"errors"</span>
)
<span class="kw">func</span> <span class="fn">getUser</span>(id <span class="typ">int</span>) (*<span class="typ">User</span>, <span class="typ">error</span>) {
row, err := db.<span class="fn">QueryRow</span>(<span class="str">"SELECT ..."</span>, id)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="kw">return</span> <span class="kw">nil</span>, <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"getUser %d: %w"</span>, id, err)
}
<span class="kw">return</span> row, <span class="kw">nil</span>
}
user, err := <span class="fn">getUser</span>(<span class="num">99</span>)
<span class="kw">if</span> <span class="pkg">errors</span>.<span class="fn">Is</span>(err, sql.ErrNoRows) {
<span class="cmt">// Correctly matches even though err is wrapped</span>
<span class="pkg">fmt</span>.<span class="fn">Println</span>(<span class="str">"user not found"</span>)
}
<span class="cmt">// DO NOT use == for wrapped errors</span>
<span class="kw">if</span> err == sql.ErrNoRows { <span class="cmt">// WRONG — misses wrapped errors</span> }</pre>
</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Custom Is method — value-based matching</span>
<span class="card-badge badge-purple">Custom Is</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Implement Is on your type when equality needs custom logic</span>
<span class="kw">type</span> <span class="typ">HTTPError</span> <span class="kw">struct</span> {
Code <span class="typ">int</span>
Message <span class="typ">string</span>
}
<span class="kw">func</span> (e *<span class="typ">HTTPError</span>) <span class="fn">Error</span>() <span class="typ">string</span> {
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Sprintf</span>(<span class="str">"HTTP %d: %s"</span>, e.Code, e.Message)
}
<span class="cmt">// Is matches on Code alone, ignoring Message</span>
<span class="kw">func</span> (e *<span class="typ">HTTPError</span>) <span class="fn">Is</span>(target <span class="typ">error</span>) <span class="typ">bool</span> {
t, ok := target.(*<span class="typ">HTTPError</span>)
<span class="kw">return</span> ok && e.Code == t.Code
}
ErrNotFound := &<span class="typ">HTTPError</span>{Code: <span class="num">404</span>}
err := <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"request failed: %w"</span>, &<span class="typ">HTTPError</span>{Code: <span class="num">404</span>, Message: <span class="str">"page gone"</span>})
<span class="pkg">errors</span>.<span class="fn">Is</span>(err, ErrNotFound) <span class="cmt">// true — Code 404 matches</span></pre>
</div>
</div>
</section>
<!-- ── 4. ERRORS.AS ──────────────────────────────────── -->
<section class="section" id="as">
<div class="section-header">
<div class="section-icon icon-gold">🎯</div>
<h2>errors.As</h2>
<span class="section-tag">Type-based extraction</span>
</div>
<div class="callout tip">
<span class="callout-icon">💡</span>
<div><code>errors.As(err, &target)</code> walks the chain and assigns the first error that can be assigned to <code>target</code>'s type. Use it when you need to read fields off a specific error type.</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">errors.As — extract a typed error from the chain</span>
<span class="card-badge badge-gold">errors.As</span>
</div>
<div class="code-wrap">
<pre><span class="kw">type</span> <span class="typ">ValidationError</span> <span class="kw">struct</span> {
Field <span class="typ">string</span>
Message <span class="typ">string</span>
}
<span class="kw">func</span> (e *<span class="typ">ValidationError</span>) <span class="fn">Error</span>() <span class="typ">string</span> {
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Sprintf</span>(<span class="str">"validation: %s %s"</span>, e.Field, e.Message)
}
<span class="kw">func</span> <span class="fn">validateAge</span>(age <span class="typ">int</span>) <span class="typ">error</span> {
<span class="kw">if</span> age < <span class="num">0</span> {
<span class="kw">return</span> &<span class="typ">ValidationError</span>{Field: <span class="str">"age"</span>, Message: <span class="str">"must be non-negative"</span>}
}
<span class="kw">return</span> <span class="kw">nil</span>
}
err := <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"createUser: %w"</span>, <span class="fn">validateAge</span>(-<span class="num">1</span>))
<span class="kw">var</span> ve *<span class="typ">ValidationError</span>
<span class="kw">if</span> <span class="pkg">errors</span>.<span class="fn">As</span>(err, &ve) {
<span class="cmt">// ve is now populated with the ValidationError</span>
<span class="pkg">fmt</span>.<span class="fn">Printf</span>(<span class="str">"bad field: %s — %s\n"</span>, ve.Field, ve.Message)
<span class="cmt">// bad field: age — must be non-negative</span>
}</pre>
</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Custom As method — control how As matches your type</span>
<span class="card-badge badge-purple">Custom As</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Implement As when your error wraps another and you need</span>
<span class="cmt">// errors.As to "see through" into a nested error field</span>
<span class="kw">type</span> <span class="typ">DBError</span> <span class="kw">struct</span> {
Query <span class="typ">string</span>
Cause <span class="typ">error</span>
}
<span class="kw">func</span> (e *<span class="typ">DBError</span>) <span class="fn">Error</span>() <span class="typ">string</span> { <span class="kw">return</span> e.Cause.<span class="fn">Error</span>() }
<span class="kw">func</span> (e *<span class="typ">DBError</span>) <span class="fn">Unwrap</span>() <span class="typ">error</span> { <span class="kw">return</span> e.Cause }
<span class="cmt">// errors.As walks Unwrap() automatically — no custom As needed here.</span>
<span class="cmt">// Only implement As when Unwrap isn't sufficient.</span></pre>
</div>
</div>
</section>
<!-- ── 5. CUSTOM ERROR TYPES ─────────────────────────── -->
<section class="section" id="custom">
<div class="section-header">
<div class="section-icon icon-purple">🏗️</div>
<h2>Custom Error Types</h2>
<span class="section-tag">error interface · Unwrap</span>
</div>
<div class="callout">
<span class="callout-icon">ℹ️</span>
<div>Any type that implements <code>Error() string</code> satisfies the <code>error</code> interface. Add an <code>Unwrap() error</code> method to make your type work seamlessly with <code>errors.Is</code> and <code>errors.As</code>.</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Structured error type with Unwrap</span>
<span class="card-badge badge-purple">Custom Type</span>
</div>
<div class="code-wrap">
<pre><span class="kw">type</span> <span class="typ">RequestError</span> <span class="kw">struct</span> {
StatusCode <span class="typ">int</span>
URL <span class="typ">string</span>
Err <span class="typ">error</span>
}
<span class="kw">func</span> (e *<span class="typ">RequestError</span>) <span class="fn">Error</span>() <span class="typ">string</span> {
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Sprintf</span>(<span class="str">"%s → HTTP %d: %v"</span>, e.URL, e.StatusCode, e.Err)
}
<span class="cmt">// Unwrap lets errors.Is and errors.As look inside</span>
<span class="kw">func</span> (e *<span class="typ">RequestError</span>) <span class="fn">Unwrap</span>() <span class="typ">error</span> { <span class="kw">return</span> e.Err }
<span class="cmt">// Usage</span>
err := &<span class="typ">RequestError</span>{
StatusCode: <span class="num">503</span>,
URL: <span class="str">"https://api.example.com/data"</span>,
Err: ErrServiceUnavailable,
}
<span class="kw">var</span> re *<span class="typ">RequestError</span>
<span class="kw">if</span> <span class="pkg">errors</span>.<span class="fn">As</span>(err, &re) {
<span class="pkg">fmt</span>.<span class="fn">Printf</span>(<span class="str">"failed URL: %s (status %d)\n"</span>, re.URL, re.StatusCode)
}</pre>
</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Sentinel errors vs. typed errors</span>
<span class="card-badge badge-blue">Pattern</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Sentinel: simple, check with errors.Is</span>
<span class="kw">var</span> ErrNotFound = <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"not found"</span>)
<span class="cmt">// Typed: carries data, check with errors.As</span>
<span class="kw">type</span> <span class="typ">NotFoundError</span> <span class="kw">struct</span>{ Resource <span class="typ">string</span>; ID <span class="typ">int</span> }
<span class="kw">func</span> (e *<span class="typ">NotFoundError</span>) <span class="fn">Error</span>() <span class="typ">string</span> {
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Sprintf</span>(<span class="str">"%s %d not found"</span>, e.Resource, e.ID)
}
<span class="cmt">// Caller pattern</span>
<span class="kw">var</span> nfe *<span class="typ">NotFoundError</span>
<span class="kw">switch</span> {
<span class="kw">case</span> <span class="pkg">errors</span>.<span class="fn">As</span>(err, &nfe):
<span class="pkg">fmt</span>.<span class="fn">Printf</span>(<span class="str">"missing %s #%d\n"</span>, nfe.Resource, nfe.ID)
<span class="kw">case</span> <span class="pkg">errors</span>.<span class="fn">Is</span>(err, ErrNotFound):
<span class="pkg">fmt</span>.<span class="fn">Println</span>(<span class="str">"something not found"</span>)
<span class="kw">default</span>:
<span class="pkg">fmt</span>.<span class="fn">Println</span>(<span class="str">"unexpected error:"</span>, err)
}</pre>
</div>
</div>
</section>
<!-- ── 6. ERRORS.JOIN ────────────────────────────────── -->
<section class="section" id="join">
<div class="section-header">
<div class="section-icon icon-blue">🔗</div>
<h2>errors.Join</h2>
<span class="section-tag">Multiple errors · Go 1.20+</span>
</div>
<div class="callout">
<span class="callout-icon">ℹ️</span>
<div><code>errors.Join</code> (Go 1.20) combines multiple errors into one. The result's <code>Unwrap</code> returns a slice, so <code>errors.Is</code> and <code>errors.As</code> check all of them.</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">errors.Join — accumulate multiple errors</span>
<span class="card-badge badge-blue">Go 1.20+</span>
</div>
<div class="code-wrap">
<pre><span class="kw">func</span> <span class="fn">validateUser</span>(u <span class="typ">User</span>) <span class="typ">error</span> {
<span class="kw">var</span> errs []<span class="typ">error</span>
<span class="kw">if</span> u.Name == <span class="str">""</span> {
errs = <span class="fn">append</span>(errs, <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"name is required"</span>))
}
<span class="kw">if</span> u.Age < <span class="num">0</span> {
errs = <span class="fn">append</span>(errs, <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"age must be non-negative"</span>))
}
<span class="kw">if</span> u.Email == <span class="str">""</span> {
errs = <span class="fn">append</span>(errs, <span class="pkg">errors</span>.<span class="fn">New</span>(<span class="str">"email is required"</span>))
}
<span class="kw">return</span> <span class="pkg">errors</span>.<span class="fn">Join</span>(errs...) <span class="cmt">// nil if errs is empty</span>
}
err := <span class="fn">validateUser</span>(<span class="typ">User</span>{Age: -<span class="num">1</span>})
<span class="cmt">// err.Error():
// name is required
// age must be non-negative
// email is required</span>
<span class="cmt">// errors.Is still works across the joined set</span>
<span class="pkg">errors</span>.<span class="fn">Is</span>(err, ErrSomeSentinel) <span class="cmt">// checks all joined errors</span></pre>
</div>
</div>
</section>
<!-- ── 7. PATTERNS ───────────────────────────────────── -->
<section class="section" id="patterns">
<div class="section-header">
<div class="section-icon icon-green">📐</div>
<h2>Patterns & Best Practices</h2>
<span class="section-tag">Idioms · dos and don'ts</span>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Idiomatic error handling flow</span>
<span class="card-badge badge-green">Pattern</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Return errors immediately — keep the happy path at the left margin</span>
<span class="kw">func</span> <span class="fn">process</span>(id <span class="typ">int</span>) (<span class="typ">string</span>, <span class="typ">error</span>) {
user, err := <span class="fn">getUser</span>(id)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="kw">return</span> <span class="str">""</span>, <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"process: %w"</span>, err)
}
data, err := <span class="fn">fetchData</span>(user.Token)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="kw">return</span> <span class="str">""</span>, <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"process fetchData: %w"</span>, err)
}
<span class="kw">return</span> <span class="fn">transform</span>(data), <span class="kw">nil</span>
}
<span class="cmt">// Handle errors at the right level — wrap when passing up,</span>
<span class="cmt">// handle (log/respond) when you can do something about it</span>
result, err := <span class="fn">process</span>(<span class="num">42</span>)
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="kw">var</span> nfe *<span class="typ">NotFoundError</span>
<span class="kw">if</span> <span class="pkg">errors</span>.<span class="fn">As</span>(err, &nfe) {
http.<span class="fn">Error</span>(w, nfe.<span class="fn">Error</span>(), http.StatusNotFound)
<span class="kw">return</span>
}
http.<span class="fn">Error</span>(w, <span class="str">"internal error"</span>, http.StatusInternalServerError)
<span class="pkg">log</span>.<span class="fn">Printf</span>(<span class="str">"process: %v"</span>, err)
<span class="kw">return</span>
}</pre>
</div>
</div>
<div class="two-col">
<div class="card">
<div class="card-header">
<span class="card-title">Do: wrap with context</span>
<span class="card-badge badge-green">Do</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Add what, where, and relevant IDs</span>
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"getOrder %d: %w"</span>, id, err)
<span class="cmt">// One wrap per function boundary is enough</span>
<span class="kw">func</span> <span class="fn">Save</span>(u *<span class="typ">User</span>) <span class="typ">error</span> {
<span class="kw">if</span> err := db.<span class="fn">Insert</span>(u); err != <span class="kw">nil</span> {
<span class="kw">return</span> <span class="pkg">fmt</span>.<span class="fn">Errorf</span>(<span class="str">"Save: %w"</span>, err)
}
<span class="kw">return</span> <span class="kw">nil</span>
}</pre>
</div>
</div>
<div class="card">
<div class="card-header">
<span class="card-title">Don't: log and return</span>
<span class="card-badge badge-red">Don't</span>
</div>
<div class="code-wrap">
<pre><span class="cmt">// Never log AND return — caller logs too → duplicate noise</span>
<span class="kw">if</span> err != <span class="kw">nil</span> {
<span class="pkg">log</span>.<span class="fn">Printf</span>(<span class="str">"error: %v"</span>, err) <span class="cmt">// BAD</span>
<span class="kw">return</span> err
}
<span class="cmt">// Never swallow errors silently</span>
result, _ := <span class="fn">riskyOp</span>() <span class="cmt">// BAD — ignoring errors</span>
<span class="cmt">// Either handle it or return it, not both</span></pre>
</div>
</div>
</div>
</section>
<!-- ── REFERENCE TABLE ───────────────────────────────── -->
<section class="section" id="reference">
<div class="section-header">
<div class="section-icon icon-blue">📋</div>
<h2>Quick Reference</h2>
<span class="section-tag">All key functions</span>
</div>
<table class="ref-table">
<thead>
<tr>
<th>Function / Method</th>
<th>Returns</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr><td>errors.New(text)</td><td>error</td><td>Create a simple error with a static message</td></tr>
<tr><td>fmt.Errorf(format, ...)</td><td>error</td><td>Create error with formatted message; use %w to wrap</td></tr>
<tr><td>errors.Is(err, target)</td><td>bool</td><td>Walk chain; true if any error equals target</td></tr>
<tr><td>errors.As(err, &target)</td><td>bool</td><td>Walk chain; assign first matching typed error</td></tr>
<tr><td>errors.Unwrap(err)</td><td>error</td><td>Return the directly wrapped error, or nil</td></tr>
<tr><td>errors.Join(errs...)</td><td>error</td><td>Combine multiple errors; nil if all nil (Go 1.20+)</td></tr>
<tr><td>Error() string</td><td>string</td><td>Interface method — implement to create custom errors</td></tr>
<tr><td>Unwrap() error</td><td>error</td><td>Implement on custom types to enable chain traversal</td></tr>
<tr><td>Is(error) bool</td><td>bool</td><td>Implement for custom equality logic in errors.Is</td></tr>
<tr><td>As(any) bool</td><td>bool</td><td>Implement for custom matching logic in errors.As</td></tr>
</tbody>
</table>
</section>
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<div class="toc-title">On this page</div>
<ul class="toc">
<li><a href="#creating">1 — Creating Errors</a></li>
<li><a href="#wrapping">2 — Wrapping</a></li>
<li><a href="#is">3 — errors.Is</a></li>
<li><a href="#as">4 — errors.As</a></li>
<li><a href="#custom">5 — Custom Types</a></li>
<li><a href="#join">6 — errors.Join</a></li>
<li><a href="#patterns">7 — Patterns</a></li>
<li><a href="#reference">99 — Quick Reference</a></li>
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