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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"><meta name="generator" content="rustdoc"><meta name="description" content="Efficient, configurable logging in Rust."><meta name="keywords" content="rust, rustlang, rust-lang, fern"><title>fern - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script>
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<noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu" role="button">☰</div><a class="sidebar-logo" href="../fern/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.png" alt="logo"></div>
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</a><h2 class="location">Crate fern</h2><div class="block version"><div class="narrow-helper"></div><p>Version 0.6.0</p></div><div class="sidebar-elems"><a id="all-types" href="all.html"><p>See all fern's items</p></a><div class="block items"><ul><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li><li><a href="#functions">Functions</a></li><li><a href="#types">Type Definitions</a></li></ul></div><div id="sidebar-vars" data-name="fern" data-ty="mod" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../fern/index.html"><img class="rust-logo" src="../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><span class="in-band">Crate <a class="mod" href="#">fern</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../src/fern/lib.rs.html#1-306" title="goto source code">[src]</a></span></h1><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Efficient, configurable logging in Rust.</p>
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<h2 id="depending-on-fern" class="section-header"><a href="#depending-on-fern">Depending on fern</a></h2>
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<p>Ensure you require both fern and log in your project’s <code>Cargo.toml</code>:</p>
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<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
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log = "0.4"
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fern = "0.5"</code></pre></div><h2 id="example-setup" class="section-header"><a href="#example-setup">Example setup</a></h2>
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<p>With fern, all logger configuration is done via builder-like methods on
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instances of the <a href="struct.Dispatch.html"><code>Dispatch</code></a> structure.</p>
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<p>Here’s an example logger which formats messages, and sends everything Debug
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and above to both stdout and an output.log file:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">log</span>::{<span class="ident">debug</span>, <span class="ident">error</span>, <span class="ident">info</span>, <span class="ident">trace</span>, <span class="ident">warn</span>};
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<span class="kw">fn</span> <span class="ident">setup_logger</span>() -> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">fern::InitError</span><span class="op">></span> {
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<span class="ident">fern::Dispatch::new</span>()
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.<span class="ident">format</span>(<span class="op">|</span><span class="ident">out</span>, <span class="ident">message</span>, <span class="ident">record</span><span class="op">|</span> {
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<span class="ident">out</span>.<span class="ident">finish</span>(<span class="macro">format_args!</span>(
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<span class="string">"{}[{}][{}] {}"</span>,
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<span class="ident">chrono::Local::now</span>().<span class="ident">format</span>(<span class="string">"[%Y-%m-%d][%H:%M:%S]"</span>),
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<span class="ident">record</span>.<span class="ident">target</span>(),
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<span class="ident">record</span>.<span class="ident">level</span>(),
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<span class="ident">message</span>
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))
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})
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.<span class="ident">level</span>(<span class="ident">log::LevelFilter::Debug</span>)
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.<span class="ident">chain</span>(<span class="ident">std::io::stdout</span>())
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.<span class="ident">chain</span>(<span class="ident">fern::log_file</span>(<span class="string">"output.log"</span>)<span class="question-mark">?</span>)
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.<span class="ident">apply</span>()<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(())
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}</code></pre></div>
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<p>Let’s unwrap this:</p>
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<hr />
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<p><a href="struct.Dispatch.html#method.new"><code>fern::Dispatch::new()</code></a></p>
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<p>Create an empty configuration.</p>
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<hr />
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<p><a href="struct.Dispatch.html#method.format"><code>.format(|...| ...)</code></a></p>
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<p>Add a formatter to the logger, modifying all messages sent through.</p>
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<hr />
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<p><a href="https://docs.rs/chrono/0.4/chrono/offset/local/struct.Local.html#method.now"><code>chrono::Local::now()</code></a></p>
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<p>Get the current time in the local timezone using the <a href="https://github.com/chronotope/chrono"><code>chrono</code></a> library.
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See the <a href="https://docs.rs/chrono/0.4/chrono/index.html#date-and-time">time-and-date docs</a>.</p>
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<hr />
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<p><a href="https://docs.rs/chrono/0.4/chrono/datetime/struct.DateTime.html#method.format"><code>.format("[%Y-%m-%d][%H:%M:%S]")</code></a></p>
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<p>Use chrono’s lazy format specifier to turn the time into a readable string.</p>
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<hr />
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<p><a href="struct.FormatCallback.html#method.finish"><code>out.finish(format_args!(...))</code></a></p>
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<p>Call the <code>fern::FormattingCallback</code> to submit the formatted message.</p>
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<p>This roundabout way is slightly odd, but it allows for very fast logging.
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No string allocation required!</p>
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<p><a href="https://doc.rust-lang.org/std/macro.format_args.html"><code>format_args!()</code></a> has the same format as <a href="https://doc.rust-lang.org/std/macro.println.html"><code>println!()</code></a> (and every other
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<a href="https://doc.rust-lang.org/std/fmt/"><code>std::fmt</code></a>-based macro).</p>
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<hr />
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<p><a href="struct.Dispatch.html#method.level"><code>.level(log::LevelFilter::Debug)</code></a></p>
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<p>Set the minimum level needed to output to <code>Debug</code>.</p>
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<hr />
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<p><a href="struct.Dispatch.html#method.chain"><code>.chain(std::io::stdout())</code></a></p>
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<p>Add a child to the logger. All messages which pass the filters will be sent
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to stdout.</p>
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<p><a href="struct.Dispatch.html#method.chain"><code>Dispatch::chain</code></a> accepts <a href="https://doc.rust-lang.org/std/io/struct.Stdout.html"><code>Stdout</code></a>, <a href="https://doc.rust-lang.org/std/io/struct.Stderr.html"><code>Stderr</code></a>, <a href="https://doc.rust-lang.org/std/fs/struct.File.html"><code>File</code></a> and other
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<a href="struct.Dispatch.html"><code>Dispatch</code></a> instances.</p>
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<hr />
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<p><a href="struct.Dispatch.html#method.chain"><code>.chain(fern::log_file(...)?)</code></a></p>
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<p>Add a second child sending messages to the file “output.log”.</p>
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<p>See <a href="fn.log_file.html"><code>fern::log_file()</code></a> for more info on file output.</p>
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<hr />
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<p><a href="struct.Dispatch.html#method.apply"><code>.apply()</code></a></p>
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<p>Consume the configuration and instantiate it as the current runtime global
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logger.</p>
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<p>This will fail if and only if <code>.apply()</code> or equivalent form another crate
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has already been used this runtime.</p>
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<p>Since the binary crate is the only one ever setting up logging, the
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<a href="struct.Dispatch.html#method.apply"><code>apply</code></a> result can be reasonably unwrapped: it’s a bug if any crate is
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calling this method more than once.</p>
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<hr />
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<p>The final output will look like:</p>
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<div class="example-wrap"><pre class="language-text"><code>[2017-01-20][12:55:04][crate-name][INFO] Hello, world!
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[2017-01-20][12:56:21][crate-name][WARN] Ahhh!
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[2017-01-20][12:58:00][crate-name][DEBUG] Something less important happened.</code></pre></div><h2 id="logging" class="section-header"><a href="#logging">Logging</a></h2>
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<p>Once the logger has been set, it will pick up all logging calls from your
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crate and all libraries you depend on.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
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<span class="ident">fern::Dispatch::new</span>()
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<span class="comment">// ...</span>
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.<span class="ident">apply</span>()<span class="question-mark">?</span>;
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<span class="macro">trace!</span>(<span class="string">"Trace message"</span>);
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<span class="macro">debug!</span>(<span class="string">"Debug message"</span>);
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<span class="macro">info!</span>(<span class="string">"Info message"</span>);
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<span class="macro">warn!</span>(<span class="string">"Warning message"</span>);
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<span class="macro">error!</span>(<span class="string">"Error message"</span>);</code></pre></div>
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<h2 id="more" class="section-header"><a href="#more">More</a></h2>
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<p>The <a href="struct.Dispatch.html"><code>Dispatch</code> documentation</a> has example usages of each method, and the
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<a href="https://github.com/daboross/fern/tree/master/examples/cmd-program.rs">full example program</a> might be useful for using fern in a larger
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application context.</p>
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<p>See the <a href="colors/index.html">colors</a> module for examples using ANSI terminal coloring.</p>
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<p>See the <a href="syslog/index.html">syslog</a> module for examples outputting to the unix syslog, or the
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<a href="https://github.com/daboross/fern/tree/master/examples/syslog.rs">syslog full example program</a> for a more realistic sample.</p>
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<p>See the <a href="meta/index.html">meta</a> module for information on getting logging-within-logging
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working correctly.</p>
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</div></details><h2 id="modules" class="small-section-header"><a href="#modules">Modules</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="mod" href="colors/index.html" title="fern::colors mod">colors</a></div><div class="item-right docblock-short"><p>Support for ANSI terminal colors via the colored crate.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="meta/index.html" title="fern::meta mod">meta</a></div><div class="item-right docblock-short"><p>Fern supports logging most things by default, except for one kind of struct: structs which make log
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calls to the global logger from within their <code>Display</code> or <code>Debug</code> implementations.</p>
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</div></div></div><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Dispatch.html" title="fern::Dispatch struct">Dispatch</a></div><div class="item-right docblock-short"><p>The base dispatch logger.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.FormatCallback.html" title="fern::FormatCallback struct">FormatCallback</a></div><div class="item-right docblock-short"><p>Callback struct for use within a formatter closure</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Output.html" title="fern::Output struct">Output</a></div><div class="item-right docblock-short"><p>Configuration for a logger output.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Panic.html" title="fern::Panic struct">Panic</a></div><div class="item-right docblock-short"><p>Logger which will panic whenever anything is logged. The panic
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will be exactly the message of the log.</p>
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</div></div></div><h2 id="enums" class="small-section-header"><a href="#enums">Enums</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="enum" href="enum.InitError.html" title="fern::InitError enum">InitError</a></div><div class="item-right docblock-short"><p>Convenience error combining possible errors which could occur while
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initializing logging.</p>
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</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.log_file.html" title="fern::log_file fn">log_file</a></div><div class="item-right docblock-short"><p>Convenience method for opening a log file with common options.</p>
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</div></div></div><h2 id="types" class="small-section-header"><a href="#types">Type Definitions</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="type" href="type.Filter.html" title="fern::Filter type">Filter</a></div><div class="item-right docblock-short"><p>A type alias for a log filter. Returning true means the record should
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succeed - false means it should fail.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="type" href="type.Formatter.html" title="fern::Formatter type">Formatter</a></div><div class="item-right docblock-short"><p>A type alias for a log formatter.</p>
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