727 lines
40 KiB
HTML
727 lines
40 KiB
HTML
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8" />
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<meta name="generator" content="pandoc" />
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<meta http-equiv="X-UA-Compatible" content="IE=EDGE" />
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<meta name="viewport" content="width=device-width, initial-scale=1" />
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<title>Changing and restoring state</title>
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<script>// Pandoc 2.9 adds attributes on both header and div. We remove the former (to
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// be compatible with the behavior of Pandoc < 2.8).
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document.addEventListener('DOMContentLoaded', function(e) {
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var hs = document.querySelectorAll("div.section[class*='level'] > :first-child");
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var i, h, a;
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for (i = 0; i < hs.length; i++) {
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h = hs[i];
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if (!/^h[1-6]$/i.test(h.tagName)) continue; // it should be a header h1-h6
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a = h.attributes;
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while (a.length > 0) h.removeAttribute(a[0].name);
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}
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});
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</script>
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<style type="text/css">
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code{white-space: pre-wrap;}
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span.smallcaps{font-variant: small-caps;}
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span.underline{text-decoration: underline;}
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div.column{display: inline-block; vertical-align: top; width: 50%;}
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div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
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ul.task-list{list-style: none;}
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</style>
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<style type="text/css">
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code {
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white-space: pre;
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}
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.sourceCode {
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overflow: visible;
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}
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</style>
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<style type="text/css" data-origin="pandoc">
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pre > code.sourceCode { white-space: pre; position: relative; }
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pre > code.sourceCode > span { line-height: 1.25; }
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pre > code.sourceCode > span:empty { height: 1.2em; }
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.sourceCode { overflow: visible; }
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code.sourceCode > span { color: inherit; text-decoration: inherit; }
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div.sourceCode { margin: 1em 0; }
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pre.sourceCode { margin: 0; }
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@media screen {
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div.sourceCode { overflow: auto; }
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}
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@media print {
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pre > code.sourceCode { white-space: pre-wrap; }
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pre > code.sourceCode > span { display: inline-block; text-indent: -5em; padding-left: 5em; }
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}
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pre.numberSource code
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{ counter-reset: source-line 0; }
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pre.numberSource code > span
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{ position: relative; left: -4em; counter-increment: source-line; }
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pre.numberSource code > span > a:first-child::before
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{ content: counter(source-line);
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position: relative; left: -1em; text-align: right; vertical-align: baseline;
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border: none; display: inline-block;
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-webkit-touch-callout: none; -webkit-user-select: none;
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-khtml-user-select: none; -moz-user-select: none;
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-ms-user-select: none; user-select: none;
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padding: 0 4px; width: 4em;
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color: #aaaaaa;
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}
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pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
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div.sourceCode
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{ }
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@media screen {
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pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
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}
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code span.al { color: #ff0000; font-weight: bold; }
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code span.an { color: #60a0b0; font-weight: bold; font-style: italic; }
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code span.at { color: #7d9029; }
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code span.bn { color: #40a070; }
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code span.bu { color: #008000; }
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code span.cf { color: #007020; font-weight: bold; }
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code span.ch { color: #4070a0; }
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code span.cn { color: #880000; }
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code span.co { color: #60a0b0; font-style: italic; }
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code span.cv { color: #60a0b0; font-weight: bold; font-style: italic; }
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code span.do { color: #ba2121; font-style: italic; }
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code span.dt { color: #902000; }
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code span.dv { color: #40a070; }
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code span.er { color: #ff0000; font-weight: bold; }
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code span.ex { }
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code span.fl { color: #40a070; }
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code span.fu { color: #06287e; }
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code span.im { color: #008000; font-weight: bold; }
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code span.in { color: #60a0b0; font-weight: bold; font-style: italic; }
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code span.kw { color: #007020; font-weight: bold; }
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code span.op { color: #666666; }
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code span.ot { color: #007020; }
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code span.pp { color: #bc7a00; }
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code span.sc { color: #4070a0; }
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code span.ss { color: #bb6688; }
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code span.st { color: #4070a0; }
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code span.va { color: #19177c; }
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code span.vs { color: #4070a0; }
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code span.wa { color: #60a0b0; font-weight: bold; font-style: italic; }
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</style>
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<script>
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// apply pandoc div.sourceCode style to pre.sourceCode instead
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(function() {
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var sheets = document.styleSheets;
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for (var i = 0; i < sheets.length; i++) {
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if (sheets[i].ownerNode.dataset["origin"] !== "pandoc") continue;
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try { var rules = sheets[i].cssRules; } catch (e) { continue; }
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var j = 0;
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while (j < rules.length) {
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var rule = rules[j];
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// check if there is a div.sourceCode rule
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if (rule.type !== rule.STYLE_RULE || rule.selectorText !== "div.sourceCode") {
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j++;
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continue;
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}
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var style = rule.style.cssText;
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// check if color or background-color is set
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if (rule.style.color === '' && rule.style.backgroundColor === '') {
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j++;
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continue;
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}
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// replace div.sourceCode by a pre.sourceCode rule
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sheets[i].deleteRule(j);
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sheets[i].insertRule('pre.sourceCode{' + style + '}', j);
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}
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}
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})();
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</script>
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<style type="text/css">body {
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background-color: #fff;
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margin: 1em auto;
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max-width: 700px;
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overflow: visible;
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padding-left: 2em;
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padding-right: 2em;
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font-family: "Open Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
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font-size: 14px;
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line-height: 1.35;
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}
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#TOC {
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clear: both;
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margin: 0 0 10px 10px;
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padding: 4px;
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width: 400px;
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border: 1px solid #CCCCCC;
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border-radius: 5px;
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background-color: #f6f6f6;
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font-size: 13px;
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line-height: 1.3;
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}
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#TOC .toctitle {
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font-weight: bold;
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font-size: 15px;
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margin-left: 5px;
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}
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#TOC ul {
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padding-left: 40px;
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margin-left: -1.5em;
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margin-top: 5px;
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margin-bottom: 5px;
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}
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#TOC ul ul {
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margin-left: -2em;
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}
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#TOC li {
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line-height: 16px;
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}
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table {
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margin: 1em auto;
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border-width: 1px;
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border-color: #DDDDDD;
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border-style: outset;
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border-collapse: collapse;
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}
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table th {
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border-width: 2px;
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padding: 5px;
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border-style: inset;
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}
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table td {
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border-width: 1px;
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border-style: inset;
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line-height: 18px;
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padding: 5px 5px;
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}
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table, table th, table td {
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border-left-style: none;
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border-right-style: none;
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}
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table thead, table tr.even {
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background-color: #f7f7f7;
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}
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p {
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margin: 0.5em 0;
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}
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blockquote {
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background-color: #f6f6f6;
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padding: 0.25em 0.75em;
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}
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hr {
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border-style: solid;
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border: none;
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border-top: 1px solid #777;
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margin: 28px 0;
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}
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dl {
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margin-left: 0;
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}
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dl dd {
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margin-bottom: 13px;
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margin-left: 13px;
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}
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dl dt {
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font-weight: bold;
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}
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ul {
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margin-top: 0;
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}
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ul li {
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list-style: circle outside;
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}
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ul ul {
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margin-bottom: 0;
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}
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pre, code {
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background-color: #f7f7f7;
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border-radius: 3px;
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color: #333;
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white-space: pre-wrap;
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}
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pre {
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border-radius: 3px;
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margin: 5px 0px 10px 0px;
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padding: 10px;
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}
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pre:not([class]) {
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background-color: #f7f7f7;
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}
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code {
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font-family: Consolas, Monaco, 'Courier New', monospace;
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font-size: 85%;
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}
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p > code, li > code {
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padding: 2px 0px;
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}
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div.figure {
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text-align: center;
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}
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img {
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background-color: #FFFFFF;
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padding: 2px;
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border: 1px solid #DDDDDD;
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border-radius: 3px;
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border: 1px solid #CCCCCC;
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margin: 0 5px;
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}
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h1 {
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margin-top: 0;
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font-size: 35px;
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line-height: 40px;
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}
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h2 {
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border-bottom: 4px solid #f7f7f7;
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padding-top: 10px;
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padding-bottom: 2px;
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font-size: 145%;
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}
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h3 {
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border-bottom: 2px solid #f7f7f7;
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padding-top: 10px;
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font-size: 120%;
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}
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h4 {
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border-bottom: 1px solid #f7f7f7;
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margin-left: 8px;
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font-size: 105%;
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}
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h5, h6 {
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border-bottom: 1px solid #ccc;
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font-size: 105%;
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}
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a {
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color: #0033dd;
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text-decoration: none;
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}
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a:hover {
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color: #6666ff; }
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a:visited {
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color: #800080; }
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a:visited:hover {
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color: #BB00BB; }
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a[href^="http:"] {
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text-decoration: underline; }
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a[href^="https:"] {
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text-decoration: underline; }
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code > span.kw { color: #555; font-weight: bold; }
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code > span.dt { color: #902000; }
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code > span.dv { color: #40a070; }
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code > span.bn { color: #d14; }
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code > span.fl { color: #d14; }
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code > span.ch { color: #d14; }
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code > span.st { color: #d14; }
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code > span.co { color: #888888; font-style: italic; }
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code > span.ot { color: #007020; }
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code > span.al { color: #ff0000; font-weight: bold; }
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code > span.fu { color: #900; font-weight: bold; }
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code > span.er { color: #a61717; background-color: #e3d2d2; }
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</style>
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</head>
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<body>
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<h1 class="title toc-ignore">Changing and restoring state</h1>
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<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" tabindex="-1"></a><span class="fu">library</span>(withr)</span></code></pre></div>
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<!-- This vignette uses a git-diff-friendly convention of ONE LINE PER SENTENCE. -->
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<p>This article explains the type of problem withr solves and shows
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typical patterns of usage. It also compares withr’s functionality to the
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<code>on.exit()</code> function from base R.</p>
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<div id="its-dangerous-to-change-state" class="section level2">
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<h2>It’s dangerous to change state</h2>
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<p>Whenever possible, it is desirable to write so-called
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<strong>pure</strong> functions. The property we focus on here is that
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the function should not change the surrounding R landscape, i.e. it
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should not change things like the search path, global options, or the
|
|||
|
|
working directory. If the behaviour of <em>other</em> functions differs
|
|||
|
|
before and after running your function, you’ve modified the landscape.
|
|||
|
|
Changing the landscape is bad because it makes code much harder to
|
|||
|
|
understand.</p>
|
|||
|
|
<p>Here’s a <code>sloppy()</code> function that prints a number with a
|
|||
|
|
specific number of significant digits, by adjusting R’s global “digits”
|
|||
|
|
option.</p>
|
|||
|
|
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" tabindex="-1"></a>sloppy <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb2-2"><a href="#cb2-2" tabindex="-1"></a> <span class="fu">options</span>(<span class="at">digits =</span> sig_digits)</span>
|
|||
|
|
<span id="cb2-3"><a href="#cb2-3" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb2-4"><a href="#cb2-4" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb2-5"><a href="#cb2-5" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb2-6"><a href="#cb2-6" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb2-7"><a href="#cb2-7" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span>
|
|||
|
|
<span id="cb2-8"><a href="#cb2-8" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb2-9"><a href="#cb2-9" tabindex="-1"></a><span class="fu">sloppy</span>(pi, <span class="dv">2</span>)</span>
|
|||
|
|
<span id="cb2-10"><a href="#cb2-10" tabindex="-1"></a><span class="co">#> [1] 3.1</span></span>
|
|||
|
|
<span id="cb2-11"><a href="#cb2-11" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb2-12"><a href="#cb2-12" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb2-13"><a href="#cb2-13" tabindex="-1"></a><span class="co">#> [1] 3.1</span></span></code></pre></div>
|
|||
|
|
<p>Notice how <code>pi</code> prints differently before and after the
|
|||
|
|
call to <code>sloppy()</code>? Calling <code>sloppy()</code> has a side
|
|||
|
|
effect: it changes the “digits” option globally, not just within its own
|
|||
|
|
scope of operations. This is what we want to avoid.</p>
|
|||
|
|
<p><em>Don’t worry, we’re restoring global state (specifically, the
|
|||
|
|
“digits” option) behind the scenes here.</em></p>
|
|||
|
|
<p>Sometimes you cannot avoid modifying the state of the world, in which
|
|||
|
|
case you just have to make sure that you put things back the way you
|
|||
|
|
found them. This is what the withr package is for.</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="the-base-solution-on.exit" class="section level2">
|
|||
|
|
<h2>The base solution: <code>on.exit()</code></h2>
|
|||
|
|
<p>The first function to know about is base R’s <code>on.exit()</code>.
|
|||
|
|
Inside your function body, every time you do something that should be
|
|||
|
|
undone <strong>on exit</strong>, you immediately register the cleanup
|
|||
|
|
code with <code>on.exit(expr, add = TRUE)</code><a href="#fn1" class="footnote-ref" id="fnref1"><sup>1</sup></a>.</p>
|
|||
|
|
<p><code>neat()</code> is an improvement over <code>sloppy()</code>,
|
|||
|
|
because it uses <code>on.exit()</code> to ensure that the “digits”
|
|||
|
|
option is restored to its original value.</p>
|
|||
|
|
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a>neat <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb3-2"><a href="#cb3-2" tabindex="-1"></a> op <span class="ot"><-</span> <span class="fu">options</span>(<span class="at">digits =</span> sig_digits)</span>
|
|||
|
|
<span id="cb3-3"><a href="#cb3-3" tabindex="-1"></a> <span class="fu">on.exit</span>(<span class="fu">options</span>(op), <span class="at">add =</span> <span class="cn">TRUE</span>)</span>
|
|||
|
|
<span id="cb3-4"><a href="#cb3-4" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb3-5"><a href="#cb3-5" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb3-6"><a href="#cb3-6" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb3-7"><a href="#cb3-7" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb3-8"><a href="#cb3-8" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span>
|
|||
|
|
<span id="cb3-9"><a href="#cb3-9" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb3-10"><a href="#cb3-10" tabindex="-1"></a><span class="fu">neat</span>(pi, <span class="dv">2</span>)</span>
|
|||
|
|
<span id="cb3-11"><a href="#cb3-11" tabindex="-1"></a><span class="co">#> [1] 3.1</span></span>
|
|||
|
|
<span id="cb3-12"><a href="#cb3-12" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb3-13"><a href="#cb3-13" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb3-14"><a href="#cb3-14" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span></code></pre></div>
|
|||
|
|
<p><code>on.exit()</code> also works when you exit the function
|
|||
|
|
abnormally, i.e. due to error. This is why official tools, like
|
|||
|
|
<code>on.exit()</code>, are a better choice than any do-it-yourself
|
|||
|
|
solution to this problem.</p>
|
|||
|
|
<p><code>on.exit()</code> is a very useful function, but it’s not very
|
|||
|
|
flexible. The withr package provides an extensible
|
|||
|
|
<code>on.exit()</code>-inspired toolkit.</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="defer-is-the-foundation-of-withr" class="section level2">
|
|||
|
|
<h2><code>defer()</code> is the foundation of withr</h2>
|
|||
|
|
<p><code>defer()</code> is the core function of withr and is very much
|
|||
|
|
like <code>on.exit()</code>, i.e. it schedules the execution of
|
|||
|
|
arbitrary code when the current function exits:</p>
|
|||
|
|
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" tabindex="-1"></a>neater <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb4-2"><a href="#cb4-2" tabindex="-1"></a> op <span class="ot"><-</span> <span class="fu">options</span>(<span class="at">digits =</span> sig_digits)</span>
|
|||
|
|
<span id="cb4-3"><a href="#cb4-3" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">options</span>(op))</span>
|
|||
|
|
<span id="cb4-4"><a href="#cb4-4" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb4-5"><a href="#cb4-5" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb4-6"><a href="#cb4-6" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb4-7"><a href="#cb4-7" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb4-8"><a href="#cb4-8" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span>
|
|||
|
|
<span id="cb4-9"><a href="#cb4-9" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb4-10"><a href="#cb4-10" tabindex="-1"></a><span class="fu">neater</span>(pi, <span class="dv">2</span>)</span>
|
|||
|
|
<span id="cb4-11"><a href="#cb4-11" tabindex="-1"></a><span class="co">#> [1] 3.1</span></span>
|
|||
|
|
<span id="cb4-12"><a href="#cb4-12" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb4-13"><a href="#cb4-13" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb4-14"><a href="#cb4-14" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span></code></pre></div>
|
|||
|
|
<p><code>withr::defer()</code> is basically a drop-in substitute for
|
|||
|
|
<code>on.exit()</code>, but with three key differences we explore
|
|||
|
|
below:</p>
|
|||
|
|
<ol style="list-style-type: decimal">
|
|||
|
|
<li>Different default behaviour around the effect of a series of two or
|
|||
|
|
more calls</li>
|
|||
|
|
<li>Control over the environment the deferred events are associated
|
|||
|
|
with</li>
|
|||
|
|
<li>Ability to work with the global environment</li>
|
|||
|
|
</ol>
|
|||
|
|
<p>Here we focus on using withr inside your functions. See the blog post
|
|||
|
|
<a href="https://www.tidyverse.org/blog/2020/04/self-cleaning-test-fixtures/">Self-cleaning
|
|||
|
|
test fixtures</a> or the testthat vignette <a href="https://testthat.r-lib.org/articles/test-fixtures.html">Test
|
|||
|
|
fixtures</a> for how to use withr inside tests.</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="last-in-first-out" class="section level2">
|
|||
|
|
<h2>Last-in, first-out</h2>
|
|||
|
|
<p>If you make more than one call to <code>defer()</code>, by default,
|
|||
|
|
it <strong>adds</strong> expressions to the <strong>top</strong> of the
|
|||
|
|
stack of deferred actions.</p>
|
|||
|
|
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" tabindex="-1"></a>defer_stack <span class="ot"><-</span> <span class="cf">function</span>() {</span>
|
|||
|
|
<span id="cb5-2"><a href="#cb5-2" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"put on socks</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb5-3"><a href="#cb5-3" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">cat</span>(<span class="st">"take off socks</span><span class="sc">\n</span><span class="st">"</span>))</span>
|
|||
|
|
<span id="cb5-4"><a href="#cb5-4" tabindex="-1"></a> </span>
|
|||
|
|
<span id="cb5-5"><a href="#cb5-5" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"put on shoes</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb5-6"><a href="#cb5-6" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">cat</span>(<span class="st">"take off shoes</span><span class="sc">\n</span><span class="st">"</span>))</span>
|
|||
|
|
<span id="cb5-7"><a href="#cb5-7" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb5-8"><a href="#cb5-8" tabindex="-1"></a><span class="fu">defer_stack</span>()</span>
|
|||
|
|
<span id="cb5-9"><a href="#cb5-9" tabindex="-1"></a><span class="co">#> put on socks</span></span>
|
|||
|
|
<span id="cb5-10"><a href="#cb5-10" tabindex="-1"></a><span class="co">#> put on shoes</span></span>
|
|||
|
|
<span id="cb5-11"><a href="#cb5-11" tabindex="-1"></a><span class="co">#> take off shoes</span></span>
|
|||
|
|
<span id="cb5-12"><a href="#cb5-12" tabindex="-1"></a><span class="co">#> take off socks</span></span></code></pre></div>
|
|||
|
|
<p>In contrast, by default, a subsequent call to <code>on.exit()</code>
|
|||
|
|
<strong>overwrites</strong> the deferred actions registered in the
|
|||
|
|
previous call.</p>
|
|||
|
|
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" tabindex="-1"></a>on_exit_last_one_wins <span class="ot"><-</span> <span class="cf">function</span>() {</span>
|
|||
|
|
<span id="cb6-2"><a href="#cb6-2" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"put on socks</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb6-3"><a href="#cb6-3" tabindex="-1"></a> <span class="fu">on.exit</span>(<span class="fu">cat</span>(<span class="st">"take off socks</span><span class="sc">\n</span><span class="st">"</span>))</span>
|
|||
|
|
<span id="cb6-4"><a href="#cb6-4" tabindex="-1"></a> </span>
|
|||
|
|
<span id="cb6-5"><a href="#cb6-5" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"put on shoes</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb6-6"><a href="#cb6-6" tabindex="-1"></a> <span class="fu">on.exit</span>(<span class="fu">cat</span>(<span class="st">"take off shoes</span><span class="sc">\n</span><span class="st">"</span>))</span>
|
|||
|
|
<span id="cb6-7"><a href="#cb6-7" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb6-8"><a href="#cb6-8" tabindex="-1"></a><span class="fu">on_exit_last_one_wins</span>()</span>
|
|||
|
|
<span id="cb6-9"><a href="#cb6-9" tabindex="-1"></a><span class="co">#> put on socks</span></span>
|
|||
|
|
<span id="cb6-10"><a href="#cb6-10" tabindex="-1"></a><span class="co">#> put on shoes</span></span>
|
|||
|
|
<span id="cb6-11"><a href="#cb6-11" tabindex="-1"></a><span class="co">#> take off shoes</span></span></code></pre></div>
|
|||
|
|
<p>Oops, we still have our socks on! The last-in, first-out, stack-like
|
|||
|
|
behaviour of <code>defer()</code> tends to be what you want in most
|
|||
|
|
applications.</p>
|
|||
|
|
<p>To get such behaviour with <code>on.exit()</code>, remember to call
|
|||
|
|
it with <code>add = TRUE, after = FALSE</code><a href="#fn2" class="footnote-ref" id="fnref2"><sup>2</sup></a>.</p>
|
|||
|
|
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1" tabindex="-1"></a>on_exit_stack <span class="ot"><-</span> <span class="cf">function</span>() {</span>
|
|||
|
|
<span id="cb7-2"><a href="#cb7-2" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"put on socks</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb7-3"><a href="#cb7-3" tabindex="-1"></a> <span class="fu">on.exit</span>(<span class="fu">cat</span>(<span class="st">"take off socks</span><span class="sc">\n</span><span class="st">"</span>), <span class="at">add =</span> <span class="cn">TRUE</span>, <span class="at">after =</span> <span class="cn">FALSE</span>)</span>
|
|||
|
|
<span id="cb7-4"><a href="#cb7-4" tabindex="-1"></a> </span>
|
|||
|
|
<span id="cb7-5"><a href="#cb7-5" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"put on shoes</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb7-6"><a href="#cb7-6" tabindex="-1"></a> <span class="fu">on.exit</span>(<span class="fu">cat</span>(<span class="st">"take off shoes</span><span class="sc">\n</span><span class="st">"</span>), <span class="at">add =</span> <span class="cn">TRUE</span>, <span class="at">after =</span> <span class="cn">FALSE</span>)</span>
|
|||
|
|
<span id="cb7-7"><a href="#cb7-7" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb7-8"><a href="#cb7-8" tabindex="-1"></a><span class="fu">on_exit_stack</span>()</span>
|
|||
|
|
<span id="cb7-9"><a href="#cb7-9" tabindex="-1"></a><span class="co">#> put on socks</span></span>
|
|||
|
|
<span id="cb7-10"><a href="#cb7-10" tabindex="-1"></a><span class="co">#> put on shoes</span></span>
|
|||
|
|
<span id="cb7-11"><a href="#cb7-11" tabindex="-1"></a><span class="co">#> take off shoes</span></span>
|
|||
|
|
<span id="cb7-12"><a href="#cb7-12" tabindex="-1"></a><span class="co">#> take off socks</span></span></code></pre></div>
|
|||
|
|
<p>Conversely, if you want <code>defer()</code> to have first-in,
|
|||
|
|
first-out behaviour, specify <code>priority = "last"</code>.</p>
|
|||
|
|
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1" tabindex="-1"></a>defer_queue <span class="ot"><-</span> <span class="cf">function</span>() {</span>
|
|||
|
|
<span id="cb8-2"><a href="#cb8-2" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"Adam gets in line for ice cream</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb8-3"><a href="#cb8-3" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">cat</span>(<span class="st">"Adam gets ice cream</span><span class="sc">\n</span><span class="st">"</span>), <span class="at">priority =</span> <span class="st">"last"</span>)</span>
|
|||
|
|
<span id="cb8-4"><a href="#cb8-4" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb8-5"><a href="#cb8-5" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"Beth gets in line for ice cream</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
|||
|
|
<span id="cb8-6"><a href="#cb8-6" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">cat</span>(<span class="st">"Beth gets ice cream</span><span class="sc">\n</span><span class="st">"</span>), <span class="at">priority =</span> <span class="st">"last"</span>)</span>
|
|||
|
|
<span id="cb8-7"><a href="#cb8-7" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb8-8"><a href="#cb8-8" tabindex="-1"></a><span class="fu">defer_queue</span>()</span>
|
|||
|
|
<span id="cb8-9"><a href="#cb8-9" tabindex="-1"></a><span class="co">#> Adam gets in line for ice cream</span></span>
|
|||
|
|
<span id="cb8-10"><a href="#cb8-10" tabindex="-1"></a><span class="co">#> Beth gets in line for ice cream</span></span>
|
|||
|
|
<span id="cb8-11"><a href="#cb8-11" tabindex="-1"></a><span class="co">#> Adam gets ice cream</span></span>
|
|||
|
|
<span id="cb8-12"><a href="#cb8-12" tabindex="-1"></a><span class="co">#> Beth gets ice cream</span></span></code></pre></div>
|
|||
|
|
</div>
|
|||
|
|
<div id="local-functions-and-with-functions" class="section level2">
|
|||
|
|
<h2>“Local” functions (and “with” functions)</h2>
|
|||
|
|
<p>Both <code>on.exit()</code> and <code>withr::defer()</code> schedule
|
|||
|
|
actions to be executed when a certain environment goes out of scope,
|
|||
|
|
most typically the execution environment of a function. But the
|
|||
|
|
<code>envir</code> argument of <code>withr::defer()</code> lets you
|
|||
|
|
specify a <em>different</em> environment, which makes it possible to
|
|||
|
|
create customised <code>on.exit()</code> extensions.</p>
|
|||
|
|
<p>Let’s look at the <code>neater()</code> function again.</p>
|
|||
|
|
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1" tabindex="-1"></a>neater <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb9-2"><a href="#cb9-2" tabindex="-1"></a> op <span class="ot"><-</span> <span class="fu">options</span>(<span class="at">digits =</span> sig_digits) <span class="co"># record orig. "digits" & change "digits"</span></span>
|
|||
|
|
<span id="cb9-3"><a href="#cb9-3" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">options</span>(op)) <span class="co"># schedule restoration of "digits"</span></span>
|
|||
|
|
<span id="cb9-4"><a href="#cb9-4" tabindex="-1"></a> </span>
|
|||
|
|
<span id="cb9-5"><a href="#cb9-5" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb9-6"><a href="#cb9-6" tabindex="-1"></a>}</span></code></pre></div>
|
|||
|
|
<p>The first two lines are typical <code>on.exit()</code> maneuvers
|
|||
|
|
where, in some order, you record an original state, arrange for its
|
|||
|
|
eventual restoration, and change it. In real life, this can be much more
|
|||
|
|
involved and you might want to wrap this logic up into a helper
|
|||
|
|
function. You can’t wrap <code>on.exit()</code> in this way, because
|
|||
|
|
there’s no way to reach back up into the correct parent frame and
|
|||
|
|
schedule cleanup there. But with <code>defer()</code>, we can! Here is
|
|||
|
|
such a custom helper, called <code>local_digits()</code>.</p>
|
|||
|
|
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1" tabindex="-1"></a>local_digits <span class="ot"><-</span> <span class="cf">function</span>(sig_digits, <span class="at">envir =</span> <span class="fu">parent.frame</span>()) {</span>
|
|||
|
|
<span id="cb10-2"><a href="#cb10-2" tabindex="-1"></a> op <span class="ot"><-</span> <span class="fu">options</span>(<span class="at">digits =</span> sig_digits)</span>
|
|||
|
|
<span id="cb10-3"><a href="#cb10-3" tabindex="-1"></a> <span class="fu">defer</span>(<span class="fu">options</span>(op), <span class="at">envir =</span> envir)</span>
|
|||
|
|
<span id="cb10-4"><a href="#cb10-4" tabindex="-1"></a>}</span></code></pre></div>
|
|||
|
|
<p>We can use <code>local_digits()</code> to keep any manipulation of
|
|||
|
|
<code>digits</code> local to a function.</p>
|
|||
|
|
<div class="sourceCode" id="cb11"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb11-1"><a href="#cb11-1" tabindex="-1"></a>neato <span class="ot"><-</span> <span class="cf">function</span>(x, digits) {</span>
|
|||
|
|
<span id="cb11-2"><a href="#cb11-2" tabindex="-1"></a> <span class="fu">local_digits</span>(digits)</span>
|
|||
|
|
<span id="cb11-3"><a href="#cb11-3" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb11-4"><a href="#cb11-4" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb11-5"><a href="#cb11-5" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb11-6"><a href="#cb11-6" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb11-7"><a href="#cb11-7" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span>
|
|||
|
|
<span id="cb11-8"><a href="#cb11-8" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb11-9"><a href="#cb11-9" tabindex="-1"></a><span class="fu">neato</span>(pi, <span class="dv">2</span>)</span>
|
|||
|
|
<span id="cb11-10"><a href="#cb11-10" tabindex="-1"></a><span class="co">#> [1] 3.1</span></span>
|
|||
|
|
<span id="cb11-11"><a href="#cb11-11" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb11-12"><a href="#cb11-12" tabindex="-1"></a><span class="fu">neato</span>(pi, <span class="dv">4</span>)</span>
|
|||
|
|
<span id="cb11-13"><a href="#cb11-13" tabindex="-1"></a><span class="co">#> [1] 3.142</span></span></code></pre></div>
|
|||
|
|
<p>You can even call <code>local_digits()</code> multiple times inside a
|
|||
|
|
function. Each call to <code>local_digits()</code> is in effect until
|
|||
|
|
the next or until the function exits, which ever comes first.</p>
|
|||
|
|
<div class="sourceCode" id="cb12"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb12-1"><a href="#cb12-1" tabindex="-1"></a>neatful <span class="ot"><-</span> <span class="cf">function</span>(x) {</span>
|
|||
|
|
<span id="cb12-2"><a href="#cb12-2" tabindex="-1"></a> <span class="fu">local_digits</span>(<span class="dv">1</span>)</span>
|
|||
|
|
<span id="cb12-3"><a href="#cb12-3" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb12-4"><a href="#cb12-4" tabindex="-1"></a> <span class="fu">local_digits</span>(<span class="dv">3</span>)</span>
|
|||
|
|
<span id="cb12-5"><a href="#cb12-5" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb12-6"><a href="#cb12-6" tabindex="-1"></a> <span class="fu">local_digits</span>(<span class="dv">5</span>)</span>
|
|||
|
|
<span id="cb12-7"><a href="#cb12-7" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb12-8"><a href="#cb12-8" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb12-9"><a href="#cb12-9" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb12-10"><a href="#cb12-10" tabindex="-1"></a><span class="fu">neatful</span>(pi)</span>
|
|||
|
|
<span id="cb12-11"><a href="#cb12-11" tabindex="-1"></a><span class="co">#> [1] 3</span></span>
|
|||
|
|
<span id="cb12-12"><a href="#cb12-12" tabindex="-1"></a><span class="co">#> [1] 3.14</span></span>
|
|||
|
|
<span id="cb12-13"><a href="#cb12-13" tabindex="-1"></a><span class="co">#> [1] 3.1416</span></span></code></pre></div>
|
|||
|
|
<p>Certain state changes, such as modifying global options, come up so
|
|||
|
|
often that withr offers pre-made helpers. These helpers come in two
|
|||
|
|
forms: <code>local_*()</code> functions, like the one we just made, and
|
|||
|
|
<code>with_*()</code> functions, which we explain below. Here are the
|
|||
|
|
state change helpers in withr that you are most likely to find
|
|||
|
|
useful:</p>
|
|||
|
|
<table>
|
|||
|
|
<thead>
|
|||
|
|
<tr class="header">
|
|||
|
|
<th>Do / undo this</th>
|
|||
|
|
<th>withr functions</th>
|
|||
|
|
</tr>
|
|||
|
|
</thead>
|
|||
|
|
<tbody>
|
|||
|
|
<tr class="odd">
|
|||
|
|
<td>Set an R option</td>
|
|||
|
|
<td><code>local_options()</code>,<code>with_options()</code></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr class="even">
|
|||
|
|
<td>Set an environment variable</td>
|
|||
|
|
<td><code>local_envvar()</code>, <code>with_envvar()</code></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr class="odd">
|
|||
|
|
<td>Change working directory</td>
|
|||
|
|
<td><code>local_dir()</code>, <code>with_dir()</code></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr class="even">
|
|||
|
|
<td>Set a graphics parameter</td>
|
|||
|
|
<td><code>local_par()</code>, <code>with_par()</code></td>
|
|||
|
|
</tr>
|
|||
|
|
</tbody>
|
|||
|
|
</table>
|
|||
|
|
<p>We didn’t really need to write our own <code>local_digits()</code>
|
|||
|
|
helper, because the built-in <code>withr::local_options()</code> also
|
|||
|
|
gets the job done:</p>
|
|||
|
|
<div class="sourceCode" id="cb13"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb13-1"><a href="#cb13-1" tabindex="-1"></a>neatest <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb13-2"><a href="#cb13-2" tabindex="-1"></a> <span class="fu">local_options</span>(<span class="fu">list</span>(<span class="at">digits =</span> sig_digits))</span>
|
|||
|
|
<span id="cb13-3"><a href="#cb13-3" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb13-4"><a href="#cb13-4" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb13-5"><a href="#cb13-5" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb13-6"><a href="#cb13-6" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb13-7"><a href="#cb13-7" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span>
|
|||
|
|
<span id="cb13-8"><a href="#cb13-8" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb13-9"><a href="#cb13-9" tabindex="-1"></a><span class="fu">neatest</span>(pi, <span class="dv">2</span>)</span>
|
|||
|
|
<span id="cb13-10"><a href="#cb13-10" tabindex="-1"></a><span class="co">#> [1] 3.1</span></span>
|
|||
|
|
<span id="cb13-11"><a href="#cb13-11" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb13-12"><a href="#cb13-12" tabindex="-1"></a><span class="fu">neatest</span>(pi, <span class="dv">4</span>)</span>
|
|||
|
|
<span id="cb13-13"><a href="#cb13-13" tabindex="-1"></a><span class="co">#> [1] 3.142</span></span></code></pre></div>
|
|||
|
|
<p>The <code>local_*()</code> functions target a slightly different use
|
|||
|
|
case from the <code>with_*()</code> functions, which are inspired by
|
|||
|
|
base R’s <code>with()</code> function:</p>
|
|||
|
|
<ul>
|
|||
|
|
<li><p><code>with_*()</code> functions are best for executing a small
|
|||
|
|
snippet of code with a modified state</p>
|
|||
|
|
<div class="sourceCode" id="cb14"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb14-1"><a href="#cb14-1" tabindex="-1"></a>neat_with <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb14-2"><a href="#cb14-2" tabindex="-1"></a> <span class="co"># imagine lots of code here</span></span>
|
|||
|
|
<span id="cb14-3"><a href="#cb14-3" tabindex="-1"></a> withr<span class="sc">::</span><span class="fu">with_options</span>(</span>
|
|||
|
|
<span id="cb14-4"><a href="#cb14-4" tabindex="-1"></a> <span class="fu">list</span>(<span class="at">digits =</span> sig_digits),</span>
|
|||
|
|
<span id="cb14-5"><a href="#cb14-5" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb14-6"><a href="#cb14-6" tabindex="-1"></a> )</span>
|
|||
|
|
<span id="cb14-7"><a href="#cb14-7" tabindex="-1"></a> <span class="co"># ... and a lot more code here</span></span>
|
|||
|
|
<span id="cb14-8"><a href="#cb14-8" tabindex="-1"></a>}</span></code></pre></div></li>
|
|||
|
|
<li><p><code>local_*()</code> functions are best for modifying state
|
|||
|
|
“from now until the function exits”</p>
|
|||
|
|
<div class="sourceCode" id="cb15"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb15-1"><a href="#cb15-1" tabindex="-1"></a>neat_local <span class="ot"><-</span> <span class="cf">function</span>(x, sig_digits) {</span>
|
|||
|
|
<span id="cb15-2"><a href="#cb15-2" tabindex="-1"></a> withr<span class="sc">::</span><span class="fu">local_options</span>(<span class="fu">list</span>(<span class="at">digits =</span> sig_digits))</span>
|
|||
|
|
<span id="cb15-3"><a href="#cb15-3" tabindex="-1"></a> <span class="fu">print</span>(x)</span>
|
|||
|
|
<span id="cb15-4"><a href="#cb15-4" tabindex="-1"></a> <span class="co"># imagine lots of code here</span></span>
|
|||
|
|
<span id="cb15-5"><a href="#cb15-5" tabindex="-1"></a>}</span></code></pre></div></li>
|
|||
|
|
</ul>
|
|||
|
|
<p>It’s best to minimize the footprint of your state modifications.
|
|||
|
|
Therefore, use <code>with_*()</code> functions where you can. But when
|
|||
|
|
this forces you to put lots of (indented) code inside
|
|||
|
|
<code>with_*()</code>, e.g. most of your function’s body, then it’s
|
|||
|
|
better to use <code>local_*()</code>.</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="deferring-events-on-the-global-environment" class="section level2">
|
|||
|
|
<h2>Deferring events on the global environment</h2>
|
|||
|
|
<p>Here is one last difference between <code>withr::defer()</code> and
|
|||
|
|
<code>on.exit()</code>: the ability to defer events on the global
|
|||
|
|
environment<a href="#fn3" class="footnote-ref" id="fnref3"><sup>3</sup></a>.</p>
|
|||
|
|
<p>At first, it sounds pretty weird to propose scheduling deferred
|
|||
|
|
actions on the global environment. It’s not ephemeral, the way function
|
|||
|
|
execution environments are. It goes out of scope very rarely, i.e. when
|
|||
|
|
you exit R. Why would you want this?</p>
|
|||
|
|
<p>The answer is: for development purposes.</p>
|
|||
|
|
<p>If you are developing functions or tests that use withr, it’s very
|
|||
|
|
useful to be able to execute that code interactively, without error, and
|
|||
|
|
with the ability to trigger the deferred events. It’s hard to develop
|
|||
|
|
with functions that work one way inside a function, but another way in
|
|||
|
|
the global environment (or, worse, throw an error).</p>
|
|||
|
|
<p>Here’s how <code>defer()</code> (and all functions based on it) works
|
|||
|
|
in an interactive session.</p>
|
|||
|
|
<div class="sourceCode" id="cb16"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb16-1"><a href="#cb16-1" tabindex="-1"></a><span class="fu">library</span>(withr)</span>
|
|||
|
|
<span id="cb16-2"><a href="#cb16-2" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb16-3"><a href="#cb16-3" tabindex="-1"></a><span class="fu">defer</span>(<span class="fu">print</span>(<span class="st">"hi"</span>))</span>
|
|||
|
|
<span id="cb16-4"><a href="#cb16-4" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb16-5"><a href="#cb16-5" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb16-6"><a href="#cb16-6" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span>
|
|||
|
|
<span id="cb16-7"><a href="#cb16-7" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb16-8"><a href="#cb16-8" tabindex="-1"></a><span class="co"># this adds another deferred event, but does not re-message</span></span>
|
|||
|
|
<span id="cb16-9"><a href="#cb16-9" tabindex="-1"></a><span class="fu">local_digits</span>(<span class="dv">3</span>)</span>
|
|||
|
|
<span id="cb16-10"><a href="#cb16-10" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb16-11"><a href="#cb16-11" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb16-12"><a href="#cb16-12" tabindex="-1"></a><span class="co">#> [1] 3.14</span></span>
|
|||
|
|
<span id="cb16-13"><a href="#cb16-13" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb16-14"><a href="#cb16-14" tabindex="-1"></a><span class="fu">deferred_run</span>()</span>
|
|||
|
|
<span id="cb16-15"><a href="#cb16-15" tabindex="-1"></a><span class="co">#> [1] "hi"</span></span>
|
|||
|
|
<span id="cb16-16"><a href="#cb16-16" tabindex="-1"></a><span class="co">#> Ran 2/2 deferred expressions</span></span>
|
|||
|
|
<span id="cb16-17"><a href="#cb16-17" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb16-18"><a href="#cb16-18" tabindex="-1"></a>pi</span>
|
|||
|
|
<span id="cb16-19"><a href="#cb16-19" tabindex="-1"></a><span class="co">#> [1] 3.141593</span></span></code></pre></div>
|
|||
|
|
<p>Note that because this example is running in a vignette, it doesn’t
|
|||
|
|
look exactly the same as what you’ll see interactively. When you defer
|
|||
|
|
events on the global environment for the first time, you get this
|
|||
|
|
message that alerts you to the situation:</p>
|
|||
|
|
<div class="sourceCode" id="cb17"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb17-1"><a href="#cb17-1" tabindex="-1"></a><span class="fu">defer</span>(<span class="fu">print</span>(<span class="st">"hi"</span>))</span>
|
|||
|
|
<span id="cb17-2"><a href="#cb17-2" tabindex="-1"></a><span class="co">#> Setting global deferred event(s).</span></span>
|
|||
|
|
<span id="cb17-3"><a href="#cb17-3" tabindex="-1"></a><span class="co">#> i These will be run:</span></span>
|
|||
|
|
<span id="cb17-4"><a href="#cb17-4" tabindex="-1"></a><span class="co">#> * Automatically, when the R session ends.</span></span>
|
|||
|
|
<span id="cb17-5"><a href="#cb17-5" tabindex="-1"></a><span class="co">#> * On demand, if you call `withr::deferred_run()`.</span></span>
|
|||
|
|
<span id="cb17-6"><a href="#cb17-6" tabindex="-1"></a><span class="co">#> i Use `withr::deferred_clear()` to clear them without executing.</span></span></code></pre></div>
|
|||
|
|
<p>If you add subsequent events, the message is <em>not</em> repeated.
|
|||
|
|
Since the global environment isn’t perishable, like a test environment
|
|||
|
|
is, you have to call <code>deferred_run()</code> explicitly to execute
|
|||
|
|
the deferred events. You can also clear them, without running, with
|
|||
|
|
<code>deferred_clear()</code>.</p>
|
|||
|
|
</div>
|
|||
|
|
<div class="footnotes footnotes-end-of-document">
|
|||
|
|
<hr />
|
|||
|
|
<ol>
|
|||
|
|
<li id="fn1"><p>It’s too bad <code>add = TRUE</code> isn’t the default,
|
|||
|
|
because you almost always want this. Without it, each call to
|
|||
|
|
<code>on.exit()</code> clobbers the effect of previous calls.<a href="#fnref1" class="footnote-back">↩︎</a></p></li>
|
|||
|
|
<li id="fn2"><p>Note: the <code>after</code> argument of
|
|||
|
|
<code>on.exit()</code> first appeared in R 3.5.0.<a href="#fnref2" class="footnote-back">↩︎</a></p></li>
|
|||
|
|
<li id="fn3"><p>This feature first appeared in withr v2.2.0.<a href="#fnref3" class="footnote-back">↩︎</a></p></li>
|
|||
|
|
</ol>
|
|||
|
|
</div>
|
|||
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script.type = "text/javascript";
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script.src = "https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML";
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document.getElementsByTagName("head")[0].appendChild(script);
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