578 lines
28 KiB
HTML
578 lines
28 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>Combining promises</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 { display: inline-block; 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 { 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">Combining promises</h1>
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<p>So far, all of our examples have involved chaining operations onto a
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single promise. In practice, you’ll often find yourself needing to
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perform tasks that require the results of more than one promise. These
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are some patterns you may find useful:</p>
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<ul>
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<li><a href="#gathering"><strong>Gathering:</strong></a> Combining
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multiple independent promises into a single computation</li>
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<li><a href="#nesting"><strong>Nesting:</strong></a> Using the result of
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one promise to affect the input or execution of another async
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operation</li>
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<li><a href="#racing"><strong>Racing:</strong></a> Using the fastest of
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multiple promises</li>
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<li><a href="#mapping"><strong>Mapping:</strong></a> Applying an async
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function to each of a list’s elements and collecting the results</li>
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|||
|
|
<li><a href="#reducing"><strong>Reducing:</strong></a> Applying an async
|
|||
|
|
function to each of a list’s elements and reducing</li>
|
|||
|
|
</ul>
|
|||
|
|
<div id="gathering" class="section level2">
|
|||
|
|
<h2>Gathering</h2>
|
|||
|
|
<p>The most common pattern for combining promises is gathering, where
|
|||
|
|
you have two or more promises in hand and you want to use all of their
|
|||
|
|
results in a computation. The <code>promise_all</code> function is
|
|||
|
|
designed for this. Its signature looks like this:</p>
|
|||
|
|
<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="fu">promise_all</span>(..., <span class="at">.list =</span> <span class="cn">NULL</span>)</span></code></pre></div>
|
|||
|
|
<p><code>promise_all</code> takes any number of promises as named
|
|||
|
|
arguments, and returns a promise of a list containing named elements
|
|||
|
|
with the results of those promises.</p>
|
|||
|
|
<p>Here’s an example using <code>promise_all</code> to combine the
|
|||
|
|
results of two async <code>read.csv</code> operations:</p>
|
|||
|
|
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(promises)</span>
|
|||
|
|
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
|
|||
|
|
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
|
|||
|
|
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a>a <span class="ot"><-</span> <span class="fu">future_promise</span>(<span class="fu">read.csv</span>(<span class="st">"a.csv"</span>))</span>
|
|||
|
|
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a>b <span class="ot"><-</span> <span class="fu">future_promise</span>(<span class="fu">read.csv</span>(<span class="st">"b.csv"</span>))</span>
|
|||
|
|
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a>result <span class="ot"><-</span> <span class="fu">promise_all</span>(<span class="at">a =</span> a, <span class="at">b =</span> b) <span class="sc">%...>%</span> {</span>
|
|||
|
|
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a> <span class="fu">rbind</span>(.<span class="sc">$</span>a, .<span class="sc">$</span>b)</span>
|
|||
|
|
<span id="cb2-10"><a href="#cb2-10" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
|
|||
|
|
<p>In this example, the value of <code>.</code> within the curly braces
|
|||
|
|
is a list whose elements <code>a</code> and <code>b</code> are both data
|
|||
|
|
frames. We use <code>rbind</code> to combine them.</p>
|
|||
|
|
<p>The <code>.$</code> prefix is a bit inelegant, so we recommend the
|
|||
|
|
use of the base R function <code>with</code>, which lets you skip the
|
|||
|
|
prefix. Here’s the same example, with <code>with</code>:</p>
|
|||
|
|
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(promises)</span>
|
|||
|
|
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
|
|||
|
|
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
|
|||
|
|
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>a <span class="ot"><-</span> <span class="fu">future_promise</span>(<span class="fu">read.csv</span>(<span class="st">"a.csv"</span>))</span>
|
|||
|
|
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a>b <span class="ot"><-</span> <span class="fu">future_promise</span>(<span class="fu">read.csv</span>(<span class="st">"b.csv"</span>))</span>
|
|||
|
|
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a><span class="fu">promise_all</span>(<span class="at">a =</span> a, <span class="at">b =</span> b) <span class="sc">%...>%</span></span>
|
|||
|
|
<span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a> <span class="fu">with</span>({</span>
|
|||
|
|
<span id="cb3-10"><a href="#cb3-10" aria-hidden="true" tabindex="-1"></a> <span class="fu">rbind</span>(a, b)</span>
|
|||
|
|
<span id="cb3-11"><a href="#cb3-11" aria-hidden="true" tabindex="-1"></a> })</span></code></pre></div>
|
|||
|
|
<p>(Note that since the <code>promise_all</code> argument names are the
|
|||
|
|
same as the variable names (<code>a = a</code>, <code>b = b</code>), the
|
|||
|
|
original variables are masked: inside the <code>with</code> block,
|
|||
|
|
<code>a</code> now refers to the <em>result</em> of the promise
|
|||
|
|
<code>a</code>, not the promise object itself. If you find this
|
|||
|
|
confusing, you can just choose a different argument name, like
|
|||
|
|
<code>promise_all(a_result = a, …)</code>.)</p>
|
|||
|
|
<p>The combination of <code>promise_all</code> and <code>with</code> is
|
|||
|
|
a concise and powerful way to gather the results of multiple
|
|||
|
|
promises.</p>
|
|||
|
|
<p><code>promise_all</code> also gives you two other options for passing
|
|||
|
|
input promises. First, if you would rather your result list be unnamed,
|
|||
|
|
you can pass in promises as unnamed arguments:
|
|||
|
|
<code>promise_all(a, b)</code> would yield <code>list(1, 2)</code>.
|
|||
|
|
Second, if you have a list of promises already in hand, you can pass the
|
|||
|
|
list as a single argument using <code>promise_all(.list = x)</code>
|
|||
|
|
(instead of, say, using <code>do.call(promise_all, x)</code>).</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="nesting" class="section level2">
|
|||
|
|
<h2>Nesting</h2>
|
|||
|
|
<p>Gathering is easy and convenient, but sometimes not flexible enough.
|
|||
|
|
For example, if you use the result of promise <code>a</code> to decide
|
|||
|
|
whether to launch a second async task, whose result you then use in
|
|||
|
|
combination with the result of <code>a</code>.</p>
|
|||
|
|
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(promises)</span>
|
|||
|
|
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
|
|||
|
|
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
|
|||
|
|
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a>a <span class="ot"><-</span> <span class="fu">future_promise</span>(<span class="dv">1</span>)</span>
|
|||
|
|
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a>a <span class="sc">%...>%</span> (<span class="cf">function</span>(a) {</span>
|
|||
|
|
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a> b <span class="ot"><-</span> <span class="fu">future_promise</span>(<span class="dv">2</span>)</span>
|
|||
|
|
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a> b <span class="sc">%...>%</span> (<span class="cf">function</span>(b) {</span>
|
|||
|
|
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a> a <span class="sc">+</span> b</span>
|
|||
|
|
<span id="cb4-11"><a href="#cb4-11" aria-hidden="true" tabindex="-1"></a> })</span>
|
|||
|
|
<span id="cb4-12"><a href="#cb4-12" aria-hidden="true" tabindex="-1"></a>})</span></code></pre></div>
|
|||
|
|
<p>(We use anonymous functions here to mask the names of the original
|
|||
|
|
promises–i.e. once inside the first anonymous function, the symbol
|
|||
|
|
<code>a</code> now refers to the result of the promise
|
|||
|
|
<code>a</code>.)</p>
|
|||
|
|
<p>The nesting pattern is effective and flexible. The main downside is
|
|||
|
|
the physical nesting of the source code; if you use this pattern to a
|
|||
|
|
depth of more than a couple of promises, your code will be quite
|
|||
|
|
indented (in programming jargon this is referred to as the “pyramid of
|
|||
|
|
doom”).</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="racing" class="section level2">
|
|||
|
|
<h2>Racing</h2>
|
|||
|
|
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(promises)</span>
|
|||
|
|
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
|
|||
|
|
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
|
|||
|
|
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>a <span class="ot"><-</span> <span class="fu">future_promise</span>({ <span class="fu">Sys.sleep</span>(<span class="dv">1</span>); <span class="dv">1</span> })</span>
|
|||
|
|
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a>b <span class="ot"><-</span> <span class="fu">future_promise</span>({ <span class="fu">Sys.sleep</span>(<span class="fl">0.5</span>); <span class="dv">2</span> })</span>
|
|||
|
|
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a>first <span class="ot"><-</span> <span class="fu">promise_race</span>(a, b)</span></code></pre></div>
|
|||
|
|
<p><code>promise_race</code> takes multiple promises and returns a new
|
|||
|
|
promise that will be fulfilled with the first promise that succeeds. In
|
|||
|
|
the example above, <code>first</code> is a promise that will be
|
|||
|
|
fulfilled with <code>2</code> after 0.5 seconds.</p>
|
|||
|
|
<p>If one of the input promises rejects before any succeed, then the
|
|||
|
|
returned promise will be rejected.</p>
|
|||
|
|
<p>Note that promises does not currently support cancellation. So losing
|
|||
|
|
promises will attempt to run to completion even after the race ends.</p>
|
|||
|
|
</div>
|
|||
|
|
<div id="mapping" class="section level2">
|
|||
|
|
<h2>Mapping</h2>
|
|||
|
|
<p>Use <code>promise_map</code> to run an async operation on each
|
|||
|
|
element of a list or vector, and collect the results in a list. It’s
|
|||
|
|
very similar to <code>lapply</code> or <code>purrr::map</code>, except
|
|||
|
|
that the function to apply can return a promise, and the return value is
|
|||
|
|
also a promise.</p>
|
|||
|
|
<p>In the example below, we iterate over a named vector of package
|
|||
|
|
names. For each package name, we launch an async task to download the
|
|||
|
|
package’s description file from CRAN pick out the last published
|
|||
|
|
date.</p>
|
|||
|
|
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(promises)</span>
|
|||
|
|
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
|
|||
|
|
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
|
|||
|
|
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a>get_pub_date <span class="ot"><-</span> <span class="cf">function</span>(pkg) {</span>
|
|||
|
|
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a> desc_url <span class="ot"><-</span> <span class="fu">paste0</span>(<span class="st">"https://cran.r-project.org/web/packages/"</span>, pkg, <span class="st">"/DESCRIPTION"</span>)</span>
|
|||
|
|
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a> <span class="fu">future_promise</span>({</span>
|
|||
|
|
<span id="cb6-8"><a href="#cb6-8" aria-hidden="true" tabindex="-1"></a> <span class="fu">read.dcf</span>(<span class="fu">url</span>(desc_url))[, <span class="st">"Date/Publication"</span>] <span class="sc">%>%</span> <span class="fu">unname</span>()</span>
|
|||
|
|
<span id="cb6-9"><a href="#cb6-9" aria-hidden="true" tabindex="-1"></a> })</span>
|
|||
|
|
<span id="cb6-10"><a href="#cb6-10" aria-hidden="true" tabindex="-1"></a>}</span>
|
|||
|
|
<span id="cb6-11"><a href="#cb6-11" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb6-12"><a href="#cb6-12" aria-hidden="true" tabindex="-1"></a>packages <span class="ot"><-</span> <span class="fu">setNames</span>(, <span class="fu">c</span>(<span class="st">"ggplot2"</span>, <span class="st">"dplyr"</span>, <span class="st">"knitr"</span>))</span>
|
|||
|
|
<span id="cb6-13"><a href="#cb6-13" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb6-14"><a href="#cb6-14" aria-hidden="true" tabindex="-1"></a>pkg_dates <span class="ot"><-</span> <span class="fu">promise_map</span>(packages, get_pub_date)</span>
|
|||
|
|
<span id="cb6-15"><a href="#cb6-15" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb6-16"><a href="#cb6-16" aria-hidden="true" tabindex="-1"></a>pkg_dates <span class="sc">%...>%</span> <span class="fu">print</span>()</span></code></pre></div>
|
|||
|
|
<p>The resulting output looks like this:</p>
|
|||
|
|
<pre><code>$ggplot2
|
|||
|
|
[1] "2016-12-30 22:45:17"
|
|||
|
|
|
|||
|
|
$dplyr
|
|||
|
|
[1] "2017-09-28 20:43:29 UTC"
|
|||
|
|
|
|||
|
|
$knitr
|
|||
|
|
[1] "2018-01-29 11:01:22 UTC"</code></pre>
|
|||
|
|
<p><code>promise_map</code> works serially; each time it calls the given
|
|||
|
|
function on an element of the list/vector, it will wait for the returned
|
|||
|
|
promise to resolve before proceeding to the next element. Furthermore,
|
|||
|
|
any error or rejected promise will cause the entire
|
|||
|
|
<code>promise_map</code> operation to reject.</p>
|
|||
|
|
<p>If you want behavior that’s similar to <code>promise_map</code> but
|
|||
|
|
for all the async operations to occur in parallel, you can achieve that
|
|||
|
|
with a combination of a regular <code>purrr::map</code> and
|
|||
|
|
<code>promise_all</code>:</p>
|
|||
|
|
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>pkg_dates <span class="ot"><-</span> purrr<span class="sc">::</span><span class="fu">map</span>(packages, get_pub_date) <span class="sc">%>%</span></span>
|
|||
|
|
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">promise_all</span>(<span class="at">.list =</span> .)</span>
|
|||
|
|
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a></span>
|
|||
|
|
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a>pkg_dates <span class="sc">%...>%</span> <span class="fu">print</span>()</span></code></pre></div>
|
|||
|
|
</div>
|
|||
|
|
<div id="reducing" class="section level2">
|
|||
|
|
<h2>Reducing</h2>
|
|||
|
|
<p>Use <code>promise_reduce</code> when you have a list where you want
|
|||
|
|
to run an async operation on each of the elements, and to do so serially
|
|||
|
|
(i.e. only one async operation runs at a time). This can be helpful when
|
|||
|
|
you’re searching through some elements using an async operation and want
|
|||
|
|
to terminate early when your search succeeds.</p>
|
|||
|
|
<p>The signature of <code>promise_reduce</code> is as follows:</p>
|
|||
|
|
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="fu">promise_reduce</span>(x, func, <span class="at">init =</span> <span class="cn">NULL</span>)</span></code></pre></div>
|
|||
|
|
<p>If you’ve worked with <code>base::Reduce()</code> or
|
|||
|
|
<code>purr:::reduce()</code>, this should seem reasonably familiar:
|
|||
|
|
<code>x</code> is a vector or list; <code>func</code> is a function that
|
|||
|
|
takes two arguments, the accumulated value and the “next” value; and
|
|||
|
|
<code>init</code> is the default accumulated value.</p>
|
|||
|
|
<p>The main difference between <code>promise_reduce</code> and
|
|||
|
|
<code>purrr:::reduce</code> is that with <code>promise_reduce</code>,
|
|||
|
|
your <code>func</code> can return a promise. If it does,
|
|||
|
|
<code>promise_reduce</code> will wait for it to resolve before updating
|
|||
|
|
the accumulated value and invoking <code>func</code> on the next
|
|||
|
|
element. The result returned from <code>promise_reduce</code> is a
|
|||
|
|
promise that resolves to the ultimate accumulated value.</p>
|
|||
|
|
<p>The following example loops through a partial list of CRAN mirrors,
|
|||
|
|
returning the first one that passes whatever check
|
|||
|
|
<code>http::http_error</code> performs.</p>
|
|||
|
|
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(promises)</span>
|
|||
|
|
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
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<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
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<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a>cran_mirrors <span class="ot"><-</span> <span class="fu">c</span>(</span>
|
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<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cloud.r-project.org"</span>,</span>
|
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<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cran.usthb.dz"</span>,</span>
|
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<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cran.csiro.au"</span>,</span>
|
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<span id="cb10-9"><a href="#cb10-9" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cran.wu.ac.at"</span></span>
|
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<span id="cb10-10"><a href="#cb10-10" aria-hidden="true" tabindex="-1"></a>)</span>
|
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<span id="cb10-11"><a href="#cb10-11" aria-hidden="true" tabindex="-1"></a></span>
|
|||
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<span id="cb10-12"><a href="#cb10-12" aria-hidden="true" tabindex="-1"></a><span class="fu">promise_reduce</span>(cran_mirrors, <span class="cf">function</span>(result, mirror) {</span>
|
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|
|
<span id="cb10-13"><a href="#cb10-13" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> (<span class="sc">!</span><span class="fu">is.null</span>(result)) {</span>
|
|||
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<span id="cb10-14"><a href="#cb10-14" aria-hidden="true" tabindex="-1"></a> result</span>
|
|||
|
|
<span id="cb10-15"><a href="#cb10-15" aria-hidden="true" tabindex="-1"></a> } <span class="cf">else</span> {</span>
|
|||
|
|
<span id="cb10-16"><a href="#cb10-16" aria-hidden="true" tabindex="-1"></a> <span class="fu">future_promise</span>({</span>
|
|||
|
|
<span id="cb10-17"><a href="#cb10-17" aria-hidden="true" tabindex="-1"></a> <span class="co"># Test the URL; return the URL on success, or NULL on failure</span></span>
|
|||
|
|
<span id="cb10-18"><a href="#cb10-18" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> (<span class="sc">!</span>httr<span class="sc">::</span><span class="fu">http_error</span>(mirror)) mirror</span>
|
|||
|
|
<span id="cb10-19"><a href="#cb10-19" aria-hidden="true" tabindex="-1"></a> })</span>
|
|||
|
|
<span id="cb10-20"><a href="#cb10-20" aria-hidden="true" tabindex="-1"></a> }</span>
|
|||
|
|
<span id="cb10-21"><a href="#cb10-21" aria-hidden="true" tabindex="-1"></a>}, <span class="at">.init =</span> <span class="cn">NULL</span>) <span class="sc">%...>%</span> <span class="fu">print</span>()</span></code></pre></div>
|
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