500 lines
30 KiB
HTML
500 lines
30 KiB
HTML
<!DOCTYPE html>
|
||
|
||
<html xmlns="http://www.w3.org/1999/xhtml">
|
||
|
||
<head>
|
||
|
||
<meta charset="utf-8" />
|
||
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
||
<meta name="generator" content="pandoc" />
|
||
|
||
<meta name="viewport" content="width=device-width, initial-scale=1">
|
||
|
||
|
||
<meta name="date" content="2019-03-15" />
|
||
|
||
<title>Lazyeval: a new approach to NSE</title>
|
||
|
||
|
||
|
||
<style type="text/css">code{white-space: pre;}</style>
|
||
<style type="text/css" data-origin="pandoc">
|
||
a.sourceLine { display: inline-block; line-height: 1.25; }
|
||
a.sourceLine { pointer-events: none; color: inherit; text-decoration: inherit; }
|
||
a.sourceLine:empty { height: 1.2em; }
|
||
.sourceCode { overflow: visible; }
|
||
code.sourceCode { white-space: pre; position: relative; }
|
||
div.sourceCode { margin: 1em 0; }
|
||
pre.sourceCode { margin: 0; }
|
||
@media screen {
|
||
div.sourceCode { overflow: auto; }
|
||
}
|
||
@media print {
|
||
code.sourceCode { white-space: pre-wrap; }
|
||
a.sourceLine { text-indent: -1em; padding-left: 1em; }
|
||
}
|
||
pre.numberSource a.sourceLine
|
||
{ position: relative; left: -4em; }
|
||
pre.numberSource a.sourceLine::before
|
||
{ content: attr(data-line-number);
|
||
position: relative; left: -1em; text-align: right; vertical-align: baseline;
|
||
border: none; pointer-events: all; display: inline-block;
|
||
-webkit-touch-callout: none; -webkit-user-select: none;
|
||
-khtml-user-select: none; -moz-user-select: none;
|
||
-ms-user-select: none; user-select: none;
|
||
padding: 0 4px; width: 4em;
|
||
color: #aaaaaa;
|
||
}
|
||
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
|
||
div.sourceCode
|
||
{ }
|
||
@media screen {
|
||
a.sourceLine::before { text-decoration: underline; }
|
||
}
|
||
code span.al { color: #ff0000; font-weight: bold; } /* Alert */
|
||
code span.an { color: #60a0b0; font-weight: bold; font-style: italic; } /* Annotation */
|
||
code span.at { color: #7d9029; } /* Attribute */
|
||
code span.bn { color: #40a070; } /* BaseN */
|
||
code span.bu { } /* BuiltIn */
|
||
code span.cf { color: #007020; font-weight: bold; } /* ControlFlow */
|
||
code span.ch { color: #4070a0; } /* Char */
|
||
code span.cn { color: #880000; } /* Constant */
|
||
code span.co { color: #60a0b0; font-style: italic; } /* Comment */
|
||
code span.cv { color: #60a0b0; font-weight: bold; font-style: italic; } /* CommentVar */
|
||
code span.do { color: #ba2121; font-style: italic; } /* Documentation */
|
||
code span.dt { color: #902000; } /* DataType */
|
||
code span.dv { color: #40a070; } /* DecVal */
|
||
code span.er { color: #ff0000; font-weight: bold; } /* Error */
|
||
code span.ex { } /* Extension */
|
||
code span.fl { color: #40a070; } /* Float */
|
||
code span.fu { color: #06287e; } /* Function */
|
||
code span.im { } /* Import */
|
||
code span.in { color: #60a0b0; font-weight: bold; font-style: italic; } /* Information */
|
||
code span.kw { color: #007020; font-weight: bold; } /* Keyword */
|
||
code span.op { color: #666666; } /* Operator */
|
||
code span.ot { color: #007020; } /* Other */
|
||
code span.pp { color: #bc7a00; } /* Preprocessor */
|
||
code span.sc { color: #4070a0; } /* SpecialChar */
|
||
code span.ss { color: #bb6688; } /* SpecialString */
|
||
code span.st { color: #4070a0; } /* String */
|
||
code span.va { color: #19177c; } /* Variable */
|
||
code span.vs { color: #4070a0; } /* VerbatimString */
|
||
code span.wa { color: #60a0b0; font-weight: bold; font-style: italic; } /* Warning */
|
||
|
||
</style>
|
||
<script>
|
||
// apply pandoc div.sourceCode style to pre.sourceCode instead
|
||
(function() {
|
||
var sheets = document.styleSheets;
|
||
for (var i = 0; i < sheets.length; i++) {
|
||
if (sheets[i].ownerNode.dataset["origin"] !== "pandoc") continue;
|
||
try { var rules = sheets[i].cssRules; } catch (e) { continue; }
|
||
for (var j = 0; j < rules.length; j++) {
|
||
var rule = rules[j];
|
||
// check if there is a div.sourceCode rule
|
||
if (rule.type !== rule.STYLE_RULE || rule.selectorText !== "div.sourceCode") continue;
|
||
var style = rule.style.cssText;
|
||
// check if color or background-color is set
|
||
if (rule.style.color === '' || rule.style.backgroundColor === '') continue;
|
||
// replace div.sourceCode by a pre.sourceCode rule
|
||
sheets[i].deleteRule(j);
|
||
sheets[i].insertRule('pre.sourceCode{' + style + '}', j);
|
||
}
|
||
}
|
||
})();
|
||
</script>
|
||
|
||
|
||
|
||
<style type="text/css">body {
|
||
background-color: #fff;
|
||
margin: 1em auto;
|
||
max-width: 700px;
|
||
overflow: visible;
|
||
padding-left: 2em;
|
||
padding-right: 2em;
|
||
font-family: "Open Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
|
||
font-size: 14px;
|
||
line-height: 1.35;
|
||
}
|
||
#header {
|
||
text-align: center;
|
||
}
|
||
#TOC {
|
||
clear: both;
|
||
margin: 0 0 10px 10px;
|
||
padding: 4px;
|
||
width: 400px;
|
||
border: 1px solid #CCCCCC;
|
||
border-radius: 5px;
|
||
background-color: #f6f6f6;
|
||
font-size: 13px;
|
||
line-height: 1.3;
|
||
}
|
||
#TOC .toctitle {
|
||
font-weight: bold;
|
||
font-size: 15px;
|
||
margin-left: 5px;
|
||
}
|
||
#TOC ul {
|
||
padding-left: 40px;
|
||
margin-left: -1.5em;
|
||
margin-top: 5px;
|
||
margin-bottom: 5px;
|
||
}
|
||
#TOC ul ul {
|
||
margin-left: -2em;
|
||
}
|
||
#TOC li {
|
||
line-height: 16px;
|
||
}
|
||
table {
|
||
margin: 1em auto;
|
||
border-width: 1px;
|
||
border-color: #DDDDDD;
|
||
border-style: outset;
|
||
border-collapse: collapse;
|
||
}
|
||
table th {
|
||
border-width: 2px;
|
||
padding: 5px;
|
||
border-style: inset;
|
||
}
|
||
table td {
|
||
border-width: 1px;
|
||
border-style: inset;
|
||
line-height: 18px;
|
||
padding: 5px 5px;
|
||
}
|
||
table, table th, table td {
|
||
border-left-style: none;
|
||
border-right-style: none;
|
||
}
|
||
table thead, table tr.even {
|
||
background-color: #f7f7f7;
|
||
}
|
||
p {
|
||
margin: 0.5em 0;
|
||
}
|
||
blockquote {
|
||
background-color: #f6f6f6;
|
||
padding: 0.25em 0.75em;
|
||
}
|
||
hr {
|
||
border-style: solid;
|
||
border: none;
|
||
border-top: 1px solid #777;
|
||
margin: 28px 0;
|
||
}
|
||
dl {
|
||
margin-left: 0;
|
||
}
|
||
dl dd {
|
||
margin-bottom: 13px;
|
||
margin-left: 13px;
|
||
}
|
||
dl dt {
|
||
font-weight: bold;
|
||
}
|
||
ul {
|
||
margin-top: 0;
|
||
}
|
||
ul li {
|
||
list-style: circle outside;
|
||
}
|
||
ul ul {
|
||
margin-bottom: 0;
|
||
}
|
||
pre, code {
|
||
background-color: #f7f7f7;
|
||
border-radius: 3px;
|
||
color: #333;
|
||
white-space: pre-wrap;
|
||
}
|
||
pre {
|
||
border-radius: 3px;
|
||
margin: 5px 0px 10px 0px;
|
||
padding: 10px;
|
||
}
|
||
pre:not([class]) {
|
||
background-color: #f7f7f7;
|
||
}
|
||
code {
|
||
font-family: Consolas, Monaco, 'Courier New', monospace;
|
||
font-size: 85%;
|
||
}
|
||
p > code, li > code {
|
||
padding: 2px 0px;
|
||
}
|
||
div.figure {
|
||
text-align: center;
|
||
}
|
||
img {
|
||
background-color: #FFFFFF;
|
||
padding: 2px;
|
||
border: 1px solid #DDDDDD;
|
||
border-radius: 3px;
|
||
border: 1px solid #CCCCCC;
|
||
margin: 0 5px;
|
||
}
|
||
h1 {
|
||
margin-top: 0;
|
||
font-size: 35px;
|
||
line-height: 40px;
|
||
}
|
||
h2 {
|
||
border-bottom: 4px solid #f7f7f7;
|
||
padding-top: 10px;
|
||
padding-bottom: 2px;
|
||
font-size: 145%;
|
||
}
|
||
h3 {
|
||
border-bottom: 2px solid #f7f7f7;
|
||
padding-top: 10px;
|
||
font-size: 120%;
|
||
}
|
||
h4 {
|
||
border-bottom: 1px solid #f7f7f7;
|
||
margin-left: 8px;
|
||
font-size: 105%;
|
||
}
|
||
h5, h6 {
|
||
border-bottom: 1px solid #ccc;
|
||
font-size: 105%;
|
||
}
|
||
a {
|
||
color: #0033dd;
|
||
text-decoration: none;
|
||
}
|
||
a:hover {
|
||
color: #6666ff; }
|
||
a:visited {
|
||
color: #800080; }
|
||
a:visited:hover {
|
||
color: #BB00BB; }
|
||
a[href^="http:"] {
|
||
text-decoration: underline; }
|
||
a[href^="https:"] {
|
||
text-decoration: underline; }
|
||
|
||
code > span.kw { color: #555; font-weight: bold; }
|
||
code > span.dt { color: #902000; }
|
||
code > span.dv { color: #40a070; }
|
||
code > span.bn { color: #d14; }
|
||
code > span.fl { color: #d14; }
|
||
code > span.ch { color: #d14; }
|
||
code > span.st { color: #d14; }
|
||
code > span.co { color: #888888; font-style: italic; }
|
||
code > span.ot { color: #007020; }
|
||
code > span.al { color: #ff0000; font-weight: bold; }
|
||
code > span.fu { color: #900; font-weight: bold; } code > span.er { color: #a61717; background-color: #e3d2d2; }
|
||
</style>
|
||
|
||
</head>
|
||
|
||
<body>
|
||
|
||
|
||
|
||
|
||
<h1 class="title toc-ignore">Lazyeval: a new approach to NSE</h1>
|
||
<h4 class="date"><em>2019-03-15</em></h4>
|
||
|
||
|
||
|
||
<p>This document outlines my previous approach to non-standard evaluation (NSE). You should avoid it unless you are working with an older version of dplyr or tidyr.</p>
|
||
<p>There are three key ideas:</p>
|
||
<ul>
|
||
<li><p>Instead of using <code>substitute()</code>, use <code>lazyeval::lazy()</code> to capture both expression and environment. (Or use <code>lazyeval::lazy_dots(...)</code> to capture promises in <code>...</code>)</p></li>
|
||
<li><p>Every function that uses NSE should have a standard evaluation (SE) escape hatch that does the actual computation. The SE-function name should end with <code>_</code>.</p></li>
|
||
<li><p>The SE-function has a flexible input specification to make it easy for people to program with.</p></li>
|
||
</ul>
|
||
<div id="lazy" class="section level2">
|
||
<h2><code>lazy()</code></h2>
|
||
<p>The key tool that makes this approach possible is <code>lazy()</code>, an equivalent to <code>substitute()</code> that captures both expression and environment associated with a function argument:</p>
|
||
<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb1-1" data-line-number="1"><span class="kw">library</span>(lazyeval)</a>
|
||
<a class="sourceLine" id="cb1-2" data-line-number="2">f <-<span class="st"> </span><span class="cf">function</span>(<span class="dt">x =</span> a <span class="op">-</span><span class="st"> </span>b) {</a>
|
||
<a class="sourceLine" id="cb1-3" data-line-number="3"> <span class="kw">lazy</span>(x)</a>
|
||
<a class="sourceLine" id="cb1-4" data-line-number="4">}</a>
|
||
<a class="sourceLine" id="cb1-5" data-line-number="5"><span class="kw">f</span>()</a>
|
||
<a class="sourceLine" id="cb1-6" data-line-number="6"><span class="co">#> <lazy></span></a>
|
||
<a class="sourceLine" id="cb1-7" data-line-number="7"><span class="co">#> expr: a - b</span></a>
|
||
<a class="sourceLine" id="cb1-8" data-line-number="8"><span class="co">#> env: <environment: 0x7ff852dfd638></span></a>
|
||
<a class="sourceLine" id="cb1-9" data-line-number="9"><span class="kw">f</span>(a <span class="op">+</span><span class="st"> </span>b)</a>
|
||
<a class="sourceLine" id="cb1-10" data-line-number="10"><span class="co">#> <lazy></span></a>
|
||
<a class="sourceLine" id="cb1-11" data-line-number="11"><span class="co">#> expr: a + b</span></a>
|
||
<a class="sourceLine" id="cb1-12" data-line-number="12"><span class="co">#> env: <environment: R_GlobalEnv></span></a></code></pre></div>
|
||
<p>As a complement to <code>eval()</code>, the lazy package provides <code>lazy_eval()</code> that uses the environment associated with the lazy object:</p>
|
||
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb2-1" data-line-number="1">a <-<span class="st"> </span><span class="dv">10</span></a>
|
||
<a class="sourceLine" id="cb2-2" data-line-number="2">b <-<span class="st"> </span><span class="dv">1</span></a>
|
||
<a class="sourceLine" id="cb2-3" data-line-number="3"><span class="kw">lazy_eval</span>(<span class="kw">f</span>())</a>
|
||
<a class="sourceLine" id="cb2-4" data-line-number="4"><span class="co">#> [1] 9</span></a>
|
||
<a class="sourceLine" id="cb2-5" data-line-number="5"><span class="kw">lazy_eval</span>(<span class="kw">f</span>(a <span class="op">+</span><span class="st"> </span>b))</a>
|
||
<a class="sourceLine" id="cb2-6" data-line-number="6"><span class="co">#> [1] 11</span></a></code></pre></div>
|
||
<p>The second argument to lazy eval is a list or data frame where names should be looked up first:</p>
|
||
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb3-1" data-line-number="1"><span class="kw">lazy_eval</span>(<span class="kw">f</span>(), <span class="kw">list</span>(<span class="dt">a =</span> <span class="dv">1</span>))</a>
|
||
<a class="sourceLine" id="cb3-2" data-line-number="2"><span class="co">#> [1] 0</span></a></code></pre></div>
|
||
<p><code>lazy_eval()</code> also works with formulas, since they contain the same information as a lazy object: an expression (only the RHS is used by convention) and an environment:</p>
|
||
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb4-1" data-line-number="1"><span class="kw">lazy_eval</span>(<span class="op">~</span><span class="st"> </span>a <span class="op">+</span><span class="st"> </span>b)</a>
|
||
<a class="sourceLine" id="cb4-2" data-line-number="2"><span class="co">#> [1] 11</span></a>
|
||
<a class="sourceLine" id="cb4-3" data-line-number="3">h <-<span class="st"> </span><span class="cf">function</span>(i) {</a>
|
||
<a class="sourceLine" id="cb4-4" data-line-number="4"> <span class="op">~</span><span class="st"> </span><span class="dv">10</span> <span class="op">+</span><span class="st"> </span>i</a>
|
||
<a class="sourceLine" id="cb4-5" data-line-number="5">}</a>
|
||
<a class="sourceLine" id="cb4-6" data-line-number="6"><span class="kw">lazy_eval</span>(<span class="kw">h</span>(<span class="dv">1</span>))</a>
|
||
<a class="sourceLine" id="cb4-7" data-line-number="7"><span class="co">#> [1] 11</span></a></code></pre></div>
|
||
</div>
|
||
<div id="standard-evaluation" class="section level2">
|
||
<h2>Standard evaluation</h2>
|
||
<p>Whenever we need a function that does non-standard evaluation, always write the standard evaluation version first. For example, let’s implement our own version of <code>subset()</code>:</p>
|
||
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb5-1" data-line-number="1">subset2_ <-<span class="st"> </span><span class="cf">function</span>(df, condition) {</a>
|
||
<a class="sourceLine" id="cb5-2" data-line-number="2"> r <-<span class="st"> </span><span class="kw">lazy_eval</span>(condition, df)</a>
|
||
<a class="sourceLine" id="cb5-3" data-line-number="3"> r <-<span class="st"> </span>r <span class="op">&</span><span class="st"> </span><span class="op">!</span><span class="kw">is.na</span>(r)</a>
|
||
<a class="sourceLine" id="cb5-4" data-line-number="4"> df[r, , drop =<span class="st"> </span><span class="ot">FALSE</span>]</a>
|
||
<a class="sourceLine" id="cb5-5" data-line-number="5">} </a>
|
||
<a class="sourceLine" id="cb5-6" data-line-number="6"></a>
|
||
<a class="sourceLine" id="cb5-7" data-line-number="7"><span class="kw">subset2_</span>(mtcars, <span class="kw">lazy</span>(mpg <span class="op">></span><span class="st"> </span><span class="dv">31</span>))</a>
|
||
<a class="sourceLine" id="cb5-8" data-line-number="8"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb5-9" data-line-number="9"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb5-10" data-line-number="10"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a></code></pre></div>
|
||
<p><code>lazy_eval()</code> will always coerce it’s first argument into a lazy object, so a variety of specifications will work:</p>
|
||
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb6-1" data-line-number="1"><span class="kw">subset2_</span>(mtcars, <span class="op">~</span>mpg <span class="op">></span><span class="st"> </span><span class="dv">31</span>)</a>
|
||
<a class="sourceLine" id="cb6-2" data-line-number="2"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb6-3" data-line-number="3"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb6-4" data-line-number="4"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb6-5" data-line-number="5"><span class="kw">subset2_</span>(mtcars, <span class="kw">quote</span>(mpg <span class="op">></span><span class="st"> </span><span class="dv">31</span>))</a>
|
||
<a class="sourceLine" id="cb6-6" data-line-number="6"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb6-7" data-line-number="7"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb6-8" data-line-number="8"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb6-9" data-line-number="9"><span class="kw">subset2_</span>(mtcars, <span class="st">"mpg > 31"</span>)</a>
|
||
<a class="sourceLine" id="cb6-10" data-line-number="10"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb6-11" data-line-number="11"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb6-12" data-line-number="12"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a></code></pre></div>
|
||
<p>Note that quoted called and strings don’t have environments associated with them, so <code>as.lazy()</code> defaults to using <code>baseenv()</code>. This will work if the expression is self-contained (i.e. doesn’t contain any references to variables in the local environment), and will otherwise fail quickly and robustly.</p>
|
||
</div>
|
||
<div id="non-standard-evaluation" class="section level2">
|
||
<h2>Non-standard evaluation</h2>
|
||
<p>With the SE version in hand, writing the NSE version is easy. We just use <code>lazy()</code> to capture the unevaluated expression and corresponding environment:</p>
|
||
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb7-1" data-line-number="1">subset2 <-<span class="st"> </span><span class="cf">function</span>(df, condition) {</a>
|
||
<a class="sourceLine" id="cb7-2" data-line-number="2"> <span class="kw">subset2_</span>(df, <span class="kw">lazy</span>(condition))</a>
|
||
<a class="sourceLine" id="cb7-3" data-line-number="3">}</a>
|
||
<a class="sourceLine" id="cb7-4" data-line-number="4"><span class="kw">subset2</span>(mtcars, mpg <span class="op">></span><span class="st"> </span><span class="dv">31</span>)</a>
|
||
<a class="sourceLine" id="cb7-5" data-line-number="5"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb7-6" data-line-number="6"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb7-7" data-line-number="7"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a></code></pre></div>
|
||
<p>This standard evaluation escape hatch is very important because it allows us to implement different NSE approaches. For example, we could create a subsetting function that finds all rows where a variable is above a threshold:</p>
|
||
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb8-1" data-line-number="1">above_threshold <-<span class="st"> </span><span class="cf">function</span>(df, var, threshold) {</a>
|
||
<a class="sourceLine" id="cb8-2" data-line-number="2"> cond <-<span class="st"> </span><span class="kw">interp</span>(<span class="op">~</span><span class="st"> </span>var <span class="op">></span><span class="st"> </span>x, <span class="dt">var =</span> <span class="kw">lazy</span>(var), <span class="dt">x =</span> threshold)</a>
|
||
<a class="sourceLine" id="cb8-3" data-line-number="3"> <span class="kw">subset2_</span>(df, cond)</a>
|
||
<a class="sourceLine" id="cb8-4" data-line-number="4">}</a>
|
||
<a class="sourceLine" id="cb8-5" data-line-number="5"><span class="kw">above_threshold</span>(mtcars, mpg, <span class="dv">31</span>)</a>
|
||
<a class="sourceLine" id="cb8-6" data-line-number="6"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb8-7" data-line-number="7"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb8-8" data-line-number="8"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a></code></pre></div>
|
||
<p>Here we’re using <code>interp()</code> to modify a formula. We use the value of <code>threshold</code> and the expression in by <code>var</code>.</p>
|
||
</div>
|
||
<div id="scoping" class="section level2">
|
||
<h2>Scoping</h2>
|
||
<p>Because <code>lazy()</code> captures the environment associated with the function argument, we automatically avoid a subtle scoping bug present in <code>subset()</code>:</p>
|
||
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb9-1" data-line-number="1">x <-<span class="st"> </span><span class="dv">31</span></a>
|
||
<a class="sourceLine" id="cb9-2" data-line-number="2">f1 <-<span class="st"> </span><span class="cf">function</span>(...) {</a>
|
||
<a class="sourceLine" id="cb9-3" data-line-number="3"> x <-<span class="st"> </span><span class="dv">30</span></a>
|
||
<a class="sourceLine" id="cb9-4" data-line-number="4"> <span class="kw">subset</span>(mtcars, ...)</a>
|
||
<a class="sourceLine" id="cb9-5" data-line-number="5">}</a>
|
||
<a class="sourceLine" id="cb9-6" data-line-number="6"><span class="co"># Uses 30 instead of 31</span></a>
|
||
<a class="sourceLine" id="cb9-7" data-line-number="7"><span class="kw">f1</span>(mpg <span class="op">></span><span class="st"> </span>x)</a>
|
||
<a class="sourceLine" id="cb9-8" data-line-number="8"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb9-9" data-line-number="9"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb9-10" data-line-number="10"><span class="co">#> 19 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2</span></a>
|
||
<a class="sourceLine" id="cb9-11" data-line-number="11"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb9-12" data-line-number="12"><span class="co">#> 28 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2</span></a>
|
||
<a class="sourceLine" id="cb9-13" data-line-number="13"></a>
|
||
<a class="sourceLine" id="cb9-14" data-line-number="14">f2 <-<span class="st"> </span><span class="cf">function</span>(...) {</a>
|
||
<a class="sourceLine" id="cb9-15" data-line-number="15"> x <-<span class="st"> </span><span class="dv">30</span></a>
|
||
<a class="sourceLine" id="cb9-16" data-line-number="16"> <span class="kw">subset2</span>(mtcars, ...)</a>
|
||
<a class="sourceLine" id="cb9-17" data-line-number="17">}</a>
|
||
<a class="sourceLine" id="cb9-18" data-line-number="18"><span class="co"># Correctly uses 31</span></a>
|
||
<a class="sourceLine" id="cb9-19" data-line-number="19"><span class="kw">f2</span>(mpg <span class="op">></span><span class="st"> </span>x)</a>
|
||
<a class="sourceLine" id="cb9-20" data-line-number="20"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb9-21" data-line-number="21"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb9-22" data-line-number="22"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a></code></pre></div>
|
||
<p><code>lazy()</code> has another advantage over <code>substitute()</code> - by default, it follows promises across function invocations. This simplifies the casual use of NSE.</p>
|
||
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb10-1" data-line-number="1">x <-<span class="st"> </span><span class="dv">31</span></a>
|
||
<a class="sourceLine" id="cb10-2" data-line-number="2">g1 <-<span class="st"> </span><span class="cf">function</span>(comp) {</a>
|
||
<a class="sourceLine" id="cb10-3" data-line-number="3"> x <-<span class="st"> </span><span class="dv">30</span></a>
|
||
<a class="sourceLine" id="cb10-4" data-line-number="4"> <span class="kw">subset</span>(mtcars, comp)</a>
|
||
<a class="sourceLine" id="cb10-5" data-line-number="5">}</a>
|
||
<a class="sourceLine" id="cb10-6" data-line-number="6"><span class="kw">g1</span>(mpg <span class="op">></span><span class="st"> </span>x)</a>
|
||
<a class="sourceLine" id="cb10-7" data-line-number="7"><span class="co">#> Error: object 'mpg' not found</span></a></code></pre></div>
|
||
<div class="sourceCode" id="cb11"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb11-1" data-line-number="1">g2 <-<span class="st"> </span><span class="cf">function</span>(comp) {</a>
|
||
<a class="sourceLine" id="cb11-2" data-line-number="2"> x <-<span class="st"> </span><span class="dv">30</span></a>
|
||
<a class="sourceLine" id="cb11-3" data-line-number="3"> <span class="kw">subset2</span>(mtcars, comp)</a>
|
||
<a class="sourceLine" id="cb11-4" data-line-number="4">}</a>
|
||
<a class="sourceLine" id="cb11-5" data-line-number="5"><span class="kw">g2</span>(mpg <span class="op">></span><span class="st"> </span>x)</a>
|
||
<a class="sourceLine" id="cb11-6" data-line-number="6"><span class="co">#> mpg cyl disp hp drat wt qsec vs am gear carb</span></a>
|
||
<a class="sourceLine" id="cb11-7" data-line-number="7"><span class="co">#> 18 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1</span></a>
|
||
<a class="sourceLine" id="cb11-8" data-line-number="8"><span class="co">#> 20 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1</span></a></code></pre></div>
|
||
<p>Note that <code>g2()</code> doesn’t have a standard-evaluation escape hatch, so it’s not suitable for programming with in the same way that <code>subset2_()</code> is.</p>
|
||
</div>
|
||
<div id="chained-promises" class="section level2">
|
||
<h2>Chained promises</h2>
|
||
<p>Take the following example:</p>
|
||
<div class="sourceCode" id="cb12"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb12-1" data-line-number="1"><span class="kw">library</span>(lazyeval)</a>
|
||
<a class="sourceLine" id="cb12-2" data-line-number="2">f1 <-<span class="st"> </span><span class="cf">function</span>(x) <span class="kw">lazy</span>(x)</a>
|
||
<a class="sourceLine" id="cb12-3" data-line-number="3">g1 <-<span class="st"> </span><span class="cf">function</span>(y) <span class="kw">f1</span>(y)</a>
|
||
<a class="sourceLine" id="cb12-4" data-line-number="4"></a>
|
||
<a class="sourceLine" id="cb12-5" data-line-number="5"><span class="kw">g1</span>(a <span class="op">+</span><span class="st"> </span>b)</a>
|
||
<a class="sourceLine" id="cb12-6" data-line-number="6"><span class="co">#> <lazy></span></a>
|
||
<a class="sourceLine" id="cb12-7" data-line-number="7"><span class="co">#> expr: a + b</span></a>
|
||
<a class="sourceLine" id="cb12-8" data-line-number="8"><span class="co">#> env: <environment: R_GlobalEnv></span></a></code></pre></div>
|
||
<p><code>lazy()</code> returns <code>a + b</code> because it always tries to find the top-level promise.</p>
|
||
<p>In this case the process looks like this:</p>
|
||
<ol style="list-style-type: decimal">
|
||
<li>Find the object that <code>x</code> is bound to.</li>
|
||
<li>It’s a promise, so find the expr it’s bound to (<code>y</code>, a symbol) and the environment in which it should be evaluated (the environment of <code>g()</code>).</li>
|
||
<li>Since <code>x</code> is bound to a symbol, look up its value: it’s bound to a promise.</li>
|
||
<li>That promise has expression <code>a + b</code> and should be evaluated in the global environment.</li>
|
||
<li>The expression is not a symbol, so stop.</li>
|
||
</ol>
|
||
<p>Occasionally, you want to avoid this recursive behaviour, so you can use <code>follow_symbol = FALSE</code>:</p>
|
||
<div class="sourceCode" id="cb13"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb13-1" data-line-number="1">f2 <-<span class="st"> </span><span class="cf">function</span>(x) <span class="kw">lazy</span>(x, <span class="dt">.follow_symbols =</span> <span class="ot">FALSE</span>)</a>
|
||
<a class="sourceLine" id="cb13-2" data-line-number="2">g2 <-<span class="st"> </span><span class="cf">function</span>(y) <span class="kw">f2</span>(y)</a>
|
||
<a class="sourceLine" id="cb13-3" data-line-number="3"></a>
|
||
<a class="sourceLine" id="cb13-4" data-line-number="4"><span class="kw">g2</span>(a <span class="op">+</span><span class="st"> </span>b)</a>
|
||
<a class="sourceLine" id="cb13-5" data-line-number="5"><span class="co">#> <lazy></span></a>
|
||
<a class="sourceLine" id="cb13-6" data-line-number="6"><span class="co">#> expr: x</span></a>
|
||
<a class="sourceLine" id="cb13-7" data-line-number="7"><span class="co">#> env: <environment: 0x7ff853abd7f8></span></a></code></pre></div>
|
||
<p>Either way, if you evaluate the lazy expression you’ll get the same result:</p>
|
||
<div class="sourceCode" id="cb14"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb14-1" data-line-number="1">a <-<span class="st"> </span><span class="dv">10</span></a>
|
||
<a class="sourceLine" id="cb14-2" data-line-number="2">b <-<span class="st"> </span><span class="dv">1</span></a>
|
||
<a class="sourceLine" id="cb14-3" data-line-number="3"></a>
|
||
<a class="sourceLine" id="cb14-4" data-line-number="4"><span class="kw">lazy_eval</span>(<span class="kw">g1</span>(a <span class="op">+</span><span class="st"> </span>b))</a>
|
||
<a class="sourceLine" id="cb14-5" data-line-number="5"><span class="co">#> [1] 11</span></a>
|
||
<a class="sourceLine" id="cb14-6" data-line-number="6"><span class="kw">lazy_eval</span>(<span class="kw">g2</span>(a <span class="op">+</span><span class="st"> </span>b))</a>
|
||
<a class="sourceLine" id="cb14-7" data-line-number="7"><span class="co">#> [1] 11</span></a></code></pre></div>
|
||
<p>Note that the resolution of chained promises only works with unevaluated objects. This is because R deletes the information about the environment associated with a promise when it has been forced, so that the garbage collector is allowed to remove the environment from memory in case it is no longer used. <code>lazy()</code> will fail with an error in such situations.</p>
|
||
<div class="sourceCode" id="cb15"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb15-1" data-line-number="1">var <-<span class="st"> </span><span class="dv">0</span></a>
|
||
<a class="sourceLine" id="cb15-2" data-line-number="2"></a>
|
||
<a class="sourceLine" id="cb15-3" data-line-number="3">f3 <-<span class="st"> </span><span class="cf">function</span>(x) {</a>
|
||
<a class="sourceLine" id="cb15-4" data-line-number="4"> <span class="kw">force</span>(x)</a>
|
||
<a class="sourceLine" id="cb15-5" data-line-number="5"> <span class="kw">lazy</span>(x)</a>
|
||
<a class="sourceLine" id="cb15-6" data-line-number="6">}</a>
|
||
<a class="sourceLine" id="cb15-7" data-line-number="7"></a>
|
||
<a class="sourceLine" id="cb15-8" data-line-number="8"><span class="kw">f3</span>(var)</a>
|
||
<a class="sourceLine" id="cb15-9" data-line-number="9"><span class="co">#> Error in lazy(x): Promise has already been forced</span></a></code></pre></div>
|
||
</div>
|
||
|
||
|
||
|
||
<!-- dynamically load mathjax for compatibility with self-contained -->
|
||
<script>
|
||
(function () {
|
||
var script = document.createElement("script");
|
||
script.type = "text/javascript";
|
||
script.src = "https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML";
|
||
document.getElementsByTagName("head")[0].appendChild(script);
|
||
})();
|
||
</script>
|
||
|
||
</body>
|
||
</html>
|