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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
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function to each of a list’s elements and reducing</li>
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</ul>
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<div id="gathering" class="section level2">
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<h2>Gathering</h2>
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<p>The most common pattern for combining promises is gathering, where
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you have two or more promises in hand and you want to use all of their
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results in a computation. The <code>promise_all</code> function is
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designed for this. Its signature looks like this:</p>
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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" 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>
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<p><code>promise_all</code> takes any number of promises as named
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arguments, and returns a promise of a list containing named elements
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with the results of those promises.</p>
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<p>Here’s an example using <code>promise_all</code> to combine the
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results of two async <code>read.csv</code> operations:</p>
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<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>
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<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
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<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
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<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a></span>
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<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>
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<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>
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||
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a></span>
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||
<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>
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||
<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>
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||
<span id="cb2-10"><a href="#cb2-10" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
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<p>In this example, the value of <code>.</code> within the curly braces
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is a list whose elements <code>a</code> and <code>b</code> are both data
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||
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>
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||
<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>
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||
<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>
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||
<span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a> <span class="fu">with</span>({</span>
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||
<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>
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||
<p>(Note that since the <code>promise_all</code> argument names are the
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||
same as the variable names (<code>a = a</code>, <code>b = b</code>), the
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||
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
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||
confusing, you can just choose a different argument name, like
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||
<code>promise_all(a_result = a, …)</code>.)</p>
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||
<p>The combination of <code>promise_all</code> and <code>with</code> is
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||
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,
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||
you can pass in promises as unnamed arguments:
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||
<code>promise_all(a, b)</code> would yield <code>list(1, 2)</code>.
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||
Second, if you have a list of promises already in hand, you can pass the
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||
list as a single argument using <code>promise_all(.list = x)</code>
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||
(instead of, say, using <code>do.call(promise_all, x)</code>).</p>
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</div>
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<div id="nesting" class="section level2">
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<h2>Nesting</h2>
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||
<p>Gathering is easy and convenient, but sometimes not flexible enough.
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||
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>
|
||
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
|
||
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a></span>
|
||
<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>
|
||
<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cloud.r-project.org"</span>,</span>
|
||
<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cran.usthb.dz"</span>,</span>
|
||
<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cran.csiro.au"</span>,</span>
|
||
<span id="cb10-9"><a href="#cb10-9" aria-hidden="true" tabindex="-1"></a> <span class="st">"https://cran.wu.ac.at"</span></span>
|
||
<span id="cb10-10"><a href="#cb10-10" aria-hidden="true" tabindex="-1"></a>)</span>
|
||
<span id="cb10-11"><a href="#cb10-11" aria-hidden="true" tabindex="-1"></a></span>
|
||
<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>
|
||
<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>
|
||
<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>
|
||
</div>
|
||
|
||
|
||
|
||
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