<p>In computer science, a <strong>first-class citizen</strong> is an entity that supports all operations available to other entities. Some of the available operations are:</p>
<ul>
<li>They may be named by variables;</li>
<li>They may be passed as arguments to procedures;</li>
<li>They may be returned as the results of procedures;</li>
<li>They may be included in data structures.</li>
</ul>
<p>It was the British computer scientist Christopher Strachey (1916-1975) <a href="https://link.springer.com/article/10.1023/A:1010052305354">who first</a> coined this notion of first-class citizen status of elements in a programming language in the 1960s.</p>
<p>In JavaScript, for example, functions are first-class citizens, as all of the operations cited above can be applied to them. Let&#39;s see some examples:</p>
<p><strong>A simple function definition in JavaScript</strong></p>
<pre><code class="highlited-code-wrapper langjavascript"><div class="highlited-code" data-language="javascript">function sum(a, b) {
  return a + b
}</div>
</code></pre><p><strong>Assigning a constant to a function</strong></p>
<pre><code class="highlited-code-wrapper langjavascript"><div class="highlited-code" data-language="javascript">const sum = (a, b) =&gt; a + b

// or
// 
// const sum = function (a, b) {
//   a + b
// }</div>
</code></pre><p><strong>Passing a function as an argument</strong></p>
<pre><code class="highlited-code-wrapper langjavascript"><div class="highlited-code" data-language="javascript">function sum(a, b, callback) {
  const result = a + b

  if (typeof callback === 'function') {
    callback(result) // pass the result as an argument of `callback`
  }

  return result
}

//        Pass `console.log` as the callback function
// -------\/
sum(2, 2, console.log) // =&gt; 4</div>
</code></pre><p><strong>Return a function</strong></p>
<pre><code class="highlited-code-wrapper langjavascript"><div class="highlited-code" data-language="javascript">function sum(a, b, callback) {
  const result = a + b

  if (callback) {
    return () =&gt; callback(result)
  }

  return result
}

//            The callback is the sum of the result with 2.
// ------------------\/
const fn = sum(2, 2, (result) =&gt; sum(2, result))
//    ^---- Store the returned function in a variable

//          Execute the function
// ---------\/
console.log(fn()) // =&gt; 6</div>
</code></pre><p><strong>Including a function in a data structure</strong></p>
<pre><code class="highlited-code-wrapper langjavascript"><div class="highlited-code" data-language="javascript">// Store the basic operations in an object
const operations = {
  sum: (a, b) =&gt; a + b,
  sub: (a, b) =&gt; a - b,
  mul: (a, b) =&gt; a * b,
  div: (a, b) =&gt; a / b,
}</div>
</code></pre>

What is a first-class citizen in computer science?

<p>A <strong>remote procedure call</strong> (RPC) is a mechanism of communication between two computational environments, where one can be identified as a client, while the other can be identified as a server.</p>
<p>From the client&#39;s point of view, the RPC is just a matter of calling a function with the desired arguments and await for the response, in order to continue the program&#39;s execution.</p>
<p><img src="https://dev-to-uploads.s3.amazonaws.com/uploads/articles/p3733ruz3n51tadloaaq.png" alt="Diagram on how a RPC (remote procedure call) works"></p>
<p>Thus, using an RPC allows one programmer to distribute the system, according to their needs.</p>
<p><strong>References:</strong></p>
<ul>
<li><a href="https://www.youtube.com/watch?v=gr7oaiUsxSU">RPC Vs Simple Procedure Call - Georgia Tech - Advanced Operating Systems</a></li>
<li><a href="https://learn.microsoft.com/en-us/windows/win32/rpc/how-rpc-works">How RPC works</a></li>
<li><a href="https://users.cs.cf.ac.uk/Dave.Marshall/C/node33.html">Remote Procedure Calls (RPC)</a></li>
<li><a href="https://web.archive.org/web/20030404113118/http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi?coll=0650&db=bks&srch=&fname=/SGI_Developer/IRIX_NetPG/sgi_html/ch04.html">Introduction to RPC Programming</a></li>
<li><a href="https://colinhacks.com/essays/painless-typesafety">Building an end-to-end typesafe API — without GraphQL</a></li>
</ul>


Douglas Moura

Douglas Moura

#beginners

What is a first-class citizen in computer science?

What is a RPC (Remote Procedure Call)?