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<h1><a href="#algebras-and-interpreters" name="algebras-and-interpreters" class="anchor"><span class="anchor-link"></span></a>Algebras and interpreters</h1>
<p>In endpoints4s, we call “algebra interfaces” (or just <strong>algebras</strong>) the traits that provide methods for constructing and combining descriptions of endpoints. You can find more details about the design of these algebras in the <a href="design.html">Design in a nutshell</a> page.</p>
<p>The concepts defined by algebras are given a concrete meaning by <strong>interpreters</strong>. In practice, we have three kinds of interpreters:</p>
<ul>
<li><strong>clients</strong> building requests (their URL, headers and entity) and decoding responses,</li>
<li><strong>servers</strong> decoding requests and building responses,</li>
<li><strong>documentation</strong> in a machine-readable format.</li>
</ul>
<h2><a href="#naming-conventions" name="naming-conventions" class="anchor"><span class="anchor-link"></span></a>Naming conventions</h2>
<ul>
<li>algebras are defined as traits in the <code>endpoints4s.algebra</code> package ;
<ul>
<li>e.g. the <a href="api/endpoints4s/algebra/Urls.html" title="endpoints4s.algebra.Urls"><code>`endpoints4s.algebra.Urls`</code></a> trait defines an algebra for describing URLs.</li>
</ul>
</li>
<li>algebras’ dependencies can be found in their super types ;</li>
<li>interpreters are traits that have the same name as the algebra they implement (they can be found by looking at the “known subclasses” of an algebra, in the Scaladoc) ;
<ul>
<li>e.g. the <a href="api/endpoints4s/http4s/client/Urls.html" title="endpoints4s.http4s.client.Urls"><code>`endpoints4s.http4s.client.Urls`</code></a> trait defines an http4s-based interpreter for the <code>Urls</code> algebra.</li>
</ul>
</li>
<li>compatible interpreters are in the same package ;</li>
<li>e.g. the <a href="api/endpoints4s/http4s/client/index.html" title="endpoints4s.http4s.client"><code>`endpoints4s.http4s.client`</code></a> package provides client interpreters that are all based on <a href="https://http4s.org">http4s</a> under the hood.</li>
</ul>
<h2><a href="#matching-algebras-and-interpreters" name="matching-algebras-and-interpreters" class="anchor"><span class="anchor-link"></span></a>Matching Algebras and Interpreters</h2>
<p>To define endpoint descriptions you need to select which algebras you need. For instance:</p>
<pre class="prettyprint"><code class="language-scala">import endpoints4s.algebra
trait MyCustomAlgebra extends algebra.Endpoints
with algebra.JsonEntitiesFromSchemas
with algebra.BasicAuthentication
</code></pre>
<p>Then, to interpret the endpoint descriptions you need to build an interpreter that matches this algebra. By following the aforementioned naming conventions, you should be looking for all the traits that have the same names as your algebra modules and that are in the package of the interpreter you want to use. For instance, to build an Pekko-HTTP server:</p>
<pre class="prettyprint"><code class="language-scala">import endpoints4s.pekkohttp.server
trait MyPekkoHttpServer extends MyCustomAlgebra
with server.Endpoints
with server.JsonEntitiesFromSchemas
with server.BasicAuthentication
</code></pre><div class="callout warning "><div class="callout-title">Warning</div>
<p>If you try to mix an interpreter module that doesn’t match your algebra the compiler will raise “conflicting inherited members” errors.</p></div>
<h2><a href="#algebras" name="algebras" class="anchor"><span class="anchor-link"></span></a>Algebras</h2>
<p>Algebras form a <strong>hierarchy</strong>: it is possible to extend an algebra with additional vocabulary, or to mix several algebras together to merge their vocabulary. This hierarchy can be seen in the <a href="api/endpoints4s/algebra/index.html#inheritance-diagram-container" title="endpoints4s.algebra"><code>diagram</code></a> generated by the API documentation.</p>
<p>The following table lists the available algebras and points to their documentation. You should start by reading the documentation of the <code>Endpoints</code> algebra, and the documentation of the <code>JsonEntities</code> and <code>JsonSchemas</code> algebras if you want to work with JSON.</p>
<table>
<thead>
<tr>
<th>Name </th>
<th>Description </th>
</tr>
</thead>
<tbody>
<tr>
<td><a href="algebras/endpoints.html">Endpoints</a></td>
<td>HTTP endpoints</td>
</tr>
<tr>
<td><a href="algebras/json-entities.html">JsonEntities</a></td>
<td>JSON request and response entities</td>
</tr>
<tr>
<td><a href="algebras/json-schemas.html">JsonSchemas</a></td>
<td>JSON schemas of data types</td>
</tr>
<tr>
<td><a href="algebras/chunked-entities.html">ChunkedEntities</a></td>
<td>Streamed requests and responses</td>
</tr>
<tr>
<td><a href="algebras/assets.html">Assets</a></td>
<td>Asset segments, endpoints serving fingerprinted assets</td>
</tr>
<tr>
<td><a href="algebras/mux-endpoints.html">MuxEndpoints</a></td>
<td>Multiplexed HTTP endpoints</td>
</tr>
</tbody>
</table>
<h2><a href="#interpreters" name="interpreters" class="anchor"><span class="anchor-link"></span></a>Interpreters</h2>
<p>Interpreters give a concrete meaning to the vocabulary and operations provided by the algebras. They usually rely on other libraries (e.g. circe, Pekko HTTP, etc.) to do so. Pick the interpreters that fit your existing stack!</p>
<table>
<thead>
<tr>
<th>Family </th>
<th>Description </th>
</tr>
</thead>
<tbody>
<tr>
<td><a href="interpreters/pekko-http.html">Pekko HTTP</a> </td>
<td>Client and server backed by <a href="https://pekko.apache.org/docs/pekko/current/">Pekko HTTP</a> </td>
</tr>
<tr>
<td><a href="https://endpoints4s.github.io/akka-http">Akka HTTP</a> </td>
<td>Client and server backed by <a href="https://doc.akka.io/docs/akka-http/current/">Akka HTTP</a> </td>
</tr>
<tr>
<td><a href="interpreters/http4s.html">http4s</a> </td>
<td>Client and server backed by <a href="https://http4s.org">http4s</a> </td>
</tr>
<tr>
<td><a href="https://endpoints4s.github.io/play">Play framework</a> </td>
<td>Client and server backed by <a href="https://www.playframework.com/">Play framework</a> </td>
</tr>
<tr>
<td><a href="interpreters/scalajs-web-fetch.html">Scala.js web (Fetch)</a> </td>
<td>Scala.js web client using <code>Fetch</code> </td>
</tr>
<tr>
<td><a href="https://endpoints4s.github.io/xhr">Scala.js web (XHR)</a> </td>
<td>Scala.js web client using <code>XMLHttpRequest</code> </td>
</tr>
<tr>
<td><a href="https://endpoints4s.github.io/scalaj">scalaj-http</a> </td>
<td>JVM client backed by <a href="https://github.com/scalaj/scalaj-http">scalaj-http</a> </td>
</tr>
<tr>
<td><a href="interpreters/sttp.html">sttp</a> </td>
<td>JVM client backed by <a href="https://github.com/softwaremill/sttp">sttp</a> </td>
</tr>
<tr>
<td><a href="interpreters/openapi.html">OpenAPI</a> </td>
<td>Generates <a href="https://github.com/OAI/OpenAPI-Specification">OpenAPI</a> documents for endpoints definitions </td>
</tr>
<tr>
<td><a href="interpreters/circe.html">circe</a> </td>
<td>Builds <a href="http://circe.github.io/circe/">circe</a> codecs out of JSON schema definitions </td>
</tr>
<tr>
<td><a href="interpreters/play-json.html">Play JSON</a> </td>
<td>Builds <a href="https://github.com/playframework/play-json">Play JSON</a> Reads and Writes out of JSON schema definitions </td>
</tr>
</tbody>
</table><div class="callout note "><div class="callout-title">Note</div>
<p>You can have different stacks on the client-side and the server-side. For instance, you can have a server backed by Play framework, a client backed by Pekko HTTP, and another client backed by Scala.js (for web browsers).</p></div>
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