https://guide.aelve.com/haskell/feed/category/rx8z5mha2017-05-01T23:35:33Zaapfhb5j2017-05-01T23:35:33Z<h1> <span class="item-name">data family</span> </h1>d1ip46q92017-05-01T23:35:29Z<h1> <span class="item-name">type family</span> </h1>mh9nz3y42017-05-01T23:34:13Z<h1> <span class="item-name">stock, newtype, anyclass</span> </h1><p>See <a href="https://downloads.haskell.org/%7Eghc/master/users-guide/glasgow_exts.html#deriving-strategies">GHC Guide on deriving strategies</a>.</p> jyk4bwqp2017-05-01T23:26:03Z<h1> <span class="item-name">pattern</span> </h1>mxrx8vgf2017-05-01T23:25:36Z<h1> <span class="item-name">mdo</span> </h1><p>hello</p> ou580jki2017-05-01T22:53:30Z<h1> <span class="item-name">proc</span> </h1>x4iz1s652017-05-01T22:50:44Z<h1> <span class="item-name">role, nominal, representational, phantom</span> </h1><p>In recent versions of GHC there is a notion of roles (see <a href="https://downloads.haskell.org/%7Eghc/latest/docs/html/users_guide/glasgow_exts.html#roles">GHC Guide</a>).</p> <p>Types that have the same representation (e.g. a type and its newtype) can be converted between each other with <code>coerce</code> or with <code>-XGeneralizedNewtypeDeriving</code>. Roles exist to constrain ways in which types can be converted. For instance, this definition will disallow converting <code>Set a</code> to <code>Set b</code> even if <code>b</code> is a newtype of <code>a</code>:</p> <div class="sourceCode"><pre class="sourceCode"><code class="sourceCode"><span class="kw">type</span> role <span class="dt">Set</span> nominal</code></pre></div> <p>The reason it's not safe to convert a <code>Set</code> is that the set will have a different structure depending on the <code>Ord</code> instance of what's stored inside, and instances can be different for newtypes even if the representations are the same.</p> <p>There are three roles:</p> <ul> <li><strong><code>nominal</code>:</strong> types must be completely identical after type family reduction</li> <li><strong><code>representational</code>:</strong> types must have the same representation</li> <li><strong><code>phantom</code>:</strong> the type can be disregarded</li> </ul> n2j51qdp2017-05-01T22:50:29Z<h1> <span class="item-name">rec</span> </h1>