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lecture 6: remove last traces of instance restrictions

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GHC has relaxed all the relevant restrictions so the material is misleading
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Joel Kaasinen
2025-09-26 08:56:42 +03:00
parent 1cb691781a
commit ff0a97af0c
1 changed files with 0 additions and 16 deletions
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@@ -3550,7 +3550,6 @@ Because <code>Nothing</code> is a constructor.
<div class="sourceCode" id="cb401"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb401-1"><a href="#cb401-1" aria-hidden="true" tabindex="-1"></a> • <span class="dt">No</span> <span class="kw">instance</span> for (<span class="dt">Check</span> a) arising from a use <span class="kw">of</span> ‘check’</span>
<span id="cb401-2"><a href="#cb401-2" aria-hidden="true" tabindex="-1"></a> <span class="dt">Possible</span> fix<span class="op">:</span></span>
<span id="cb401-3"><a href="#cb401-3" aria-hidden="true" tabindex="-1"></a> add (<span class="dt">Check</span> a) to the context <span class="kw">of</span> the <span class="kw">instance</span> declaration</span></code></pre></div>
<p>If you think about it, this instance hierarchy allows us to circumvent the limitation that we can’t make a <code>Check [Int]</code> instance.</p>
<p>Finally, sometimes multiple constraints are needed. Consider for example the <code>Eq</code> instance for <code>Either</code>:</p>
<div class="sourceCode" id="cb402"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb402-1"><a href="#cb402-1" aria-hidden="true" tabindex="-1"></a><span class="kw">instance</span> (<span class="dt">Eq</span> a, <span class="dt">Eq</span> b) <span class="ot">=&gt;</span> <span class="dt">Eq</span> (<span class="dt">Either</span> a b) <span class="kw">where</span></span>
<span id="cb402-2"><a href="#cb402-2" aria-hidden="true" tabindex="-1"></a> <span class="dt">Left</span> x <span class="op">==</span> <span class="dt">Left</span> y <span class="ot">=</span> x<span class="op">==</span>y</span>
@@ -3595,21 +3594,6 @@ Because <code>Nothing</code> is a constructor.
<code>Size</code>
</li>
</ol>
<p>Which of the following instance declarations is legal?</p>
<ol class="quiz">
<li>
<code>instance Eq Maybe</code>
</li>
<li>
<code>instance Eq (a,a)</code>
</li>
<li>
<code>instance Eq (Maybe Int)</code>
</li>
<li class="correct">
<code>instance Eq (a,b)</code>
</li>
</ol>
<p>Given the following definition of the class <code>BitOperations</code></p>
<div class="sourceCode" id="cb405"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb405-1"><a href="#cb405-1" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> <span class="dt">BitOperations</span> a <span class="kw">where</span></span>
<span id="cb405-2"><a href="#cb405-2" aria-hidden="true" tabindex="-1"></a><span class="ot"> bitNot ::</span> a <span class="ot">-&gt;</span> a</span>