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<!DOCTYPE html>
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<meta name="author" content="Ilya Kostyuchenko">
<title>4</title>
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<body>
<div class="reveal">
<div class="slides">
<section id="функциональное-программирование" class="slide level1">
<h1>Функциональное программирование</h1>
</section>
<section class="slide level1">
<h2 id="напоминалочка">Напоминалочка</h2>
<div class="sourceCode" id="cb1"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">class</span> <span class="dt">Functor</span> f <span class="kw">where</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="ot"> fmap ::</span> (a <span class="ot">-></span> b) <span class="ot">-></span> f a <span class="ot">-></span> f b</span></code></pre></div>
<div class="sourceCode" id="cb2"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">class</span> <span class="dt">Functor</span> f <span class="ot">=></span> <span class="dt">Applicative</span> f <span class="kw">where</span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="ot"> (<*>) ::</span> f (a <span class="ot">-></span> b) <span class="ot">-></span> f a <span class="ot">-></span> f b</span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="ot"> pure ::</span> a <span class="ot">-></span> f a</span></code></pre></div>
<div class="sourceCode" id="cb3"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">class</span> <span class="dt">Applicative</span> f <span class="ot">=></span> <span class="dt">Monad</span> f <span class="kw">where</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="ot"> (>>=) ::</span> f a <span class="ot">-></span> (a <span class="ot">-></span> f b) <span class="ot">-></span> f b</span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="ot"> return ::</span> a <span class="ot">-></span> f a</span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="either">Either</h2>
<div class="sourceCode" id="cb4"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">data</span> <span class="dt">Either</span> a b <span class="ot">=</span> <span class="dt">Left</span> a <span class="op">|</span> <span class="dt">Right</span> b</span></code></pre></div>
<div class="sourceCode" id="cb5"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb5-1"><a href="#cb5-1"></a><span class="ot">throwError ::</span> e <span class="ot">-></span> <span class="dt">Either</span> e a</span>
<span id="cb5-2"><a href="#cb5-2"></a>throwError <span class="ot">=</span> <span class="dt">Left</span></span></code></pre></div>
<div class="sourceCode" id="cb6"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">data</span> <span class="dt">PaymentError</span> <span class="ot">=</span> <span class="dt">InsufficientBalance</span></span>
<span id="cb6-2"><a href="#cb6-2"></a></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="ot">pay ::</span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">PaymentError</span> <span class="dt">Balance</span></span>
<span id="cb6-4"><a href="#cb6-4"></a>pay balance price <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb6-5"><a href="#cb6-5"></a> <span class="kw">let</span> newBalance <span class="ot">=</span> balance <span class="op">-</span> price</span>
<span id="cb6-6"><a href="#cb6-6"></a> when (newBalance <span class="op"><</span> <span class="dv">0</span>) (throwError <span class="dt">InsufficientBalance</span>)</span>
<span id="cb6-7"><a href="#cb6-7"></a> <span class="fu">return</span> newBalance</span></code></pre></div>
<div class="sourceCode" id="cb7"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb7-1"><a href="#cb7-1"></a><span class="ot">when ::</span> (<span class="dt">Applicative</span> f) <span class="ot">=></span> <span class="dt">Bool</span> <span class="ot">-></span> f () <span class="ot">-></span> f ()</span>
<span id="cb7-2"><a href="#cb7-2"></a>when p s <span class="ot">=</span> <span class="kw">if</span> p <span class="kw">then</span> s <span class="kw">else</span> <span class="fu">pure</span> ()</span></code></pre></div>
</section>
<section id="state" class="slide level1">
<h1>State</h1>
<div class="sourceCode" id="cb8"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">data</span> <span class="dt">State</span> s a <span class="ot">=</span> <span class="dt">State</span> {<span class="ot"> runState ::</span> s <span class="ot">-></span> (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb9"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb9-1"><a href="#cb9-1"></a><span class="ot">get ::</span> <span class="dt">State</span> s s</span>
<span id="cb9-2"><a href="#cb9-2"></a>get <span class="ot">=</span> <span class="dt">State</span> (\s <span class="ot">-></span> (s, s))</span>
<span id="cb9-3"><a href="#cb9-3"></a></span>
<span id="cb9-4"><a href="#cb9-4"></a><span class="ot">put ::</span> s <span class="ot">-></span> <span class="dt">State</span> s ()</span>
<span id="cb9-5"><a href="#cb9-5"></a>put s <span class="ot">=</span> <span class="dt">State</span> (\_ <span class="ot">-></span> (_, s))</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb10"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb10-1"><a href="#cb10-1"></a><span class="ot">getNewId ::</span> <span class="dt">State</span> <span class="dt">TransactionId</span> <span class="dt">TransactionId</span></span>
<span id="cb10-2"><a href="#cb10-2"></a>getNewId <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb10-3"><a href="#cb10-3"></a> newId <span class="ot"><-</span> get</span>
<span id="cb10-4"><a href="#cb10-4"></a> put (<span class="fu">succ</span> newId)</span>
<span id="cb10-5"><a href="#cb10-5"></a> <span class="fu">return</span> newId</span>
<span id="cb10-6"><a href="#cb10-6"></a></span>
<span id="cb10-7"><a href="#cb10-7"></a><span class="kw">data</span> <span class="dt">Transaction</span> <span class="ot">=</span> <span class="dt">Transaction</span></span>
<span id="cb10-8"><a href="#cb10-8"></a> {<span class="ot"> transactionId ::</span> <span class="dt">TransactionId</span>,</span>
<span id="cb10-9"><a href="#cb10-9"></a><span class="ot"> transactionAmount ::</span> <span class="dt">Price</span></span>
<span id="cb10-10"><a href="#cb10-10"></a> }</span></code></pre></div>
<div class="sourceCode" id="cb11"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb11-1"><a href="#cb11-1"></a><span class="ot">registerPayment ::</span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">State</span> <span class="dt">TransactionId</span> <span class="dt">Transaction</span></span>
<span id="cb11-2"><a href="#cb11-2"></a>registerPayment price <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb11-3"><a href="#cb11-3"></a> newId <span class="ot"><-</span> getNewId</span>
<span id="cb11-4"><a href="#cb11-4"></a> <span class="fu">return</span> <span class="dt">Transaction</span></span>
<span id="cb11-5"><a href="#cb11-5"></a> { transactionId <span class="ot">=</span> newId</span>
<span id="cb11-6"><a href="#cb11-6"></a> transactionAmount <span class="ot">=</span> price</span>
<span id="cb11-7"><a href="#cb11-7"></a> }</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb12"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb12-1"><a href="#cb12-1"></a><span class="ot">registerPayment ::</span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">State</span> <span class="dt">TransactionId</span> <span class="dt">Transaction</span></span>
<span id="cb12-2"><a href="#cb12-2"></a><span class="ot">pay ::</span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">PaymentError</span> <span class="dt">Balance</span></span></code></pre></div>
<div class="sourceCode" id="cb13"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb13-1"><a href="#cb13-1"></a>payAndRegister balance price <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb13-2"><a href="#cb13-2"></a> newBalance <span class="ot"><-</span> pay balance price</span>
<span id="cb13-3"><a href="#cb13-3"></a> transacation <span class="ot"><-</span> registerPayment price</span>
<span id="cb13-4"><a href="#cb13-4"></a> <span class="fu">return</span> (newBalance, transaction)</span></code></pre></div>
<div class="sourceCode" id="cb14"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb14-1"><a href="#cb14-1"></a><span class="ot">(>=>) ::</span> <span class="dt">Monad</span> m <span class="ot">=></span> (a <span class="ot">-></span> m b) <span class="ot">-></span> (b <span class="ot">-></span> m c) <span class="ot">-></span> (a <span class="ot">-></span> m c)</span></code></pre></div>
<div class="sourceCode" id="cb15"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb15-1"><a href="#cb15-1"></a><span class="co">-- Такого нет</span></span>
<span id="cb15-2"><a href="#cb15-2"></a>(<span class="op">>=></span>)</span>
<span id="cb15-3"><a href="#cb15-3"></a><span class="ot"> ::</span> (<span class="dt">Monad</span> m, <span class="dt">Monad</span> n)</span>
<span id="cb15-4"><a href="#cb15-4"></a> <span class="ot">=></span> (a <span class="ot">-></span> m b) <span class="ot">-></span> (b <span class="ot">-></span> n c) <span class="ot">-></span> (a <span class="ot">-></span> <span class="op">?</span> c)</span></code></pre></div>
<p><span class="fragment">На каждый чих нужно создавать новый тип?</span></p>
</section>
<section class="slide level1">
<p>Мы хотим:</p>
<ol type="1">
<li class="fragment">Отделить эффекты от конкретной монады</li>
<li class="fragment">Получить возможность комбинировать разные эффекты</li>
</ol>
<p><span class="fragment">(И чтобы не надо было на каждый случай новый тип создавать)</span></p>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb16"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb16-1"><a href="#cb16-1"></a><span class="ot">registerPayment ::</span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">State</span> <span class="dt">TransactionId</span> <span class="dt">Transaction</span></span>
<span id="cb16-2"><a href="#cb16-2"></a><span class="ot">pay ::</span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">PaymentError</span> <span class="dt">Balance</span></span></code></pre></div>
<div class="sourceCode" id="cb17"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb17-1"><a href="#cb17-1"></a>registerPayment</span>
<span id="cb17-2"><a href="#cb17-2"></a><span class="ot"> ::</span> <span class="dt">MonadState</span> <span class="dt">TransactionId</span> m</span>
<span id="cb17-3"><a href="#cb17-3"></a> <span class="ot">=></span> <span class="dt">Price</span> <span class="ot">-></span> m <span class="dt">Transaction</span></span>
<span id="cb17-4"><a href="#cb17-4"></a>pay</span>
<span id="cb17-5"><a href="#cb17-5"></a><span class="ot"> ::</span> <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m</span>
<span id="cb17-6"><a href="#cb17-6"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m <span class="dt">Balance</span></span></code></pre></div>
<div class="sourceCode" id="cb18"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb18-1"><a href="#cb18-1"></a>payAndRegister</span>
<span id="cb18-2"><a href="#cb18-2"></a><span class="ot"> ::</span> (<span class="dt">MonadState</span> <span class="dt">TransactionId</span> m, <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m)</span>
<span id="cb18-3"><a href="#cb18-3"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span></code></pre></div>
</section>
<section class="slide level1">
<p>На самом деле мы хотим чтобы понятие “state” не было привязано к конкретной монаде.</p>
<p><span class="fragment">Мы хотим чтобы “базовые операции” не были привязаны к конкретной монаде.</span></p>
<div class="sourceCode" id="cb19"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb19-1"><a href="#cb19-1"></a><span class="co">-- Было</span></span>
<span id="cb19-2"><a href="#cb19-2"></a><span class="ot">get ::</span> <span class="dt">State</span> s s</span>
<span id="cb19-3"><a href="#cb19-3"></a><span class="ot">put ::</span> s <span class="ot">-></span> <span class="dt">State</span> s ()</span></code></pre></div>
<div class="sourceCode" id="cb20"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb20-1"><a href="#cb20-1"></a><span class="co">-- Стало</span></span>
<span id="cb20-2"><a href="#cb20-2"></a><span class="ot">get ::</span> <span class="dt">MonadState</span> s m <span class="ot">=></span> m s</span>
<span id="cb20-3"><a href="#cb20-3"></a><span class="ot">put ::</span> <span class="dt">MonadState</span> s m <span class="ot">=></span> s <span class="ot">-></span> m ()</span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="monadstate">MonadState</h2>
</section>
<section class="slide level1">
<!--
```{ .haskell .fragment }
class MonadState s m | m -> s where
get :: m s
put :: s -> m ()
```
[`m -> s` означает что для каждого `m` может быть строго один `s`.]{ .fragment }
[(Это нужно чтобы когда вы делаете `get` было понятно что вам возвращать.)]{ .fragment } -->
<div class="sourceCode" id="cb21"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb21-1"><a href="#cb21-1"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadState</span> s m <span class="kw">where</span></span>
<span id="cb21-2"><a href="#cb21-2"></a><span class="ot"> get ::</span> m s</span>
<span id="cb21-3"><a href="#cb21-3"></a><span class="ot"> put ::</span> s <span class="ot">-></span> m ()</span></code></pre></div>
<div class="sourceCode" id="cb22"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb22-1"><a href="#cb22-1"></a><span class="kw">data</span> <span class="dt">State</span> s a <span class="ot">=</span> <span class="dt">State</span> {<span class="ot"> runState ::</span> s <span class="ot">-></span> (a, s) }</span>
<span id="cb22-2"><a href="#cb22-2"></a></span>
<span id="cb22-3"><a href="#cb22-3"></a><span class="kw">instance</span> <span class="dt">MonadState</span> s (<span class="dt">State</span> s) <span class="kw">where</span></span>
<span id="cb22-4"><a href="#cb22-4"></a> get <span class="ot">=</span> <span class="dt">State</span> (\s <span class="ot">-></span> (s, s))</span>
<span id="cb22-5"><a href="#cb22-5"></a> put s <span class="ot">=</span> <span class="dt">State</span> (\_ <span class="ot">-></span> ((), s))</span></code></pre></div>
<div class="sourceCode" id="cb23"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb23-1"><a href="#cb23-1"></a><span class="ot">getNewId ::</span> <span class="dt">MonadState</span> <span class="dt">TransactionId</span> m <span class="ot">=></span> m <span class="dt">TransactionId</span></span>
<span id="cb23-2"><a href="#cb23-2"></a>getNewId <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb23-3"><a href="#cb23-3"></a> newId <span class="ot"><-</span> get</span>
<span id="cb23-4"><a href="#cb23-4"></a> put (<span class="fu">succ</span> newId)</span>
<span id="cb23-5"><a href="#cb23-5"></a> <span class="fu">return</span> newId</span></code></pre></div>
<div class="sourceCode" id="cb24"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb24-1"><a href="#cb24-1"></a><span class="ot">registerPayment ::</span> <span class="dt">Price</span> <span class="ot">-></span> <span class="dt">State</span> <span class="dt">TransactionId</span> <span class="dt">Transaction</span></span>
<span id="cb24-2"><a href="#cb24-2"></a>registerPayment price <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb24-3"><a href="#cb24-3"></a> newId <span class="ot"><-</span> getNewId</span>
<span id="cb24-4"><a href="#cb24-4"></a> <span class="fu">return</span> <span class="dt">Transaction</span></span>
<span id="cb24-5"><a href="#cb24-5"></a> { transactionId <span class="ot">=</span> newId</span>
<span id="cb24-6"><a href="#cb24-6"></a> transactionAmount <span class="ot">=</span> price</span>
<span id="cb24-7"><a href="#cb24-7"></a> }</span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="monaderror">MonadError</h2>
<div class="sourceCode" id="cb25"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb25-1"><a href="#cb25-1"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadError</span> e m <span class="kw">where</span></span>
<span id="cb25-2"><a href="#cb25-2"></a><span class="ot"> throwError ::</span> e <span class="ot">-></span> m a</span>
<span id="cb25-3"><a href="#cb25-3"></a><span class="ot"> catchError ::</span> m a <span class="ot">-></span> (e <span class="ot">-></span> m a) <span class="ot">-></span> m a</span></code></pre></div>
<div class="sourceCode" id="cb26"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb26-1"><a href="#cb26-1"></a><span class="kw">data</span> <span class="dt">Either</span> e a <span class="ot">=</span> <span class="dt">Left</span> e <span class="op">|</span> <span class="dt">Right</span> a</span>
<span id="cb26-2"><a href="#cb26-2"></a></span>
<span id="cb26-3"><a href="#cb26-3"></a><span class="kw">instance</span> <span class="dt">MonadError</span> e (<span class="dt">Either</span> e) <span class="kw">where</span></span>
<span id="cb26-4"><a href="#cb26-4"></a> throwError e <span class="ot">=</span> <span class="dt">Left</span> e</span>
<span id="cb26-5"><a href="#cb26-5"></a></span>
<span id="cb26-6"><a href="#cb26-6"></a> catchError (<span class="dt">Left</span> e) f <span class="ot">=</span> f e</span>
<span id="cb26-7"><a href="#cb26-7"></a> catchError (<span class="dt">Right</span> a) _ <span class="ot">=</span> <span class="dt">Right</span> a</span></code></pre></div>
</section>
<section class="slide level1">
<p>Мы хотим:</p>
<ol type="1">
<li class="fragment"><del>Отделить эффекты от конкретной монады</del></li>
<li class="fragment">Получить возможность комбинировать разные эффекты</li>
</ol>
</section>
<section class="slide level1">
<p>Начнем с функторов</p>
<div class="sourceCode" id="cb27"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb27-1"><a href="#cb27-1"></a><span class="kw">data</span> <span class="dt">Compose</span> f g a <span class="ot">=</span> <span class="dt">Compose</span> {<span class="ot"> getCompose ::</span> f (g a) }</span></code></pre></div>
<div class="sourceCode" id="cb28"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb28-1"><a href="#cb28-1"></a><span class="kw">type</span> <span class="dt">ErrorStateFunctor</span> a <span class="ot">=</span></span>
<span id="cb28-2"><a href="#cb28-2"></a> <span class="dt">Compose</span> (<span class="dt">Either</span> <span class="dt">PaymentError</span>) (<span class="dt">State</span> <span class="dt">TransactionId</span>) a</span></code></pre></div>
<div class="sourceCode" id="cb29"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb29-1"><a href="#cb29-1"></a><span class="co">-- Compose</span></span>
<span id="cb29-2"><a href="#cb29-2"></a><span class="co">-- { getCompose ::</span></span>
<span id="cb29-3"><a href="#cb29-3"></a><span class="co">-- Either PaymentError (State TransactionId a)</span></span>
<span id="cb29-4"><a href="#cb29-4"></a><span class="co">-- }</span></span></code></pre></div>
<div class="sourceCode" id="cb30"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb30-1"><a href="#cb30-1"></a><span class="kw">instance</span> (<span class="dt">Functor</span> f, <span class="dt">Functor</span> g) <span class="ot">=></span><span class="dt">Functor</span> (<span class="dt">Compose</span> f g) <span class="kw">where</span></span>
<span id="cb30-2"><a href="#cb30-2"></a> <span class="fu">fmap</span> f (<span class="dt">Compose</span> x) <span class="ot">=</span> <span class="dt">Compose</span> (<span class="fu">fmap</span> (<span class="fu">fmap</span> f) x)</span></code></pre></div>
<p><span class="fragment">🎉</span></p>
<p><span class="fragment">(<code>Applicative</code> тоже можно – можете сами попробовать)</span></p>
</section>
<section class="slide level1">
<p>К монадам!</p>
<div class="sourceCode" id="cb31"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb31-1"><a href="#cb31-1"></a><span class="kw">instance</span> (<span class="dt">Monad</span> f, <span class="dt">Monad</span> g) <span class="ot">=></span> <span class="dt">Monad</span> (<span class="dt">Compose</span> f g) <span class="kw">where</span></span>
<span id="cb31-2"><a href="#cb31-2"></a> <span class="fu">return</span> x <span class="ot">=</span> <span class="dt">Compose</span> (<span class="fu">return</span> (<span class="fu">return</span> x))</span>
<span id="cb31-3"><a href="#cb31-3"></a> (<span class="dt">Compose</span> x) <span class="op">>>=</span> f <span class="ot">=</span> <span class="op">???</span></span></code></pre></div>
<p><span class="fragment">😢</span></p>
<p><span class="fragment">Все равно хочется!</span></p>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb32"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb32-1"><a href="#cb32-1"></a><span class="kw">data</span> <span class="dt">Compose</span> f g a <span class="ot">=</span> <span class="dt">Compose</span> {<span class="ot"> getCompose ::</span> f (g a) }</span></code></pre></div>
<p><span class="fragment">Для такого не можем</span></p>
<div class="sourceCode" id="cb33"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb33-1"><a href="#cb33-1"></a><span class="kw">data</span> <span class="dt">ErrorState</span> e s a <span class="ot">=</span></span>
<span id="cb33-2"><a href="#cb33-2"></a> <span class="dt">ErrorState</span> {<span class="ot"> runErrorState ::</span> <span class="dt">State</span> s (<span class="dt">Either</span> e a) }</span></code></pre></div>
<p><span class="fragment">А для такого можем!</span></p>
<div class="sourceCode" id="cb34"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb34-1"><a href="#cb34-1"></a>(<span class="dt">State</span> s (<span class="dt">Either</span> e a)) <span class="op">~</span> (s <span class="ot">-></span> <span class="dt">Either</span> e (a, s))</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb35"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb35-1"><a href="#cb35-1"></a><span class="kw">data</span> <span class="dt">ErrorState</span> e s a <span class="ot">=</span></span>
<span id="cb35-2"><a href="#cb35-2"></a> <span class="dt">ErrorState</span> {<span class="ot"> runErrorState ::</span> <span class="dt">State</span> s (<span class="dt">Either</span> e a) }</span></code></pre></div>
<p><span class="fragment">Хочется чтобы можно было не только <code>Either</code>!</span></p>
<div class="sourceCode" id="cb36"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb36-1"><a href="#cb36-1"></a><span class="kw">data</span> <span class="dt">State</span> s a <span class="ot">=</span> <span class="dt">State</span> {<span class="ot"> runState ::</span> s <span class="ot">-></span> (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb37"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb37-1"><a href="#cb37-1"></a><span class="kw">data</span> <span class="dt">StateT</span> s m a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> m (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb38"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb38-1"><a href="#cb38-1"></a><span class="kw">instance</span> (<span class="dt">Monad</span> m) <span class="ot">=></span> <span class="dt">Monad</span> (<span class="dt">StateT</span> s m) <span class="kw">where</span></span>
<span id="cb38-2"><a href="#cb38-2"></a> <span class="fu">return</span> a <span class="ot">=</span> <span class="dt">StateT</span> <span class="op">$</span> \ s <span class="ot">-></span> <span class="fu">return</span> (a, s)</span>
<span id="cb38-3"><a href="#cb38-3"></a></span>
<span id="cb38-4"><a href="#cb38-4"></a> m <span class="op">>>=</span> f <span class="ot">=</span> <span class="dt">StateT</span> (\s <span class="ot">-></span> <span class="kw">do</span></span>
<span id="cb38-5"><a href="#cb38-5"></a> (a, s') <span class="ot"><-</span> runStateT m s</span>
<span id="cb38-6"><a href="#cb38-6"></a> runStateT (f a) s'</span>
<span id="cb38-7"><a href="#cb38-7"></a> )</span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="transformers">Transformers!</h2>
<p><img data-src="images/transformer.gif" height="400" /></p>
</section>
<section class="slide level1">
<h2 id="вернем-операции">Вернем операции!</h2>
<div class="sourceCode" id="cb39"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb39-1"><a href="#cb39-1"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadState</span> s m <span class="kw">where</span></span>
<span id="cb39-2"><a href="#cb39-2"></a><span class="ot"> get ::</span> m s</span>
<span id="cb39-3"><a href="#cb39-3"></a><span class="ot"> put ::</span> s <span class="ot">-></span> m ()</span></code></pre></div>
<div class="sourceCode" id="cb40"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb40-1"><a href="#cb40-1"></a><span class="kw">data</span> <span class="dt">StateT</span> s n a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> n (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb41"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb41-1"><a href="#cb41-1"></a><span class="kw">instance</span> <span class="dt">Monad</span> n <span class="ot">=></span> <span class="dt">MonadState</span> s (<span class="dt">StateT</span> s n) <span class="kw">where</span></span>
<span id="cb41-2"><a href="#cb41-2"></a> get <span class="ot">=</span> <span class="dt">StateT</span> (\s <span class="ot">-></span> <span class="fu">return</span> (s, s))</span>
<span id="cb41-3"><a href="#cb41-3"></a> put s <span class="ot">=</span> <span class="dt">StateT</span> (\_ <span class="ot">-></span> <span class="fu">return</span> ((), s)</span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="error">Error</h2>
<div class="sourceCode" id="cb42"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb42-1"><a href="#cb42-1"></a><span class="kw">data</span> <span class="dt">ExceptT</span> e n a <span class="ot">=</span> <span class="dt">ExceptT</span> {<span class="ot"> runExceptT ::</span> n (<span class="dt">Either</span> e a) }</span></code></pre></div>
<div class="sourceCode" id="cb43"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb43-1"><a href="#cb43-1"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadError</span> e m <span class="kw">where</span></span>
<span id="cb43-2"><a href="#cb43-2"></a><span class="ot"> throwError ::</span> e <span class="ot">-></span> m a</span>
<span id="cb43-3"><a href="#cb43-3"></a><span class="ot"> catchError ::</span> m a <span class="ot">-></span> (e <span class="ot">-></span> m a) <span class="ot">-></span> m a</span></code></pre></div>
<div class="sourceCode" id="cb44"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb44-1"><a href="#cb44-1"></a><span class="kw">instance</span> <span class="dt">Monad</span> n <span class="ot">=></span> <span class="dt">MonadError</span> e (<span class="dt">ExceptT</span> e n) <span class="kw">where</span></span>
<span id="cb44-2"><a href="#cb44-2"></a> throwError e <span class="ot">=</span> <span class="dt">ExceptT</span> (<span class="fu">return</span> (<span class="dt">Left</span> e))</span>
<span id="cb44-3"><a href="#cb44-3"></a></span>
<span id="cb44-4"><a href="#cb44-4"></a> catchError (<span class="dt">ExceptT</span> n) f <span class="ot">=</span> <span class="dt">ExceptT</span> <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb44-5"><a href="#cb44-5"></a> x <span class="ot"><-</span> n</span>
<span id="cb44-6"><a href="#cb44-6"></a> <span class="kw">case</span> x <span class="kw">of</span></span>
<span id="cb44-7"><a href="#cb44-7"></a> <span class="dt">Left</span> e <span class="ot">-></span> runExceptT (f e)</span>
<span id="cb44-8"><a href="#cb44-8"></a> <span class="dt">Right</span> a <span class="ot">-></span> <span class="fu">return</span> (<span class="dt">Right</span> a)</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb45"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb45-1"><a href="#cb45-1"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadError</span> e m <span class="kw">where</span></span>
<span id="cb45-2"><a href="#cb45-2"></a><span class="ot"> throwError ::</span> e <span class="ot">-></span> m a</span>
<span id="cb45-3"><a href="#cb45-3"></a><span class="ot"> catchError ::</span> m a <span class="ot">-></span> (e <span class="ot">-></span> m a) <span class="ot">-></span> m a</span></code></pre></div>
<div class="sourceCode" id="cb46"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb46-1"><a href="#cb46-1"></a><span class="kw">data</span> <span class="dt">PaymentError</span> <span class="ot">=</span> <span class="dt">InsufficientBalance</span></span>
<span id="cb46-2"><a href="#cb46-2"></a></span>
<span id="cb46-3"><a href="#cb46-3"></a><span class="ot">pay ::</span> <span class="dt">MonadError</span> e m <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m <span class="dt">Balance</span></span>
<span id="cb46-4"><a href="#cb46-4"></a>pay balance price <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb46-5"><a href="#cb46-5"></a> <span class="kw">let</span> newBalance <span class="ot">=</span> balance <span class="op">-</span> price</span>
<span id="cb46-6"><a href="#cb46-6"></a> when (newBalance <span class="op"><</span> <span class="dv">0</span>) (throwError <span class="dt">InsufficientBalance</span>)</span>
<span id="cb46-7"><a href="#cb46-7"></a> <span class="fu">return</span> newBalance</span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="the-states">The states</h2>
<div class="sourceCode" id="cb47"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb47-1"><a href="#cb47-1"></a><span class="kw">data</span> <span class="dt">State</span> s a <span class="ot">=</span> <span class="dt">State</span> {<span class="ot"> runState ::</span> s <span class="ot">-></span> (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb48"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb48-1"><a href="#cb48-1"></a><span class="kw">data</span> <span class="dt">StateT</span> s m a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> m (a, s) }</span></code></pre></div>
<p><span class="fragment">🤔</span></p>
</section>
<section class="slide level1">
<p>Напоминалочка:</p>
<div class="sourceCode" id="cb49"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb49-1"><a href="#cb49-1"></a><span class="kw">data</span> <span class="dt">Identity</span> a <span class="ot">=</span> <span class="dt">Identity</span> {<span class="ot"> runIdentity ::</span> a }</span></code></pre></div>
<div class="sourceCode" id="cb50"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb50-1"><a href="#cb50-1"></a>(<span class="dt">Identity</span> a) <span class="op">~</span> a</span></code></pre></div>
<div class="sourceCode" id="cb51"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb51-1"><a href="#cb51-1"></a><span class="kw">data</span> <span class="dt">State</span> s a <span class="ot">=</span> <span class="dt">State</span> {<span class="ot"> runState ::</span> s <span class="ot">-></span> (a, s) }</span>
<span id="cb51-2"><a href="#cb51-2"></a></span>
<span id="cb51-3"><a href="#cb51-3"></a><span class="kw">data</span> <span class="dt">StateT</span> s m a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> m (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb52"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb52-1"><a href="#cb52-1"></a>(<span class="dt">StateT</span> s <span class="dt">Identity</span> a) <span class="op">~</span> (s <span class="ot">-></span> <span class="dt">Identity</span> (a, s))</span></code></pre></div>
<p><span class="fragment">🤔</span></p>
<div class="sourceCode" id="cb53"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb53-1"><a href="#cb53-1"></a>(s <span class="ot">-></span> <span class="dt">Identity</span> (a, s)) <span class="op">~</span> (s <span class="ot">-></span> (a, s)) <span class="op">~</span> (<span class="dt">State</span> s a)</span></code></pre></div>
<div class="sourceCode" id="cb54"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb54-1"><a href="#cb54-1"></a><span class="kw">type</span> <span class="dt">State</span> s a <span class="ot">=</span> <span class="dt">StateT</span> s <span class="dt">Identity</span> a</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb55"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb55-1"><a href="#cb55-1"></a><span class="kw">data</span> <span class="dt">ExceptT</span> e n a <span class="ot">=</span> <span class="dt">ExceptT</span> {<span class="ot"> runExceptT ::</span> n (<span class="dt">Either</span> e a) }</span>
<span id="cb55-2"><a href="#cb55-2"></a></span>
<span id="cb55-3"><a href="#cb55-3"></a><span class="kw">data</span> <span class="dt">StateT</span> s m a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> m (a, s) }</span>
<span id="cb55-4"><a href="#cb55-4"></a></span>
<span id="cb55-5"><a href="#cb55-5"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadError</span> e m <span class="kw">where</span></span>
<span id="cb55-6"><a href="#cb55-6"></a><span class="ot"> throwError ::</span> e <span class="ot">-></span> m a</span>
<span id="cb55-7"><a href="#cb55-7"></a><span class="ot"> catchError ::</span> m a <span class="ot">-></span> (e <span class="ot">-></span> m a) <span class="ot">-></span> m a</span>
<span id="cb55-8"><a href="#cb55-8"></a></span>
<span id="cb55-9"><a href="#cb55-9"></a><span class="kw">class</span> <span class="dt">Monad</span> m <span class="ot">=></span> <span class="dt">MonadState</span> s m <span class="kw">where</span></span>
<span id="cb55-10"><a href="#cb55-10"></a><span class="ot"> get ::</span> m s</span>
<span id="cb55-11"><a href="#cb55-11"></a><span class="ot"> put ::</span> s <span class="ot">-></span> m ()</span></code></pre></div>
<p>Пришли к тому, с чего начинали, только еще сложнее</p>
</section>
<section class="slide level1">
<ol type="1">
<li class="fragment"><del>Отделить эффекты от конкретной монады</del></li>
<li class="fragment">Получить возможность комбинировать разные эффекты</li>
</ol>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb56"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb56-1"><a href="#cb56-1"></a><span class="ot">foo ::</span> <span class="dt">ExceptT</span> <span class="dt">Bool</span> (<span class="dt">State</span> <span class="dt">Char</span>) <span class="dt">Int</span></span>
<span id="cb56-2"><a href="#cb56-2"></a>foo <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb56-3"><a href="#cb56-3"></a> c <span class="ot"><-</span> get <span class="co">-- :: State Char Char</span></span>
<span id="cb56-4"><a href="#cb56-4"></a> throwError <span class="dt">True</span> <span class="co">-- :: ExceptT Bool (State Char) Int</span></span></code></pre></div>
<div class="sourceCode" id="cb57"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb57-1"><a href="#cb57-1"></a><span class="ot">(>=>) ::</span> <span class="dt">Monad</span> m <span class="ot">=></span> (a <span class="ot">-></span> m b) <span class="ot">-></span> (b <span class="ot">-></span> m c) <span class="ot">-></span> (a <span class="ot">-></span> m c)</span>
<span id="cb57-2"><a href="#cb57-2"></a></span>
<span id="cb57-3"><a href="#cb57-3"></a><span class="co">-- Такого нет</span></span>
<span id="cb57-4"><a href="#cb57-4"></a>(<span class="op">>=></span>)</span>
<span id="cb57-5"><a href="#cb57-5"></a><span class="ot"> ::</span> (<span class="dt">Monad</span> m, <span class="dt">Monad</span> n)</span>
<span id="cb57-6"><a href="#cb57-6"></a> <span class="ot">=></span> (a <span class="ot">-></span> m b) <span class="ot">-></span> (b <span class="ot">-></span> n c) <span class="ot">-></span> (a <span class="ot">-></span> <span class="op">?</span> c)</span></code></pre></div>
<div class="sourceCode" id="cb58"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb58-1"><a href="#cb58-1"></a><span class="ot">lift ::</span> <span class="dt">State</span> <span class="dt">Char</span> a <span class="ot">-></span> <span class="dt">ExceptT</span> <span class="dt">Bool</span> (<span class="dt">State</span> <span class="dt">Char</span>) a</span></code></pre></div>
<div class="sourceCode" id="cb59"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb59-1"><a href="#cb59-1"></a><span class="ot">foo ::</span> <span class="dt">ExceptT</span> <span class="dt">Bool</span> (<span class="dt">State</span> <span class="dt">Char</span>) <span class="dt">Int</span></span>
<span id="cb59-2"><a href="#cb59-2"></a>foo <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb59-3"><a href="#cb59-3"></a> c <span class="ot"><-</span> lift get <span class="co">-- :: ExceptT Bool (State Char) Int</span></span>
<span id="cb59-4"><a href="#cb59-4"></a> throwError <span class="dt">True</span> <span class="co">-- :: ExceptT Bool (State Char) Int</span></span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb60"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb60-1"><a href="#cb60-1"></a><span class="kw">class</span> <span class="dt">MonadTrans</span> t <span class="kw">where</span></span>
<span id="cb60-2"><a href="#cb60-2"></a><span class="ot"> lift ::</span> (<span class="dt">Monad</span> m) <span class="ot">=></span> m a <span class="ot">-></span> t m a</span></code></pre></div>
<div class="sourceCode" id="cb61"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb61-1"><a href="#cb61-1"></a><span class="ot">lift ::</span> <span class="dt">State</span> <span class="dt">Char</span> a <span class="ot">-></span> <span class="dt">ExceptT</span> <span class="dt">Bool</span> (<span class="dt">State</span> <span class="dt">Char</span>) a</span>
<span id="cb61-2"><a href="#cb61-2"></a><span class="co">-- (m ) a -> (t ) (m ) a</span></span></code></pre></div>
<div class="sourceCode" id="cb62"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb62-1"><a href="#cb62-1"></a><span class="kw">instance</span> <span class="dt">MonadTrans</span> (<span class="dt">StateT</span> s) <span class="kw">where</span></span>
<span id="cb62-2"><a href="#cb62-2"></a> lift m <span class="ot">=</span> <span class="dt">StateT</span> <span class="op">$</span> \ s <span class="ot">-></span> <span class="kw">do</span></span>
<span id="cb62-3"><a href="#cb62-3"></a> a <span class="ot"><-</span> m</span>
<span id="cb62-4"><a href="#cb62-4"></a> <span class="fu">return</span> (a, s)</span></code></pre></div>
<div class="sourceCode" id="cb63"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb63-1"><a href="#cb63-1"></a><span class="kw">instance</span> <span class="dt">MonadTrans</span> (<span class="dt">ExceptT</span> e) <span class="kw">where</span></span>
<span id="cb63-2"><a href="#cb63-2"></a> lift m <span class="ot">=</span> <span class="dt">ExceptT</span> (<span class="fu">fmap</span> <span class="dt">Right</span> m)</span></code></pre></div>
<div class="sourceCode" id="cb64"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb64-1"><a href="#cb64-1"></a><span class="ot">foo ::</span> <span class="dt">ExceptT</span> <span class="dt">Bool</span> (<span class="dt">State</span> <span class="dt">Char</span>) <span class="dt">Int</span></span>
<span id="cb64-2"><a href="#cb64-2"></a>foo <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb64-3"><a href="#cb64-3"></a> c <span class="ot"><-</span> lift get</span>
<span id="cb64-4"><a href="#cb64-4"></a> throwError <span class="dt">True</span></span></code></pre></div>
</section>
<section class="slide level1">
<p>Но хочется чтобы без <code>lift</code> 🙂</p>
<div class="sourceCode" id="cb65"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb65-1"><a href="#cb65-1"></a><span class="ot">foo ::</span> <span class="dt">ExceptT</span> <span class="dt">Bool</span> (<span class="dt">State</span> <span class="dt">Char</span>) <span class="dt">Int</span></span>
<span id="cb65-2"><a href="#cb65-2"></a>foo <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb65-3"><a href="#cb65-3"></a> c <span class="ot"><-</span> get</span>
<span id="cb65-4"><a href="#cb65-4"></a> throwError <span class="dt">True</span></span></code></pre></div>
<p><span class="fragment">Нужен инстанс <code>MonadState</code> для <code>ExceptT</code></span></p>
<div class="sourceCode" id="cb66"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb66-1"><a href="#cb66-1"></a><span class="kw">instance</span> <span class="dt">MonadState</span> s n <span class="ot">=></span> <span class="dt">MonadState</span> s (<span class="dt">ExceptT</span> e n) <span class="kw">where</span></span>
<span id="cb66-2"><a href="#cb66-2"></a> get <span class="ot">=</span> lift get</span>
<span id="cb66-3"><a href="#cb66-3"></a> put s <span class="ot">=</span> lift (put s)</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb67"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb67-1"><a href="#cb67-1"></a>registerPayment</span>
<span id="cb67-2"><a href="#cb67-2"></a><span class="ot"> ::</span> <span class="dt">State</span> <span class="dt">TransactionId</span> m</span>
<span id="cb67-3"><a href="#cb67-3"></a> <span class="ot">=></span> <span class="dt">Price</span> <span class="ot">-></span> m <span class="dt">Transaction</span></span>
<span id="cb67-4"><a href="#cb67-4"></a>pay</span>
<span id="cb67-5"><a href="#cb67-5"></a><span class="ot"> ::</span> <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m</span>
<span id="cb67-6"><a href="#cb67-6"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m <span class="dt">Balance</span></span></code></pre></div>
<div class="sourceCode" id="cb68"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb68-1"><a href="#cb68-1"></a>payAndRegister</span>
<span id="cb68-2"><a href="#cb68-2"></a><span class="ot"> ::</span> (<span class="dt">State</span> <span class="dt">TransactionId</span> m, <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m)</span>
<span id="cb68-3"><a href="#cb68-3"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span>
<span id="cb68-4"><a href="#cb68-4"></a>payAndRegister balance price <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb68-5"><a href="#cb68-5"></a> newBalance <span class="ot"><-</span> pay balance price</span>
<span id="cb68-6"><a href="#cb68-6"></a> transacation <span class="ot"><-</span> registerPayment price</span>
<span id="cb68-7"><a href="#cb68-7"></a> <span class="fu">return</span> (newBalance, transaction)</span></code></pre></div>
</section>
<section class="slide level1">
<p>Как это “запускать”?</p>
<div class="sourceCode" id="cb69"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb69-1"><a href="#cb69-1"></a>payAndRegister</span>
<span id="cb69-2"><a href="#cb69-2"></a><span class="ot"> ::</span> (<span class="dt">State</span> <span class="dt">TransactionId</span> m, <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m)</span>
<span id="cb69-3"><a href="#cb69-3"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span></code></pre></div>
<div class="sourceCode" id="cb70"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb70-1"><a href="#cb70-1"></a><span class="ot">myBalance ::</span> <span class="dt">Balance</span></span>
<span id="cb70-2"><a href="#cb70-2"></a><span class="ot">myPrice ::</span> <span class="dt">Price</span></span>
<span id="cb70-3"><a href="#cb70-3"></a></span>
<span id="cb70-4"><a href="#cb70-4"></a>balanceAndTransaction</span>
<span id="cb70-5"><a href="#cb70-5"></a><span class="ot"> ::</span> <span class="dt">Either</span> <span class="dt">PaymentError</span> (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span></code></pre></div>
<div class="sourceCode" id="cb71"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb71-1"><a href="#cb71-1"></a><span class="kw">data</span> <span class="dt">ExceptT</span> e n a <span class="ot">=</span> <span class="dt">ExceptT</span> {<span class="ot"> runExceptT ::</span> n (<span class="dt">Either</span> e a) }</span>
<span id="cb71-2"><a href="#cb71-2"></a><span class="kw">data</span> <span class="dt">StateT</span> s n a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> n (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb72"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb72-1"><a href="#cb72-1"></a>balanceAndTransaction <span class="ot">=</span></span>
<span id="cb72-2"><a href="#cb72-2"></a> runStateT <span class="dv">0</span> (payAndRegister myBalance myPrice)</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb73"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb73-1"><a href="#cb73-1"></a>payAndRegister</span>
<span id="cb73-2"><a href="#cb73-2"></a><span class="ot"> ::</span> (<span class="dt">State</span> <span class="dt">TransactionId</span> m, <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m)</span>
<span id="cb73-3"><a href="#cb73-3"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span>
<span id="cb73-4"><a href="#cb73-4"></a></span>
<span id="cb73-5"><a href="#cb73-5"></a><span class="kw">data</span> <span class="dt">StateT</span> s n a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> n (a, s) }</span></code></pre></div>
<div class="sourceCode" id="cb74"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb74-1"><a href="#cb74-1"></a>balanceAndTransaction</span>
<span id="cb74-2"><a href="#cb74-2"></a><span class="ot"> ::</span> <span class="dt">Either</span> <span class="dt">PaymentError</span> (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span>
<span id="cb74-3"><a href="#cb74-3"></a>balanceAndTransaction <span class="ot">=</span></span>
<span id="cb74-4"><a href="#cb74-4"></a> <span class="fu">flip</span> runStateT <span class="dv">0</span> (payAndRegister myBalance myPrice)</span></code></pre></div>
<div class="sourceCode" id="cb75"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb75-1"><a href="#cb75-1"></a> <span class="fu">flip</span> runStateT <span class="dv">0</span> (payAndRegister myBalance myPrice)</span>
<span id="cb75-2"><a href="#cb75-2"></a><span class="co">-- ^-------------^</span></span>
<span id="cb75-3"><a href="#cb75-3"></a><span class="co">-- StateT TransactionId n a -> n a</span></span></code></pre></div>
<div class="sourceCode" id="cb76"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb76-1"><a href="#cb76-1"></a><span class="co">-- (n a) ~ (Either PaymentError (Balance, Transaction))</span></span></code></pre></div>
<div class="sourceCode" id="cb77"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb77-1"><a href="#cb77-1"></a><span class="fu">flip</span> runStateT <span class="dv">0</span> (payAndRegister myBalance myPrice)</span>
<span id="cb77-2"><a href="#cb77-2"></a><span class="co">-- ^--------------------------------^</span></span>
<span id="cb77-3"><a href="#cb77-3"></a><span class="co">-- StateT TransactionId</span></span>
<span id="cb77-4"><a href="#cb77-4"></a><span class="co">-- (Either PaymentError)</span></span>
<span id="cb77-5"><a href="#cb77-5"></a><span class="co">-- (Balance, Transaction)</span></span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb78"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb78-1"><a href="#cb78-1"></a>payAndRegister</span>
<span id="cb78-2"><a href="#cb78-2"></a><span class="ot"> ::</span> (<span class="dt">State</span> <span class="dt">TransactionId</span> m, <span class="dt">MonadError</span> <span class="dt">PaymentError</span> m)</span>
<span id="cb78-3"><a href="#cb78-3"></a> <span class="ot">=></span> <span class="dt">Balance</span> <span class="ot">-></span> <span class="dt">Price</span> <span class="ot">-></span> m (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span>
<span id="cb78-4"><a href="#cb78-4"></a></span>
<span id="cb78-5"><a href="#cb78-5"></a><span class="kw">data</span> <span class="dt">ExceptT</span> e n a <span class="ot">=</span> <span class="dt">ExceptT</span> {<span class="ot"> runExceptT ::</span> n (<span class="dt">Either</span> e a) }</span>
<span id="cb78-6"><a href="#cb78-6"></a><span class="kw">data</span> <span class="dt">StateT</span> s n a <span class="ot">=</span> <span class="dt">StateT</span> {<span class="ot"> runStateT ::</span> s <span class="ot">-></span> n (a, s) }</span>
<span id="cb78-7"><a href="#cb78-7"></a><span class="kw">data</span> <span class="dt">Identity</span> a <span class="ot">=</span> <span class="dt">Identity</span> {<span class="ot"> runIdentity ::</span> a }</span></code></pre></div>
<div class="sourceCode" id="cb79"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb79-1"><a href="#cb79-1"></a>balanceAndTransaction</span>
<span id="cb79-2"><a href="#cb79-2"></a><span class="ot"> ::</span> <span class="dt">Either</span> <span class="dt">PaymentError</span> (<span class="dt">Balance</span>, <span class="dt">Transaction</span>)</span>
<span id="cb79-3"><a href="#cb79-3"></a>balanceAndTransaction <span class="ot">=</span></span>
<span id="cb79-4"><a href="#cb79-4"></a> (runIdentity <span class="op">.</span> runExceptT <span class="op">.</span> <span class="fu">flip</span> runStateT <span class="dv">0</span>)</span>
<span id="cb79-5"><a href="#cb79-5"></a> (payAndRegister myBalance myPrice)</span></code></pre></div>
</section>
<section class="slide level1">
<p>А теперь мы пойдем в совершенно другом направлении!</p>
</section>
<section class="slide level1">
<h2 id="напоминалочка-1">Напоминалочка</h2>
<div class="sourceCode" id="cb80"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb80-1"><a href="#cb80-1"></a><span class="kw">class</span> <span class="dt">Show</span> a <span class="kw">where</span></span>
<span id="cb80-2"><a href="#cb80-2"></a><span class="ot"> show ::</span> a <span class="ot">-></span> <span class="dt">String</span></span></code></pre></div>
<div class="sourceCode" id="cb81"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb81-1"><a href="#cb81-1"></a><span class="kw">data</span> <span class="dt">Foo</span> <span class="ot">=</span> <span class="dt">Bar</span> {<span class="ot"> barInt ::</span> <span class="dt">Int</span> }</span>
<span id="cb81-2"><a href="#cb81-2"></a> <span class="kw">deriving</span> <span class="dt">Show</span></span></code></pre></div>
<div class="sourceCode" id="cb82"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb82-1"><a href="#cb82-1"></a><span class="fu">show</span> (<span class="dt">Bar</span> <span class="dv">8</span>)</span>
<span id="cb82-2"><a href="#cb82-2"></a><span class="co">-- Bar {barInt = 8}</span></span></code></pre></div>
</section>
<section class="slide level1">
<h2 id="read">Read</h2>
<p>Обратная операция к <code>show</code></p>
<div class="sourceCode" id="cb83"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb83-1"><a href="#cb83-1"></a><span class="fu">read</span><span class="ot"> ::</span> <span class="dt">Read</span> a <span class="ot">=></span> <span class="dt">String</span> <span class="ot">-></span> a</span></code></pre></div>
<div class="sourceCode" id="cb84"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb84-1"><a href="#cb84-1"></a><span class="kw">class</span> <span class="dt">Read</span> a <span class="kw">where</span></span>
<span id="cb84-2"><a href="#cb84-2"></a> <span class="fu">readsPrec</span></span>
<span id="cb84-3"><a href="#cb84-3"></a><span class="ot"> ::</span> <span class="co">-- | Приоритет контекста выражения</span></span>
<span id="cb84-4"><a href="#cb84-4"></a> <span class="dt">Int</span></span>
<span id="cb84-5"><a href="#cb84-5"></a> <span class="ot">-></span> <span class="dt">String</span></span>
<span id="cb84-6"><a href="#cb84-6"></a> <span class="ot">-></span> [(a, <span class="dt">String</span>)]</span></code></pre></div>
<div class="sourceCode" id="cb85"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb85-1"><a href="#cb85-1"></a><span class="kw">data</span> <span class="dt">Foo</span> <span class="ot">=</span> <span class="dt">Bar</span> {<span class="ot"> barInt ::</span> <span class="dt">Int</span> }</span>
<span id="cb85-2"><a href="#cb85-2"></a> <span class="kw">deriving</span> (<span class="dt">Show</span>, <span class="dt">Read</span>)</span></code></pre></div>
<div class="sourceCode" id="cb86"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb86-1"><a href="#cb86-1"></a><span class="fu">read</span> <span class="st">"Bar {barInt = 8}"</span></span>
<span id="cb86-2"><a href="#cb86-2"></a><span class="co">-- Bar { barInt = 8 }</span></span></code></pre></div>
</section>
<section id="монады" class="slide level1">
<h1>Монады</h1>
<ol type="1">
<li class="fragment">Базовые операции монады (эффекты)</li>
</ol>
<p><span class="fragment"><code>MonadState</code>, <code>MonadErrror</code></span></p>
<ol start="2" type="1">
<li class="fragment">Конкретные монады (переносчик) (и способы их “разворачивать”)</li>
</ol>
<p><span class="fragment"><code>StateT</code> (<code>runStateT</code>), <code>Either</code>, <code>ExceptT</code> (<code>runExceptT</code>)</span></p>
<p><span class="fragment">Переводят эффекты в pure код</span></p>
</section>
<section id="монада-io" class="slide level1">
<h1>Монада <code>IO</code></h1>
<p><span class="fragment">Обладает только базовыми операциями</span> <span class="fragment">(Невозможно перевести в pure код)</span></p>
<div class="sourceCode" id="cb87"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb87-1"><a href="#cb87-1"></a><span class="fu">getLine</span><span class="ot"> ::</span> <span class="dt">IO</span> <span class="dt">String</span></span>
<span id="cb87-2"><a href="#cb87-2"></a></span>
<span id="cb87-3"><a href="#cb87-3"></a><span class="fu">putStrLn</span><span class="ot"> ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">IO</span> ()</span></code></pre></div>
<div class="sourceCode" id="cb88"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb88-1"><a href="#cb88-1"></a><span class="kw">type</span> <span class="dt">FilePath</span> <span class="ot">=</span> <span class="dt">String</span></span>
<span id="cb88-2"><a href="#cb88-2"></a></span>
<span id="cb88-3"><a href="#cb88-3"></a><span class="fu">readFile</span><span class="ot"> ::</span> <span class="dt">FilePath</span> <span class="ot">-></span> <span class="dt">IO</span> <span class="dt">String</span></span>
<span id="cb88-4"><a href="#cb88-4"></a></span>
<span id="cb88-5"><a href="#cb88-5"></a><span class="fu">writeFile</span><span class="ot"> ::</span> <span class="dt">FilePath</span> <span class="ot">-></span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">IO</span> ()</span></code></pre></div>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb89"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb89-1"><a href="#cb89-1"></a><span class="ot">add10FromConsole ::</span> <span class="dt">IO</span> ()</span>
<span id="cb89-2"><a href="#cb89-2"></a>add10FromConsole <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb89-3"><a href="#cb89-3"></a> x <span class="ot"><-</span> <span class="fu">getLine</span></span>
<span id="cb89-4"><a href="#cb89-4"></a> <span class="kw">let</span></span>
<span id="cb89-5"><a href="#cb89-5"></a><span class="ot"> n ::</span> <span class="dt">Int</span></span>
<span id="cb89-6"><a href="#cb89-6"></a> n <span class="ot">=</span> <span class="fu">read</span> x</span>
<span id="cb89-7"><a href="#cb89-7"></a> <span class="fu">putStrLn</span> (<span class="fu">show</span> (n <span class="op">+</span> <span class="dv">10</span>))</span></code></pre></div>
</section>
<section class="slide level1">
<p>“Разворачивать” <code>IO</code> умеет только рантайм.</p>
<div class="sourceCode" id="cb90"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb90-1"><a href="#cb90-1"></a><span class="ot">main ::</span> <span class="dt">IO</span> ()</span>
<span id="cb90-2"><a href="#cb90-2"></a>main <span class="ot">=</span> add10FromConsole</span></code></pre></div>
<pre class="fragment"><code>> 10
20</code></pre>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb92"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb92-1"><a href="#cb92-1"></a><span class="co">-- Как 'read', но не взрывается в рантайме</span></span>
<span id="cb92-2"><a href="#cb92-2"></a><span class="ot">readMaybe ::</span> <span class="dt">Read</span> a <span class="ot">=></span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Maybe</span> a</span></code></pre></div>
<div class="sourceCode" id="cb93"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb93-1"><a href="#cb93-1"></a><span class="ot">accumulateNums ::</span> <span class="dt">StateT</span> <span class="dt">Int</span> <span class="dt">IO</span> ()</span>
<span id="cb93-2"><a href="#cb93-2"></a>accumulateNums <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb93-3"><a href="#cb93-3"></a> x <span class="ot"><-</span> lift <span class="fu">getLine</span></span>
<span id="cb93-4"><a href="#cb93-4"></a> <span class="kw">case</span> (readMaybe<span class="ot"> x ::</span> <span class="dt">Int</span>) <span class="kw">of</span></span>
<span id="cb93-5"><a href="#cb93-5"></a> <span class="dt">Nothing</span> <span class="ot">-></span> <span class="kw">do</span></span>
<span id="cb93-6"><a href="#cb93-6"></a> s <span class="ot"><-</span> get</span>
<span id="cb93-7"><a href="#cb93-7"></a> lift (<span class="fu">putStrLn</span> (<span class="fu">show</span> s))</span>
<span id="cb93-8"><a href="#cb93-8"></a> <span class="dt">Just</span> x' <span class="ot">-></span> <span class="kw">do</span></span>
<span id="cb93-9"><a href="#cb93-9"></a> modify (<span class="op">+</span> x')</span>
<span id="cb93-10"><a href="#cb93-10"></a> accumulateNums</span></code></pre></div>
<div class="sourceCode" id="cb94"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb94-1"><a href="#cb94-1"></a><span class="ot">main ::</span> <span class="dt">IO</span> ()</span>
<span id="cb94-2"><a href="#cb94-2"></a>main <span class="ot">=</span> <span class="fu">flip</span> runStateT <span class="dv">0</span> accumulateNums</span></code></pre></div>
<pre class="fragment"><code>> 8
> 3
> a
11</code></pre>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb96"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb96-1"><a href="#cb96-1"></a><span class="ot">accumulateNums ::</span> <span class="dt">StateT</span> <span class="dt">Int</span> <span class="dt">IO</span> ()</span></code></pre></div>
<div class="sourceCode" id="cb97"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb97-1"><a href="#cb97-1"></a><span class="co">-- Хочется так, но где тут взять 'IO'?</span></span>
<span id="cb97-2"><a href="#cb97-2"></a><span class="ot">accumulateNums ::</span> <span class="dt">MonadState</span> <span class="dt">Int</span> m <span class="ot">=></span> m ()</span></code></pre></div>
<p><span class="fragment">Трансформера <code>IO</code> нет.</span></p>
<p><span class="fragment">Тогда <code>lift</code>!</span></p>
<p><span class="fragment">Но <code>lift</code> поднимает строго на один уровень.</span></p>
</section>
<section class="slide level1">
<div class="sourceCode" id="cb98"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb98-1"><a href="#cb98-1"></a><span class="kw">class</span> <span class="dt">MonadIO</span> m <span class="kw">where</span></span>
<span id="cb98-2"><a href="#cb98-2"></a><span class="ot"> liftIO ::</span> <span class="dt">IO</span> a <span class="ot">-></span> m a</span></code></pre></div>
<div class="sourceCode" id="cb99"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb99-1"><a href="#cb99-1"></a><span class="ot">accumulateNums ::</span> (<span class="dt">MonadState</span> <span class="dt">Int</span> m, <span class="dt">MonadIO</span> m) <span class="ot">=></span> m ()</span>
<span id="cb99-2"><a href="#cb99-2"></a>accumulateNums <span class="ot">=</span> <span class="kw">do</span></span>
<span id="cb99-3"><a href="#cb99-3"></a> x <span class="ot"><-</span> liftIO <span class="fu">getLine</span></span>
<span id="cb99-4"><a href="#cb99-4"></a> <span class="kw">case</span> (readMaybe<span class="ot"> x ::</span> <span class="dt">Int</span>) <span class="kw">of</span></span>
<span id="cb99-5"><a href="#cb99-5"></a> <span class="dt">Nothing</span> <span class="ot">-></span> <span class="kw">do</span></span>
<span id="cb99-6"><a href="#cb99-6"></a> s <span class="ot"><-</span> get</span>
<span id="cb99-7"><a href="#cb99-7"></a> liftIO (<span class="fu">putStrLn</span> (<span class="fu">show</span> s))</span>
<span id="cb99-8"><a href="#cb99-8"></a> <span class="dt">Just</span> x' <span class="ot">-></span> <span class="kw">do</span></span>
<span id="cb99-9"><a href="#cb99-9"></a> modify (<span class="op">+</span> x')</span>
<span id="cb99-10"><a href="#cb99-10"></a> accumulateNums</span></code></pre></div>
<div class="sourceCode" id="cb100"><pre class="sourceCode haskell fragment"><code class="sourceCode haskell"><span id="cb100-1"><a href="#cb100-1"></a><span class="ot">main ::</span> <span class="dt">IO</span> ()</span>
<span id="cb100-2"><a href="#cb100-2"></a>main <span class="ot">=</span> <span class="fu">flip</span> runStateT <span class="dv">0</span> accumulateNums</span></code></pre></div>
<pre class="fragment"><code>> 8
> 3
> a
11</code></pre>
</section>
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