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<title>Automatic type-class derivation with Shapeless - Das Keyboard Shredder</title>
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<meta name="description" content="We had a knowledge sharing session at work recently on Shapeless for automatic type class derivation. Here is a little write-up for the topic. Scala List First let’s review how List works in Scala. A List is a linked list with head and tail, plus Nil for empty list. It can be represented with the following abstract data type: sealed trait List[+A] { def ::[B >: A](head: B): List[B] = Cons(head, this) } case object Nil extends List[Nothing] // Nothing is a sub-type of every other type case class Cons[+A](head: A, tail: List[A]) extends List[A] Notice that ::, the list concatenation operation, is just a method on trait List[+A]. Since Scala operators that end with : are right-associative, we can conveniently create lists by chaining multiple ::s. Therefore the following expressions are equivalent: 1 :: 2 :: Nil 1 :: (2 :: Nil) Nil.::(2).::(1) Cons(1, Cons(2, Nil)) It’s important to point out here that Scala List is homogeneous, i.e. it has a single type parameter A and thus can only store elements of A and its sub-types. On the other hand, it can have varying numbers of elements at runtime. Shapeless HList Since List …" />
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<meta property="og:description" content="We had a knowledge sharing session at work recently on Shapeless for automatic type class derivation. Here is a little write-up for the topic. Scala List First let’s review how List works in Scala. A List is a linked list with head and tail, plus Nil for empty list. It can be represented with the following abstract data type: sealed trait List[+A] { def ::[B >: A](head: B): List[B] = Cons(head, this) } case object Nil extends List[Nothing] // Nothing is a sub-type of every other type case class Cons[+A](head: A, tail: List[A]) extends List[A] Notice that ::, the list concatenation operation, is just a method on trait List[+A]. Since Scala operators that end with : are right-associative, we can conveniently create lists by chaining multiple ::s. Therefore the following expressions are equivalent: 1 :: 2 :: Nil 1 :: (2 :: Nil) Nil.::(2).::(1) Cons(1, Cons(2, Nil)) It’s important to point out here that Scala List is homogeneous, i.e. it has a single type parameter A and thus can only store elements of A and its sub-types. On the other hand, it can have varying numbers of elements at runtime. Shapeless HList Since List …"/>
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<p>We had a knowledge sharing session at work recently on <a href="https://github.com/milessabin/shapeless/">Shapeless</a> for automatic type class derivation. Here is a little write-up for the topic.</p>
<h2>Scala List</h2>
<p>First let’s review how <code>List</code> works in Scala. A <code>List</code> is a linked list with <code>head</code> and <code>tail</code>, plus <code>Nil</code> for empty list. It can be represented with the following abstract data type:</p>
<div class="highlight"><pre><span></span><span class="k">sealed</span> <span class="k">trait</span> <span class="nc">List</span><span class="o">[</span><span class="kt">+A</span><span class="o">]</span> <span class="o">{</span>
<span class="k">def</span> <span class="o">::[</span><span class="kt">B</span> <span class="k">>:</span> <span class="kt">A</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">B</span><span class="o">)</span><span class="k">:</span> <span class="kt">List</span><span class="o">[</span><span class="kt">B</span><span class="o">]</span> <span class="k">=</span> <span class="nc">Cons</span><span class="o">(</span><span class="n">head</span><span class="o">,</span> <span class="k">this</span><span class="o">)</span>
<span class="o">}</span>
<span class="k">case</span> <span class="k">object</span> <span class="nc">Nil</span> <span class="k">extends</span> <span class="nc">List</span><span class="o">[</span><span class="kt">Nothing</span><span class="o">]</span> <span class="c1">// Nothing is a sub-type of every other type</span>
<span class="k">case</span> <span class="k">class</span> <span class="nc">Cons</span><span class="o">[</span><span class="kt">+A</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">A</span><span class="o">,</span> <span class="n">tail</span><span class="k">:</span> <span class="kt">List</span><span class="o">[</span><span class="kt">A</span><span class="o">])</span> <span class="k">extends</span> <span class="nc">List</span><span class="o">[</span><span class="kt">A</span><span class="o">]</span>
</pre></div>
<p>Notice that <code>::</code>, the list concatenation operation, is just a method on trait <code>List[+A]</code>. Since Scala operators that end with <code>:</code> are right-associative, we can conveniently create lists by chaining multiple <code>::</code>s. Therefore the following expressions are equivalent:</p>
<div class="highlight"><pre><span></span><span class="mi">1</span> <span class="o">::</span> <span class="mi">2</span> <span class="o">::</span> <span class="nc">Nil</span>
<span class="mi">1</span> <span class="o">::</span> <span class="o">(</span><span class="mi">2</span> <span class="o">::</span> <span class="nc">Nil</span><span class="o">)</span>
<span class="nc">Nil</span><span class="o">.::(</span><span class="mi">2</span><span class="o">).::(</span><span class="mi">1</span><span class="o">)</span>
<span class="nc">Cons</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="nc">Cons</span><span class="o">(</span><span class="mi">2</span><span class="o">,</span> <span class="nc">Nil</span><span class="o">))</span>
</pre></div>
<p>It’s important to point out here that Scala <code>List</code> is homogeneous, i.e. it has a single type parameter <code>A</code> and thus can only store elements of <code>A</code> and its sub-types. On the other hand, it can have varying numbers of elements at runtime.</p>
<h2>Shapeless HList</h2>
<p>Since <code>List</code> is homogeneous, the most common way to represent lists of different types in Scala is tuples. However, since tuples of different arities are different types (<code>Tuple2[A, B]</code>, <code>Tuple3[A, B, C]</code>, …) and limited to 22 elements, it’s hard to write generic code that operates on tuples of varying arities.</p>
<p>Heterogenous list, or <code>HList</code>, is the core type in Shapeless and can represent lists of varying lengths with different element types.</p>
<div class="highlight"><pre><span></span><span class="k">sealed</span> <span class="k">trait</span> <span class="nc">HList</span>
<span class="c1">// HNil is both a type (trait) and an object</span>
<span class="k">sealed</span> <span class="k">trait</span> <span class="nc">HNil</span> <span class="k">extends</span> <span class="nc">HList</span> <span class="o">{</span>
<span class="k">def</span> <span class="o">::[</span><span class="kt">H</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">H</span><span class="o">)</span><span class="k">:</span> <span class="kt">H</span> <span class="kt">::</span> <span class="kt">HNil</span> <span class="o">=</span> <span class="k">new</span> <span class="o">::(</span><span class="n">head</span><span class="o">,</span> <span class="k">this</span><span class="o">)</span>
<span class="o">}</span>
<span class="k">case</span> <span class="k">object</span> <span class="nc">HNil</span> <span class="k">extends</span> <span class="nc">HNil</span>
<span class="k">case</span> <span class="k">class</span> <span class="nc">::</span><span class="o">[</span><span class="kt">+H</span>, <span class="kt">+T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">H</span><span class="o">,</span> <span class="n">tail</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span> <span class="k">extends</span> <span class="nc">HList</span>
<span class="k">implicit</span> <span class="k">class</span> <span class="nc">HListOps</span><span class="o">[</span><span class="kt">T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="k">val</span> <span class="n">self</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span> <span class="k">extends</span> <span class="nc">AnyVal</span> <span class="o">{</span>
<span class="k">def</span> <span class="o">::[</span><span class="kt">H</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">H</span><span class="o">)</span><span class="k">:</span> <span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span> <span class="o">=</span> <span class="k">new</span> <span class="o">::(</span><span class="n">head</span><span class="o">,</span> <span class="n">self</span><span class="o">)</span>
<span class="o">}</span>
</pre></div>
<p>And the following expressions are equivalent:</p>
<div class="highlight"><pre><span></span><span class="mi">1</span> <span class="o">::</span> <span class="mf">3.14</span> <span class="o">::</span> <span class="s">"foo"</span> <span class="o">::</span> <span class="kc">true</span> <span class="o">::</span> <span class="nc">HNil</span>
<span class="mi">1</span> <span class="o">::</span> <span class="o">(</span><span class="mf">3.14</span> <span class="o">::</span> <span class="o">(</span><span class="s">"foo"</span> <span class="o">::</span> <span class="o">(</span><span class="kc">true</span> <span class="o">::</span> <span class="nc">HNil</span><span class="o">)))</span>
<span class="nc">HNil</span><span class="o">.::(</span><span class="kc">true</span><span class="o">).::(</span><span class="s">"foo"</span><span class="o">).::(</span><span class="mf">3.14</span><span class="o">).::(</span><span class="mi">1</span><span class="o">)</span>
<span class="o">::(</span><span class="mi">1</span><span class="o">,</span> <span class="o">::(</span><span class="mf">3.14</span><span class="o">,</span> <span class="o">::(</span><span class="s">"foo"</span><span class="o">,</span> <span class="o">::(</span><span class="kc">true</span><span class="o">,</span> <span class="nc">HNil</span><span class="o">))))</span>
</pre></div>
<p>Note that there are two <code>::(head: H)</code> implementations with different return types <code>H :: HNil</code> and <code>H :: T</code>. This way types of all elements are retained and not erased to <code>HList</code> if <code>::</code> was only defined in <code>trait HList</code>.</p>
<p>Also note that in Scala, generic types with 2 type paramemters can be used in an infix position, i.e. <code>::[H, T]</code> == <code>H :: T</code>, and those that end with <code>:</code> are right-associative, we can conveniently create unique <code>HList</code> types in a syntax similar to <code>List</code> creation.</p>
<div class="highlight"><pre><span></span><span class="c1">// ::[Int, ::[Double, HNil]]</span>
<span class="k">type</span> <span class="kt">L1</span> <span class="o">=</span> <span class="nc">Int</span> <span class="o">::</span> <span class="nc">Double</span> <span class="o">::</span> <span class="nc">HNil</span>
<span class="c1">// ::(1, ::(3.14, HNil))</span>
<span class="k">val</span> <span class="n">l1</span><span class="k">:</span> <span class="kt">L1</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">::</span> <span class="mf">3.14</span> <span class="o">::</span> <span class="nc">HNil</span>
<span class="c1">// ::[Int, ::[Double, ::[String, HNil]]]</span>
<span class="k">type</span> <span class="kt">L2</span> <span class="o">=</span> <span class="nc">Int</span> <span class="o">::</span> <span class="nc">Double</span> <span class="o">::</span> <span class="nc">String</span> <span class="o">::</span> <span class="nc">HNil</span>
<span class="c1">// ::(1, ::(3.14, ::("foo", HNil)))</span>
<span class="k">val</span> <span class="n">l2</span><span class="k">:</span> <span class="kt">L2</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">::</span> <span class="mf">3.14</span> <span class="o">::</span> <span class="s">"foo"</span> <span class="o">::</span> <span class="nc">HNil</span>
<span class="c1">// ::[Int, ::[Double, ::[String, ::[Boolean, HNil]]]]</span>
<span class="k">type</span> <span class="kt">L3</span> <span class="o">=</span> <span class="nc">Int</span> <span class="o">::</span> <span class="nc">Double</span> <span class="o">::</span> <span class="nc">String</span> <span class="o">::</span> <span class="nc">Boolean</span> <span class="o">::</span> <span class="nc">HNil</span>
<span class="c1">// ::(1, ::(3.14, ::("foo", ::(true, HNil))))</span>
<span class="k">val</span> <span class="n">l3</span><span class="k">:</span> <span class="kt">L3</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">::</span> <span class="mf">3.14</span> <span class="o">::</span> <span class="s">"foo"</span> <span class="o">::</span> <span class="kc">true</span> <span class="o">::</span> <span class="nc">HNil</span>
</pre></div>
<p>We can see that each <code>HList</code> instance, i.e. <code>L1</code>, <code>L2</code>, <code>L3</code>, is a unique type with fixed but varying number of element types determined at compile time. But since all of them are instances of <code>HList</code> and recursively <code>H :: T</code>, we can operate them in a generic way with implicits.</p>
<h2>Implicit type-class derivation</h2>
<p>Now let’s look at how we can operate <code>HList</code> by recursively process <code>head</code> and <code>tail</code>. Say we have a <code>Flip[T]</code> type class that flips the value of type <code>T</code>, and implicit instances for <code>Int</code>, <code>Double</code>, <code>Boolean</code> and <code>String</code>.</p>
<div class="highlight"><pre><span></span><span class="k">trait</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">T</span>
<span class="o">}</span>
<span class="k">object</span> <span class="nc">Flip</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">apply</span><span class="o">[</span><span class="kt">T</span><span class="o">](</span><span class="n">f</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=></span> <span class="n">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="n">f</span><span class="o">(</span><span class="n">x</span><span class="o">)</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">intFlip</span> <span class="k">=</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">Int</span><span class="o">](-</span><span class="k">_</span><span class="o">)</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">doubleFlip</span> <span class="k">=</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">Double</span><span class="o">](-</span><span class="k">_</span><span class="o">)</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">booleanFlip</span> <span class="k">=</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">Boolean</span><span class="o">](!</span><span class="k">_</span><span class="o">)</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">stringFlip</span> <span class="k">=</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">String</span><span class="o">](</span><span class="k">_</span><span class="o">.</span><span class="n">reverse</span><span class="o">)</span>
</pre></div>
<p>And we want to apply <code>Flip[T]</code> to an <code>Hlist</code> of arbitrary length and types. For an <code>HList</code> of <code>A :: B :: C :: D :: HNil</code>, we want to recursively summon implicit instances of <code>Flip[T]</code> for the heads, i.e. <code>A</code>, <code>B</code>, <code>C</code>, <code>D</code>, and the tails, i.e. <code>B :: C :: D :: HNil</code>, <code>C :: D :: HNil</code>, <code>D :: HNil</code>, <code>HNil</code>. As any recursive approach it’s easy to start with the terminal case <code>HNil</code>.</p>
<div class="highlight"><pre><span></span><span class="k">import</span> <span class="nn">shapeless._</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">hnilFlip</span> <span class="k">=</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">HNil</span><span class="o">](</span><span class="k">_</span> <span class="k">=></span> <span class="nc">HNil</span><span class="o">)</span>
<span class="k">implicit</span> <span class="k">def</span> <span class="n">hconsFlip</span><span class="o">[</span><span class="kt">H</span>, <span class="kt">T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">]</span>
<span class="o">(</span><span class="k">implicit</span> <span class="n">hf</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">H</span><span class="o">],</span> <span class="n">tf</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span> <span class="c1">// summon implicit instances, tf is computed recursively</span>
<span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span> <span class="o">=</span> <span class="n">hf</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">head</span><span class="o">)</span> <span class="o">::</span> <span class="n">tf</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">tail</span><span class="o">)</span>
<span class="o">}</span>
</pre></div>
<p>Now we can summon an implicit <code>Flip[T]</code> with any <code>HList</code> instances.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">flip</span><span class="o">[</span><span class="kt">T</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)(</span><span class="k">implicit</span> <span class="n">f</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span> <span class="k">=</span> <span class="n">f</span><span class="o">(</span><span class="n">x</span><span class="o">)</span>
<span class="n">flip</span><span class="o">(</span><span class="mi">1</span> <span class="o">::</span> <span class="mf">3.14</span> <span class="o">::</span> <span class="s">"foo"</span> <span class="o">::</span> <span class="kc">true</span> <span class="o">::</span> <span class="nc">HNil</span><span class="o">)</span>
<span class="c1">// Int :: Double :: String :: Boolean :: HNil = -1 :: -3.14 :: "oof" :: false :: HNil</span>
</pre></div>
<p>The above code is expanded at compile time to:</p>
<div class="highlight"><pre><span></span><span class="k">val</span> <span class="n">f</span> <span class="k">=</span> <span class="n">hconsFlip</span><span class="o">(</span><span class="n">intFlip</span><span class="o">,</span>
<span class="n">hconsFlip</span><span class="o">(</span><span class="n">doubleFlip</span><span class="o">,</span>
<span class="n">hconsFlip</span><span class="o">(</span><span class="n">stringFlip</span><span class="o">,</span>
<span class="n">hconsFlip</span><span class="o">(</span><span class="n">booleanFlip</span><span class="o">,</span> <span class="n">hnilFlip</span><span class="o">))))</span>
<span class="n">f</span><span class="o">.</span><span class="n">apply</span><span class="o">(</span><span class="mi">1</span> <span class="o">::</span> <span class="mf">3.14</span> <span class="o">::</span> <span class="s">"foo"</span> <span class="o">::</span> <span class="kc">true</span> <span class="o">::</span> <span class="nc">HNil</span><span class="o">)</span>
</pre></div>
<h2>Generic and LabelledGeneric</h2>
<p>Now that we know how to operate <code>HList</code>s in a generic way, Shapeless also offers <code>Generic</code> and <code>LabelledGeneric</code> for operating tuples and case classes in the same manner. <code>Generic</code> is a type-class for conversion between Scala types and <code>HList</code>s.</p>
<div class="highlight"><pre><span></span><span class="c1">// (Int, Double, String, Boolean) <=> Int :: Double :: String :: Boolean :: HNil</span>
<span class="k">val</span> <span class="n">gen</span> <span class="k">=</span> <span class="nc">Generic</span><span class="o">[(</span><span class="kt">Int</span>, <span class="kt">Double</span>, <span class="kt">String</span>, <span class="kt">Boolean</span><span class="o">)]</span>
<span class="k">val</span> <span class="n">l</span> <span class="k">=</span> <span class="n">gen</span><span class="o">.</span><span class="n">to</span><span class="o">((</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span> <span class="c1">// 1 :: 3.14 :: "foo" :: true :: HNil</span>
<span class="k">val</span> <span class="n">t</span> <span class="k">=</span> <span class="n">gen</span><span class="o">.</span><span class="n">from</span><span class="o">(</span><span class="n">l</span><span class="o">)</span> <span class="c1">// (1, 3.14, "foo", true)</span>
</pre></div>
<p>We can now extend <code>flip</code> to any tuples.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">flip</span><span class="o">[</span><span class="kt">T</span>, <span class="kt">L</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span>
<span class="o">(</span><span class="k">implicit</span> <span class="n">gen</span><span class="k">:</span> <span class="kt">Generic.Aux</span><span class="o">[</span><span class="kt">T</span>, <span class="kt">L</span><span class="o">],</span> <span class="n">f</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">L</span><span class="o">])</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span>
<span class="n">gen</span><span class="o">.</span><span class="n">from</span><span class="o">(</span><span class="n">f</span><span class="o">(</span><span class="n">gen</span><span class="o">.</span><span class="n">to</span><span class="o">(</span><span class="n">x</span><span class="o">)))</span>
<span class="c1">// T = (Int, Double, String, Boolean)</span>
<span class="c1">// L = Int :: Double :: String :: Boolean :: HNil</span>
<span class="c1">// gen = Generic[(Int, Double, String, Boolean)]</span>
<span class="c1">// f = implicitly[Flip[Int :: Double :: String :: Boolean :: HNil]]</span>
<span class="n">flip</span><span class="o">((</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span>
<span class="c1">// (Int, Double, String, Boolean) = (-1, -3.14, "oof", false)</span>
</pre></div>
<p><code>Generic</code> also works with case classes, so our <code>flip</code> method works automatically supports them as well.</p>
<div class="highlight"><pre><span></span><span class="k">case</span> <span class="k">class</span> <span class="nc">Record</span><span class="o">(</span><span class="n">i</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">d</span><span class="k">:</span> <span class="kt">Double</span><span class="o">,</span> <span class="n">s</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">b</span><span class="k">:</span> <span class="kt">Boolean</span><span class="o">)</span>
<span class="c1">// Record <=> Int :: Double :: String :: Boolean :: HNil</span>
<span class="k">val</span> <span class="n">gen</span> <span class="k">=</span> <span class="nc">Generic</span><span class="o">[</span><span class="kt">Record</span><span class="o">]</span>
<span class="k">val</span> <span class="n">l</span> <span class="k">=</span> <span class="n">gen</span><span class="o">.</span><span class="n">to</span><span class="o">(</span><span class="nc">Record</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span> <span class="c1">// 1 :: 3.14 :: "foo" :: true :: HNil</span>
<span class="k">val</span> <span class="n">g</span> <span class="k">=</span> <span class="n">gen</span><span class="o">.</span><span class="n">from</span><span class="o">(</span><span class="n">l</span><span class="o">)</span> <span class="c1">// Record(1, 3.14, "foo", true)</span>
<span class="n">flip</span><span class="o">(</span><span class="nc">Record</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span> <span class="c1">// Record(-1, -3.14, "oof", false)</span>
</pre></div>
<p>You might have noticed that so far we’ve only operated on the types and values of individual fields, but not field names in a case class. <code>LabelledGeneric</code> is designed just for that by giving us access field names via the type system.</p>
<div class="highlight"><pre><span></span><span class="k">val</span> <span class="n">gen</span> <span class="k">=</span> <span class="nc">LabelledGeneric</span><span class="o">[</span><span class="kt">Record</span><span class="o">]</span>
<span class="c1">// Int with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("i")], Int]</span>
<span class="c1">// :: Double with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("d")], Double]</span>
<span class="c1">// :: String with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("s")], String]</span>
<span class="c1">// :: Boolean with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("b")], Boolean]</span>
<span class="c1">// :: shapeless.HNil</span>
</pre></div>
<p>Each field type e.g. <code>Int</code>, <code>Double</code> is extended with <code>KeyTag[K, V]</code> where <code>K</code> is a macro generated singleton type that uniquely represents the string value. Without diving too deep into the topic, we can retrieve the field name with the <code>Witness</code> type class.</p>
<div class="highlight"><pre><span></span><span class="k">import</span> <span class="nn">shapeless.labelled.FieldType</span>
<span class="k">def</span> <span class="n">name</span><span class="o">[</span><span class="kt">K</span> <span class="k"><:</span> <span class="kt">Symbol</span>, <span class="kt">V</span><span class="o">](</span><span class="n">f</span><span class="k">:</span> <span class="kt">FieldType</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">V</span><span class="o">])</span>
<span class="o">(</span><span class="k">implicit</span> <span class="n">wit</span><span class="k">:</span> <span class="kt">Witness.Aux</span><span class="o">[</span><span class="kt">K</span><span class="o">])</span><span class="k">:</span> <span class="kt">String</span> <span class="o">=</span> <span class="n">wit</span><span class="o">.</span><span class="n">value</span><span class="o">.</span><span class="n">name</span>
<span class="c1">// l.head: Int with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("i")], Int]</span>
<span class="n">name</span><span class="o">(</span><span class="n">l</span><span class="o">.</span><span class="n">head</span><span class="o">)</span> <span class="c1">// "i"</span>
</pre></div>
<p>We can now derive type classes that also depends on field names, for example <code>ToMap[T]</code> that converts <code>T</code> to <code>Map[String, Any]</code>.</p>
<div class="highlight"><pre><span></span><span class="k">import</span> <span class="nn">shapeless._</span>
<span class="k">import</span> <span class="nn">shapeless.labelled.FieldType</span>
<span class="k">trait</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">Map</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Any</span><span class="o">]</span>
<span class="o">}</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">hnilToMap</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">HNil</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">HNil</span><span class="o">)</span> <span class="k">=</span> <span class="nc">Map</span><span class="o">.</span><span class="n">empty</span>
<span class="o">}</span>
<span class="k">implicit</span> <span class="k">def</span> <span class="n">hconsToMap</span><span class="o">[</span><span class="kt">K</span> <span class="k"><:</span> <span class="kt">Symbol</span>, <span class="kt">H</span>, <span class="kt">T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">]</span>
<span class="o">(</span><span class="k">implicit</span> <span class="n">wit</span><span class="k">:</span> <span class="kt">Witness.Aux</span><span class="o">[</span><span class="kt">K</span><span class="o">],</span> <span class="n">ttm</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span>
<span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">FieldType</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">H</span><span class="o">]</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">FieldType</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">H</span><span class="o">]</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">FieldType</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">H</span><span class="o">]</span> <span class="o">::</span> <span class="n">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">Map</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Any</span><span class="o">]</span> <span class="k">=</span>
<span class="n">ttm</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">tail</span><span class="o">)</span> <span class="o">+</span> <span class="o">(</span><span class="n">wit</span><span class="o">.</span><span class="n">value</span><span class="o">.</span><span class="n">name</span> <span class="o">-></span> <span class="n">x</span><span class="o">.</span><span class="n">head</span><span class="o">)</span>
<span class="o">}</span>
<span class="k">def</span> <span class="n">toMap</span><span class="o">[</span><span class="kt">T</span>, <span class="kt">L</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span>
<span class="o">(</span><span class="k">implicit</span>
<span class="n">gen</span><span class="k">:</span> <span class="kt">LabelledGeneric.Aux</span><span class="o">[</span><span class="kt">T</span>, <span class="kt">L</span><span class="o">],</span>
<span class="n">tm</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">L</span><span class="o">])</span><span class="k">:</span> <span class="kt">Map</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Any</span><span class="o">]</span> <span class="k">=</span>
<span class="n">tm</span><span class="o">(</span><span class="n">gen</span><span class="o">.</span><span class="n">to</span><span class="o">(</span><span class="n">x</span><span class="o">))</span>
<span class="k">case</span> <span class="k">class</span> <span class="nc">Record</span><span class="o">(</span><span class="n">i</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">d</span><span class="k">:</span> <span class="kt">Double</span><span class="o">,</span> <span class="n">s</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">b</span><span class="k">:</span> <span class="kt">Boolean</span><span class="o">)</span>
<span class="n">toMap</span><span class="o">(</span><span class="nc">Record</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span> <span class="c1">// Map("b" -> true, "s" -> "foo", d -> 3.14, i -> 1)</span>
</pre></div>
<h2>Type class companions</h2>
<p>Now we know how to write generic code that works with tuples and case classes, but the above examples are still pretty verbose. Luckily there’re are some helpers in shapeless to reduce boilerplate for these problems, namely <code>ProductTypeClassCompanion</code> and <code>LabelledProductTypeClassCompanion</code>.</p>
<p>The <code>ProductTypeClassCompanion</code> skeleton for <code>Flip[T]</code> looks like this:</p>
<div class="highlight"><pre><span></span><span class="k">import</span> <span class="nn">shapeless._</span>
<span class="k">object</span> <span class="nc">FlipDerivedOrphans</span> <span class="k">extends</span> <span class="nc">ProductTypeClassCompanion</span><span class="o">[</span><span class="kt">Flip</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">val</span> <span class="n">typeClass</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ProductTypeClass</span><span class="o">[</span><span class="kt">Flip</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">product</span><span class="o">[</span><span class="kt">H</span>, <span class="kt">T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="n">ch</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">H</span><span class="o">],</span> <span class="n">ct</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="o">???</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">emptyProduct</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">HNil</span><span class="o">]</span> <span class="k">=</span> <span class="o">???</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">project</span><span class="o">[</span><span class="kt">F</span>, <span class="kt">G</span><span class="o">](</span><span class="n">instance</span><span class="k">:</span> <span class="o">=></span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">G</span><span class="o">],</span> <span class="n">to</span><span class="k">:</span> <span class="kt">F</span> <span class="o">=></span> <span class="n">G</span><span class="o">,</span> <span class="n">from</span><span class="k">:</span> <span class="kt">G</span> <span class="o">=></span> <span class="n">F</span><span class="o">)</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">F</span><span class="o">]</span> <span class="k">=</span> <span class="o">???</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>It’s easy to fill in the blanks.</p>
<div class="highlight"><pre><span></span><span class="k">object</span> <span class="nc">FlipDerivedOrphans</span> <span class="k">extends</span> <span class="nc">ProductTypeClassCompanion</span><span class="o">[</span><span class="kt">Flip</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">val</span> <span class="n">typeClass</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ProductTypeClass</span><span class="o">[</span><span class="kt">Flip</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">product</span><span class="o">[</span><span class="kt">H</span>, <span class="kt">T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="n">ch</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">H</span><span class="o">],</span> <span class="n">ct</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="k">=</span>
<span class="nc">Flip</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">](</span><span class="n">x</span> <span class="k">=></span> <span class="n">ch</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">head</span><span class="o">)</span> <span class="o">::</span> <span class="n">ct</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">tail</span><span class="o">))</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">emptyProduct</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">HNil</span><span class="o">]</span> <span class="k">=</span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">HNil</span><span class="o">](</span><span class="k">_</span> <span class="k">=></span> <span class="nc">HNil</span><span class="o">)</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">project</span><span class="o">[</span><span class="kt">F</span>, <span class="kt">G</span><span class="o">](</span><span class="n">instance</span><span class="k">:</span> <span class="o">=></span> <span class="nc">Flip</span><span class="o">[</span><span class="kt">G</span><span class="o">],</span> <span class="n">to</span><span class="k">:</span> <span class="kt">F</span> <span class="o">=></span> <span class="n">G</span><span class="o">,</span> <span class="n">from</span><span class="k">:</span> <span class="kt">G</span> <span class="o">=></span> <span class="n">F</span><span class="o">)</span><span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">F</span><span class="o">]</span> <span class="k">=</span>
<span class="nc">Flip</span><span class="o">[</span><span class="kt">F</span><span class="o">](</span><span class="n">f</span> <span class="k">=></span> <span class="n">from</span><span class="o">(</span><span class="n">instance</span><span class="o">(</span><span class="n">to</span><span class="o">(</span><span class="n">f</span><span class="o">))))</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="k">implicit</span> <span class="k">def</span> <span class="n">deriveFlip</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span>
<span class="o">(</span><span class="k">implicit</span> <span class="n">orphan</span><span class="k">:</span> <span class="kt">Orphan</span><span class="o">[</span><span class="kt">Flip</span>, <span class="kt">FlipDerivedOrphans.</span><span class="k">type</span>, <span class="kt">T</span><span class="o">])</span>
<span class="k">:</span> <span class="kt">Flip</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="n">orphan</span><span class="o">.</span><span class="n">instance</span>
<span class="k">case</span> <span class="k">class</span> <span class="nc">Record</span><span class="o">(</span><span class="n">i</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">d</span><span class="k">:</span> <span class="kt">Double</span><span class="o">,</span> <span class="n">s</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">b</span><span class="k">:</span> <span class="kt">Boolean</span><span class="o">)</span>
<span class="k">val</span> <span class="n">f</span> <span class="k">=</span> <span class="n">implicitly</span><span class="o">[</span><span class="kt">Flip</span><span class="o">[</span><span class="kt">Record</span><span class="o">]]</span>
<span class="n">f</span><span class="o">(</span><span class="nc">Record</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span>
</pre></div>
<p>Likewise we can use <code>LabelledProductTypeClassCompanion</code> for <code>ToMap[T]</code>.</p>
<div class="highlight"><pre><span></span><span class="k">object</span> <span class="nc">ToMapDerivedOrphans</span> <span class="k">extends</span> <span class="nc">LabelledProductTypeClassCompanion</span><span class="o">[</span><span class="kt">ToMap</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">val</span> <span class="n">typeClass</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">LabelledProductTypeClass</span><span class="o">[</span><span class="kt">ToMap</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">product</span><span class="o">[</span><span class="kt">H</span>, <span class="kt">T</span> <span class="k"><:</span> <span class="kt">HList</span><span class="o">](</span><span class="n">name</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">ch</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">H</span><span class="o">],</span> <span class="n">ct</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="k">=</span>
<span class="k">new</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">H</span> <span class="kt">::</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">Map</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Any</span><span class="o">]</span> <span class="k">=</span> <span class="n">ct</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">tail</span><span class="o">)</span> <span class="o">+</span> <span class="o">(</span><span class="n">name</span> <span class="o">-></span> <span class="n">x</span><span class="o">.</span><span class="n">head</span><span class="o">)</span>
<span class="o">}</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">emptyProduct</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">HNil</span><span class="o">]</span> <span class="k">=</span>
<span class="k">new</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">HNil</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">HNil</span><span class="o">)</span><span class="k">:</span> <span class="kt">Map</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Any</span><span class="o">]</span> <span class="k">=</span> <span class="nc">Map</span><span class="o">.</span><span class="n">empty</span>
<span class="o">}</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">project</span><span class="o">[</span><span class="kt">F</span>, <span class="kt">G</span><span class="o">](</span><span class="n">instance</span><span class="k">:</span> <span class="o">=></span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">G</span><span class="o">],</span> <span class="n">to</span><span class="k">:</span> <span class="kt">F</span> <span class="o">=></span> <span class="n">G</span><span class="o">,</span> <span class="n">from</span><span class="k">:</span> <span class="kt">G</span> <span class="o">=></span> <span class="n">F</span><span class="o">)</span><span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">F</span><span class="o">]</span> <span class="k">=</span>
<span class="k">new</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">F</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">F</span><span class="o">)</span><span class="k">:</span> <span class="kt">Map</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Any</span><span class="o">]</span> <span class="k">=</span> <span class="n">instance</span><span class="o">(</span><span class="n">to</span><span class="o">(</span><span class="n">x</span><span class="o">))</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="k">implicit</span> <span class="k">def</span> <span class="n">deriveToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span>
<span class="o">(</span><span class="k">implicit</span> <span class="n">orphan</span><span class="k">:</span> <span class="kt">Orphan</span><span class="o">[</span><span class="kt">ToMap</span>, <span class="kt">ToMapDerivedOrphans.</span><span class="k">type</span>, <span class="kt">T</span><span class="o">])</span>
<span class="k">:</span> <span class="kt">ToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="n">orphan</span><span class="o">.</span><span class="n">instance</span>
<span class="k">case</span> <span class="k">class</span> <span class="nc">Record</span><span class="o">(</span><span class="n">i</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">d</span><span class="k">:</span> <span class="kt">Double</span><span class="o">,</span> <span class="n">s</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">b</span><span class="k">:</span> <span class="kt">Boolean</span><span class="o">)</span>
<span class="k">val</span> <span class="n">f</span> <span class="k">=</span> <span class="n">implicitly</span><span class="o">[</span><span class="kt">ToMap</span><span class="o">[</span><span class="kt">Record</span><span class="o">]]</span>
<span class="n">f</span><span class="o">(</span><span class="nc">Record</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mf">3.14</span><span class="o">,</span> <span class="s">"foo"</span><span class="o">,</span> <span class="kc">true</span><span class="o">))</span>
</pre></div>
<p>However this doesn’t work right away. The compiler complains about <code>could not find implicit value for parameter e: ToMap[Record]</code>. Upon closer inspection, we can see that even though the <code>ch: ToMap[H]</code> argument in <code>def product</code> is unused, it’s still summoning implicits for <code>Int</code>, <code>Double</code>, <code>String</code> and <code>Boolean</code>. This can be worked around by introducing dummy instances like below. Since <code>dummyToMap</code> is in the companion <code>object ToMap</code> which has lower priority than <code>deriveToMap</code> in the current scope, it’s only used for <code>ch: TopMap[H]</code> and not <code>implicitly[ToMap[Record]]</code>. This is obviously not the most elegant solution and only used here to demonstrate common problems when designing and deriving type classes. A better solution might be using an <span class="caps">ADT</span> as <code>ToMap#apply</code> return type instead of <code>Map[String, Any]</code>, and use pattern matching to handle head (single field) vs. tail (<code>Map</code>) cases.</p>
<div class="highlight"><pre><span></span><span class="k">object</span> <span class="nc">ToMap</span> <span class="o">{</span>
<span class="k">implicit</span> <span class="k">def</span> <span class="n">dummyToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ToMap</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">apply</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span> <span class="k">=</span> <span class="o">???</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>This concludes the write-up. However there’re still some topics not covered, like <code>Coproduct</code> and more complex scenarios for implicit lookup.</p>
<h2>References</h2>
<ul>
<li><a href="https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0">Shapeless feature overview</a></li>
<li><a href="https://github.com/milessabin/shapeless/blob/master/examples/src/main/scala/shapeless/examples/shows.scala">Shows</a> and <a href="https://github.com/milessabin/shapeless/blob/master/examples/src/main/scala/shapeless/examples/monoids.scala">Monoid</a> examples in Shapeless</li>
<li><a href="https://stackoverflow.com/questions/25517069/what-is-the-purpose-of-the-emptycoproduct-and-coproduct-methods-of-the-typeclass">StackOverflow question on Coproduct</a></li>
<li><a href="http://underscore.io/books/shapeless-guide/">The Type Astronaut’s Guide to Shapeless</a></li>
<li><a href="http://eed3si9n.com/implicit-parameter-precedence-again">Implicit parameter precedence again</a></li>
</ul>
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