an> a
  = <span class="hljs-type">Just</span> a
  | <span class="hljs-type">Nothing</span>

<span class="hljs-comment">-- Just 3.14 : Maybe Float</span>
<span class="hljs-comment">-- Just &quot;hi&quot; : Maybe String</span>
<span class="hljs-comment">-- Just True : Maybe Bool</span>
<span class="hljs-comment">-- Nothing   : Maybe a</span>
</code></pre>
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This is a type with two variants. You either have `Nothing` or you have `Just` a value. The type variable makes it possible to have a `Maybe Float` and