text-display-0.0.4.0: doc/book/Tutorial.html
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<ol class="chapter"><li class="chapter-item expanded affix "><a href="Introduction.html">Introduction</a></li><li class="chapter-item expanded affix "><a href="Tutorial.html" class="active">Tutorial</a></li><li class="chapter-item expanded affix "><a href="https://hackage.haskell.org/package/text-display/docs/Data-Text-Display.html">API Reference (→ Hackage)</a></li><li class="chapter-item expanded affix "><a href="Design.html">Design</a></li><li class="chapter-item expanded affix "><a href="Comparison.html">Comparison</a></li></ol>
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<h1 class="menu-title">text-display</h1>
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<h1 id="tutorial"><a class="header" href="#tutorial">Tutorial</a></h1>
<p>This tutorial will teach you how to use the <code>Display</code> typeclass for your own data types.</p>
<h2 id="the-display-typeclass"><a class="header" href="#the-display-typeclass">The Display typeclass</a></h2>
<p>The <code>Display</code> typeclass is an interface to create user-facing output.
As opposed to the <code>Show</code> typeclass, it does not have to output Haskell syntax at all
time. This enables the programmer to separate concerns between textual output
that needs to be parsable by derived instances of the <code>Read</code> typeclass, and
output whose prime goal is to convey information to humans.</p>
<p>The main way to get started with it is to call the <code>display</code> function on any data type
that implements the <code>Display</code> typeclass. We are going to see how to implement it in
the next sections.</p>
<h2 id="implementing-the-typeclass"><a class="header" href="#implementing-the-typeclass">Implementing the typeclass</a></h2>
<p>The easiest way to implement the typeclass is to provide an implementation for its
<code>displayBuilder</code> method:</p>
<pre><code class="language-haskell">import Data.Text.Display
data MyType = MyType Int
-- >>> display (MyType 32)
-- "MyType 32"
instance Display MyType where
displayBuilder (MyType i) = "MyType " <> display i
</code></pre>
<p>But this can be quite time-consuming, especially if your datatype already has
an existing <code>Show</code> that you wish to reuse. In which case, you can piggy-back
on this instance like this:</p>
<pre><code class="language-haskell">{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE DerivingStrategies #-}
data AutomaticallyDerived = AD
deriving stock Show
deriving Display
via (ShowInstance AutomaticallyDerived)
</code></pre>
<p>This derivation takes advantage of the <a href="https://hackage.haskell.org/package/text-display/docs/Data-Text-Display.html#t:ShowInstance">ShowInstance</a> helper to reuse
the <code>Show</code> instance of <code>AutomaticallyDerived</code>.
In this case, <code>show</code> and <code>display</code> will give the same result on the
<code>AutomaticallyDerived</code> datatype.</p>
<p>But let's say you want to redact an instance of <code>Display</code>? You can do it locally, through
the <code>OpaqueInstance</code> helper. It is most useful to hide tokens or passwords:</p>
<pre><code class="language-haskell">data UserToken = UserToken UUID
deriving Display
via (OpaqueInstance "[REDACTED]" UserToken)
display $ UserToken "7a01d2ce-31ff-11ec-8c10-5405db82c3cd"
-- => "[REDACTED]"
</code></pre>
<p>By now you should be comfortable with the base concepts of <code>Display</code>. Have fun. :) </p>
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