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<h3 id="tl-dr" style="margin: 1.3em 0px 1em; padding: 0px;
font-weight: bold;font-size: 1.3em;">tl;dr</h3>
<p style="margin: 1.2em 0px ! important;">We have <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">instance IsString [Char]</code>
and <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">instance IsList (NonEmpty a)</code>.
Let’s also have <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsString (NonEmpty Char)</code>.</p>
<h3 id="background" style="margin: 1.3em 0px 1em; padding: 0px;
font-weight: bold;font-size: 1.3em;">Background</h3>
<p style="margin: 1.2em 0px ! important;"><code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsString</code>
is a class that is used with the <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">-XOverloadedStrings</code>
extension to support string literals of types other than <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">String</code>
- for instance, with the <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsString Text</code>
instance in scope, you can write <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">"foobar" :: Text</code>
and it will compile.</p>
<p style="margin: 1.2em 0px ! important;">For reference, the
standard libraries supply the following instances of <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsString</code>:</p>
<pre style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-haskell" style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block ! important;display: block; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 255) none repeat scroll 0% 0%;"><span class="hljs-class"><span class="hljs-keyword">instance</span> <span class="hljs-container">(<span class="hljs-title">a</span> ~ <span class="hljs-type">Char</span>)</span> => <span class="hljs-type">IsString</span> [a]
<span class="hljs-keyword">instance</span> <span class="hljs-type">IsString</span> a => <span class="hljs-type">IsString</span> <span class="hljs-container">(<span class="hljs-type">Const</span> <span class="hljs-title">a</span> <span class="hljs-title">b</span>)</span>
<span class="hljs-keyword">instance</span> <span class="hljs-type">IsString</span> a => <span class="hljs-type">IsString</span> <span class="hljs-container">(<span class="hljs-type">Identity</span> <span class="hljs-title">a</span>)</span></span>
</code></pre>
<h3 id="proposal" style="margin: 1.3em 0px 1em; padding: 0px;
font-weight: bold;font-size: 1.3em;">Proposal</h3>
<p style="margin: 1.2em 0px ! important;">I propose adding a new
instance of <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsString</code>
for <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">NonEmpty</code>
lists of characters. <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">NonEmpty</code>
has been in <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">base</code>
starting from version 4.9, and for that reason people are
starting to use it more often - e.g. the popular <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">megaparsec</code>
library defines its custom error type like this:</p>
<pre style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-haskell" style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block ! important;display: block; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 255) none repeat scroll 0% 0%;"><span class="hljs-typedef"><span class="hljs-keyword">data</span> <span class="hljs-type">ErrorItem</span> t</span>
= <span class="hljs-type">Tokens</span> (<span class="hljs-type">NonEmpty</span> t) <span class="hljs-comment">-- ^ Non-empty stream of tokens</span>
| <span class="hljs-type">Label</span> (<span class="hljs-type">NonEmpty</span> <span class="hljs-type">Char</span>) <span class="hljs-comment">-- ^ Label (cannot be empty)</span>
| <span class="hljs-type">EndOfInput</span> <span class="hljs-comment">-- ^ End of input</span>
</code></pre>
<p style="margin: 1.2em 0px ! important;">Here <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">NonEmpty Char</code>
stands for “non-empty string”. Without the <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsString</code>
instance users are forced to write non-empty strings in an
inconvenient and awkward-looking way (e.g. <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">Label ('f' :| "oobar")</code>);
the instance makes <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">Label "foobar"</code>
an acceptable notation, thus making the <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">NonEmpty Char</code>
type more viable for use in libraries and user-facing APIs.</p>
<h3 id="implementation" style="margin: 1.3em 0px 1em; padding:
0px; font-weight: bold;font-size: 1.3em;">Implementation</h3>
<p style="margin: 1.2em 0px ! important;">Here’s a sample
implementation:</p>
<pre style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-haskell" style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block ! important;display: block; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 255) none repeat scroll 0% 0%;"><span class="hljs-class"><span class="hljs-keyword">instance</span> <span class="hljs-container">(<span class="hljs-title">a</span> ~ <span class="hljs-type">Char</span>)</span> => <span class="hljs-type">IsString</span> <span class="hljs-container">(<span class="hljs-type">NonEmpty</span> <span class="hljs-title">a</span>)</span> <span class="hljs-keyword">where</span></span>
fromString (a:<span class="hljs-keyword">as</span>) = a :| <span class="hljs-keyword">as</span>
fromString <span class="hljs-string">""</span> = errorWithoutStackTrace <span class="hljs-string">"NonEmpty.fromString: empty string"</span>
</code></pre>
<p style="margin: 1.2em 0px ! important;">This mirrors the <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">IsList</code>
instance for <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">NonEmpty</code>.
(The reason I haven’t used <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">fromList</code>
is that I want the error message to say “fromString” instead of
“fromList”.)</p>
<p style="margin: 1.2em 0px ! important;">If this is accepted, I
can make a patch.</p>
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