Even better might be to use GHC 8 custom type errors in the constraint head to explain what &#39;Impossible&#39; means.<br><br><div class="gmail_quote"><div dir="ltr">On Tue, 22 Mar 2016 10:21 am Henning Thielemann, &lt;<a href="mailto:lemming@henning-thielemann.de">lemming@henning-thielemann.de</a>&gt; wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
On Wed, 24 Feb 2016, Chris Allen wrote:<br>
<br>
&gt; You can&#39;t not-include the instances because we&#39;ll just end up with orphans so that&#39;s not cricket I think.<br>
<br>
We could define<br>
<br>
   instance Impossible a =&gt; Functor ((,,) a b) where<br>
<br>
where Impossible is a non exported class.<br>
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