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List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Haskell spec vs FlexibleInstances
From:       Dan Doel <dan.doel () gmail ! com>
Date:       2014-02-28 16:14:02
Message-ID: CAHEA9tOxJroDy_phc-T52s0B39vfEz5o5C8H6P-6HuvhiYh3Ew () mail ! gmail ! com
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On Fri, Feb 28, 2014 at 9:44 AM, Casey McCann <cam@uptoisomorphism.net>wrote:

> The restriction to distinct type variables, and the resulting issues
> with unification, apply equally to instances for applied type
> constructors. This disallows e.g. writing a Monoid instance for (a ->
> a) directly without a newtype wrapper, not that we'd want to do that
> anyway.
>

This was once the instance defined in Data.Monoid. It was changed to the
current one in GHC 6.6.

--
 Dan

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<div dir="ltr"><br><div class="gmail_extra"><div class="gmail_quote">On Fri, Feb 28, \
2014 at 9:44 AM, Casey McCann <span dir="ltr">&lt;<a \
href="mailto:cam@uptoisomorphism.net" \
target="_blank">cam@uptoisomorphism.net</a>&gt;</span> wrote:<br> <blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex">The restriction to distinct type variables, and the resulting \
issues<br> with unification, apply equally to instances for applied type<br>
constructors. This disallows e.g. writing a Monoid instance for (a -&gt;<br>
a) directly without a newtype wrapper, not that we&#39;d want to do that<br>
anyway.<span class="HOEnZb"><font \
color="#888888"><br></font></span></blockquote><div><br><div class="gmail_default" \
style="font-family:arial,helvetica,sans-serif;display:inline">This was once the \
instance defined in Data.Monoid. It was changed to the current one in GHC 6.6.</div> \
</div></div><br>--<div class="gmail_default" \
style="font-family:arial,helvetica,sans-serif;display:inline"> \
Dan</div><br></div></div>



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