Hi,<div>that's a bug in GHC---it erroneously accepts polymorphic instances which violate the FD of a class.</div><div>-Iavor<br><br><div class="gmail_quote">On Fri, Mar 18, 2011 at 7:08 AM, Daniel Fischer <span dir="ltr"><<a href="mailto:daniel.is.fischer@googlemail.com">daniel.is.fischer@googlemail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="im">On Friday 18 March 2011 14:40:40, JP Moresmau wrote:<br>
> Thanks to you all, I think I understand better.<br>
> instance Search Id Id where<br>
> search _ _ i = only (FoundId i)<br>
><br>
> Is too restrictive on the first type, so declaring instead:<br>
> instance Search id Id where<br>
> search _ _ i = only (FoundId i)<br>
<br>
</div>Not sure what GHC does with that, but at least in spirit that violates the<br>
FunDep of<br>
<br>
class Search id a | a -> id where ...<br>
<br>
Even if it works now, it may well not work in the future.<br>
I'd go for a more stable solution respecting the intent of FunDeps<br>
(i.e. there should only be one type t with an instance Search t Id).<br>
<div class="im"><br>
><br>
> Fixed the issue!! Now the initial "id" is not Id and everybody is<br>
> happy (and the code still seems to work as intended)<br>
><br>
> thanks again<br>
><br>
> JP<br>
<br>
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