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<p class=MsoNormal style='margin-bottom:12.0pt'>Thanks, this makes sense and
helps. Is this described in the wobbly types paper, or is paper covering
a different topics? I would like to have a cite-able reference.<o:p></o:p></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Excellent question. I believe that what I describe below is a <b>restriction</b>
of the system described in the POPL’06 paper<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> <a
href="http://research.microsoft.com/%7Esimonpj/papers/gadt/gadt-icfp.pdf">http://research.microsoft.com/%7Esimonpj/papers/gadt/gadt-icfp.pdf</a><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>At this moment I can’t remember when we changed to the
additional restrictions below. I think the reason was that we wanted to avoid
the complexity of “fresh” mgus described in the paper, but memory is failing
me. Dimitrios or Stephanie may have a better memory.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Simon<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> glasgow-haskell-users-bounces@haskell.org
[mailto:glasgow-haskell-users-bounces@haskell.org] <b>On Behalf Of </b>Jason
Dagit<br>
<b>Sent:</b> 29 July 2008 15:14<br>
<b>To:</b> Simon Peyton-Jones<br>
<b>Cc:</b> glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> Re: GHC 6.6 GADT type unification vs GHC 6.8<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><o:p> </o:p></p>
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<p class=MsoNormal>On Tue, Jul 29, 2008 at 1:07 AM, Simon Peyton-Jones <<a
href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p><span style='font-size:11.0pt;color:#1F497D'>GHC 6.6 was a bit more generous
than GHC 6.8, but erroneously so. Specifically, GHC 6.8 and all
subsequent versions require that when you pattern match on a value of GADT
type,</span><o:p></o:p></p>
<p style='margin-left:72.0pt;text-indent:-18.0pt'><span style='font-size:11.0pt;
font-family:Symbol;color:#1F497D'>·</span><span style='font-size:11.0pt;
color:#1F497D'> </span><span
style='font-size:11.0pt;font-family:Symbol;color:#1F497D'> </span><span
style='font-size:11.0pt;color:#1F497D'>the type of the scrutinee</span><o:p></o:p></p>
<p style='margin-left:72.0pt;text-indent:-18.0pt'><span style='font-size:11.0pt;
font-family:Symbol;color:#1F497D'>·</span><span style='font-size:11.0pt;
color:#1F497D'> </span><span
style='font-size:11.0pt;font-family:Symbol;color:#1F497D'> </span><span
style='font-size:11.0pt;color:#1F497D'>the type of the result of the case</span><o:p></o:p></p>
<p style='margin-left:72.0pt;text-indent:-18.0pt'><span style='font-size:11.0pt;
font-family:Symbol;color:#1F497D'>·</span><span style='font-size:11.0pt;
color:#1F497D'> </span><span
style='font-size:11.0pt;font-family:Symbol;color:#1F497D'> </span><span
style='font-size:11.0pt;color:#1F497D'>the types of any free variables used
inside the case alternatives</span><o:p></o:p></p>
<p><span style='font-size:11.0pt;color:#1F497D'>are all completely known
("rigid" in GHC's terminology) at the case expression.</span><o:p></o:p></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'>Thanks, this makes sense and
helps. Is this described in the wobbly types paper, or is paper covering
a different topics? I would like to have a cite-able reference.<o:p></o:p></p>
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<p><span style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p><span style='font-size:11.0pt;color:#1F497D'>The easiest way to make a
variable have a rigid type is to give it a type signature. </span><o:p></o:p></p>
<p><span style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p><span style='font-size:11.0pt;color:#1F497D'>I don't expect to revert to GHC
6.6's behaviour because I don't know how to do (robust, complete) type
inference for that.</span><o:p></o:p></p>
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<p class=MsoNormal><br>
In that case, good thing I finally figured out what changes to make to darcs so
that our type witness code compiles in both 6.6 and 6.8. My changes match
what you describe.<br>
<o:p></o:p></p>
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<p><span style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p><span style='font-size:11.0pt;color:#1F497D'>I'll add these remarks to the
user manual.</span><o:p></o:p></p>
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<p class=MsoNormal>Great!<br>
<br>
Thanks,<br>
Jason<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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