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.
Excellent question. I believe that what I describe below is a restriction of
the system described in the POPL'06 paper
http://research.microsoft.com/%7Esimonpj/papers/gadt/gadt-icfp.pdf
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.
Simon
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jason Dagit
Sent: 29 July 2008 15:14
To: Simon Peyton-Jones
Cc: glasgow-haskell-users@haskell.org
Subject: Re: GHC 6.6 GADT type unification vs GHC 6.8
On Tue, Jul 29, 2008 at 1:07 AM, Simon Peyton-Jones <[EMAIL
PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
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,
* the type of the scrutinee
* the type of the result of the case
* the types of any free variables used inside the case alternatives
are all completely known ("rigid" in GHC's terminology) at the case expression.
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.
The easiest way to make a variable have a rigid type is to give it a type
signature.
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.
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.
I'll add these remarks to the user manual.
Great!
Thanks,
Jason
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