GADT pattern match in non-rigid context

[Simon Peyton-Jones]

| >> You should be giving a type signature to rewrap!  That should fix it.
| >
| > Thanks, all works fine now :-)
|
| it would be great if the error message suggested giving a type signature
| as a solution (or maybe it already does, in the HEAD?)

Good point: I'll do that.

Simon


| >> | -----Original Message-----
| >> | From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-
| users-bounces at haskell.org] On Behalf Of
| >> | Neil Mitchell
| >> | Sent: 17 December 2007 16:23
| >> | To: glasgow-haskell-users at haskell.org
| >> | Subject: GADT pattern match in non-rigid context
| >> |
| >> | Hi,
| >> |
| >> | Upgrading from GHC 6.6 to 6.8 has caused some code to stop working:
| >> |
| >> | ----------------------------------------------------------------------
| >> | {-# OPTIONS_GHC -fglasgow-exts #-}
| >> |
| >> | module Data2 where
| >> |
| >> | data CCompany
| >> |
| >> | data Paradise :: * -> * where
| >> |     CC :: Paradise CCompany
| >> |
| >> | rewrapCC CC = []
| >> | ----------------------------------------------------------------------
| >> |
| >> | [1 of 1] Compiling Data2            ( Data2.hs, interpreted )
| >> |
| >> | Data2.hs:12:9:
| >> |     GADT pattern match in non-rigid context for `CC'
| >> |       Tell GHC HQ if you'd like this to unify the context
| >> |     In the pattern: CC
| >> |     In the definition of `rewrapCC': rewrapCC CC = []
| >> |
| >> | This code is from the Uniplate benchmarking code, which runs the
| >> | Paradise benchmark from SYB on Uniplate, Compos and SYB. The Compos
| >> | code uses GADT's, so the program first needs to convert from standard
| >> | data structures to GADT's before it can work, then back at the end.
| >> | It's the problem of converting from a GADT to a normal data structure
| >> | that is failing.
| >> |
| >> | So is there an easy workaround? Or should I be asking GHC HQ to unify
| things?
| >> |
| >> | Thanks
| >> |
| >> | Neil
| >> | _______________________________________________
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| >> | Glasgow-haskell-users at haskell.org
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| >>
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