GHC 6.10.2 the 'impossible' panic with type families

[Simon Peyton-Jones]

Definitely a bug, thank you!  I've created a Trac ticket for it.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Jan Jakubuv
| Sent: 05 May 2009 11:06
| To: glasgow-haskell-users at haskell.org
| Subject: GHC 6.10.2 the 'impossible' panic with type families
|
| Dear haskellers,
|
| with the following code:
|
|     {-# OPTIONS -fglasgow-exts #-}
|
|     class SUBST s where
|         type STerm s
|
|     class OBJECT o where
|         type OTerm o
|         apply :: (SUBST s, OTerm o ~ STerm s) => s -> o
|
|     fce' f = fce . apply $ f
|
|     fce f = fce' f
|
| I have the following message in both GHCi and GHC (tested on GHC 6.10.1
| linux and win, and 6.10.2 linux):
|
|     ghc: panic! (the 'impossible' happened)
|       (GHC version 6.10.2 for i386-unknown-linux):
|             idInfo co{v agz} [tv]
|
|     Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
|
| The above is the smallest example I have found that behaves in this way. It
| is a striped version of an original and more complicated code. What seems
| interesting to me is that when one defines `fce` as follows (instead of the
| above):
|
|    fce = fce'
|
| then everything is ok (= no panic message). In my original code `fce'` was a
| monad computation, something like:
|
|     fce' f = fce f >>= \s → fce (apply s)
|
| and this produces the same result (= the panic message).
|
| Is this a bug or a known issue?
|
| Sincerely,
|   Jan.
|
|
|
| --
| Heriot-Watt University is a Scottish charity
| registered under charity number SC000278.
|
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