So this is definitely a GHC bug, but I think the problem is probably
triggered by this line:
instance Serializable a b => IResource a
I don't think this is a valid instance declaration without a functional
dependency on Serializable, as it's impossible to know which type 'b' to use
in the methods of IResource.
-- ryan
On Tue, Jun 28, 2011 at 3:43 AM, Alberto G. Corona
I have an "'impossible' happened" error.
The code may look a little bit convoluted but it is part of my real code.:
--------- begin of code------
{-# LANGUAGE FlexibleInstances, UndecidableInstances , MultiParamTypeClasses #-}
class Serializable a b
class IResource a --The rest of the instance definitions does not matter for the error
instance Serializable a b => IResource a
data DBRef a= DBRef String a
instance (IResource a) => Read (DBRef a)
data Votation a= Votation{ content :: DBRef a } deriving (Read)
------------------------------- end of code ---
gives the following error at compilation time:
tests>runghc impossiblelloop.hs ghc: panic! (the 'impossible' happened) (GHC version 7.0.3 for i386-unknown-mingw32): solveDerivEqns: probable loop (impossiblelloop.hs:20:13-16 main:Main.$fReadVotation{v rhI} [a{tv abB} [tv] ] base:GHC.Read.Read{tc 2d} [main:Main.Votation{tc rbo}
a{tv abB} [tv]] = [base:GHC.Read.Read{tc 2d}
(main:Main.DBRef{tc rbu}
a{tv abB} [tv])]) [[main:Main.Serializable{tc rbA} a{tv abB} [tv] b{tv ajE} [tcs]]]
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe