The 'impossible' happened. solveDerivEqns: probable loop
This 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)
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