#4083: Generalized newtype deriving fails for newtypes that are type family
instances
----------------------------------------+-----------------------------------
Reporter: RANDY Ruler of Zexernet | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.12.1 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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With FlexibleContexts, FlexibleInstances, GeneralizedNewtypeDeriving,
StandaloneDeriving, & TypeFamilies the following code produces an
"impossible" error (genDerivBinds: bad derived class):
{{{
data family F a
newtype instance F [a] = Maybe a
class C a where data D a
deriving instance C (Maybe a) => C (F [a])
}}}
Whereas I expected something like this (which works):
{{{
instance C (Maybe a) => C (F [a]) where newtype D (F [a]) = SomeCon (D
(Maybe a))
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4083>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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