#4028: Derived Data instance requires Data instances for unused type parameters
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Reporter: igloo | Owner:
Type: bug | Status: new
Priority: normal | Milestone: Not GHC
Component: Compiler (Type checker) | Version: 6.12.2
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by dreixel):
Consider the following different choices of Data instances for FSVec:
{{{
newtype FSVec s a = FSVec {unFSVec :: [a]} deriving (Eq, Typeable)
instance (Typeable s, Data a) => Data (FSVec s a)
newtype FSVec1 s a = FSVec1 {unFSVec1 :: [a]} deriving (Eq, Typeable)
instance (Typeable s, Data a) => Data (FSVec1 s a) where
dataCast1 f = Data.Typeable.gcast1 f
newtype FSVec2 s a = FSVec2 {unFSVec2 :: [a]} deriving (Eq, Typeable)
instance (Data s, Data a) => Data (FSVec2 s a) where
dataCast2 f = Data.Typeable.gcast2 f
}}}
Let's now test their behavior when extending a generic function with an
adhoc case:
{{{
-- We need to define ext2Q, which uses dataCast2.
-- ext1Q, which is defined in the library, uses dataCast1.
newtype Q r a = Q { unQ :: a -> r }
ext2Q :: (Data d, Typeable2 t)
=> (d -> q) -> (forall d1 d2. (Data d1, Data d2) => t d1 d2 -> q) ->
d -> q
ext2Q def ext arg =
case dataCast2 (Q ext) of
Just (Q ext') -> ext' arg
Nothing -> def arg
test, test1, test2 :: Char
test = (const 'p') `ext2Q` (\(FSVec _) -> 'q') $ (FSVec "" :: FSVec ()
Char)
test1 = (const 'p') `ext2Q` (\(FSVec1 _) -> 'q') $ (FSVec1 "" :: FSVec1 ()
Char)
test2 = (const 'p') `ext2Q` (\(FSVec2 _) -> 'q') $ (FSVec2 "" :: FSVec2 ()
Char)
main = print (test, test1, test2)
}}}
From my perspective, only test3 behaves correctly, and hence we should
define dataCast2 for FSVec as ghc 6.12 does. Note also that FSVec is an
instance of Typeable2, so it's only sensible to use dataCast2 for it.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4028#comment:5>
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