#4896: Deriving Data does not work for attached code
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Reporter: mitar | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Changes (by simonpj):
* cc: jpm@…, andres@… (added)
Comment:
I had a quick look. The `Data`/`Typeable` instances for type families
make my head hurt badly.
Two facts. Currently GHC generates a `Data` instance looking like this
{{{
instance (Typeable (Bar (D a b)), Data a, Data b) => Data (Bar (D a b))
where
dataCast2 f = gcast2 f
}}}
That fails. But it passes if we said `dataCast1 f = gcast1 f`.
Secondly, we are generating too many `Typeable` instances. If you try
{{{
instance Foo (D a b) where
data Bar (D a b) = B deriving( Typeable )
instance Foo [a] where
data Bar [a] = C deriving( Typeable )
}}}
you get
{{{
Duplicate instance declarations:
instance Typeable1 Bar -- Defined at T4896.hs:36:34-41
instance Typeable1 Bar -- Defined at T4896.hs:39:30-37
}}}
I'd like someone to work out a clear story for what should happen for
`Data` and `Typeable` instances for `data` families. Then I could
implement it. Pedro perhaps? Or Andres?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4896#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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