#2721: Newtype deriving doesn't work with type families
---------------------------------+------------------------------------------
Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown/Multiple | Os: Unknown/Multiple
---------------------------------+------------------------------------------
This assumes `-XTypeFamiles -XGeneralizedNewtypeDeriving`. Example:
{{{
class C a where
type T a
foo :: a -> T a
instance C Int where
type T Int = Int
foo = id
newtype N = N Int deriving(C)
}}}
This happily produces an `instance C N` but no `type instance T N`. It
should either (preferably) generate
{{{
type instance T N = Int
}}}
or fail. The example also compiles if `T` is a data family (the `Int`
instance needs to be change accordingly). It should probably fail in this
case.
BTW, this also compiles fine, with rather dramatic consequences:
{{{
type family T a
class C a where
foo :: a -> T a
type instance T Int = Int
instance C Int where
foo = id
type instance T N = Double
newtype N = N Int deriving(C)
}}}
I guess this last example is the same bug as `#1496`. I wonder if the
deriving clause could generate something like:
{{{
instance T Int ~ T N => C Int
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2721>
GHC <http://www.haskell.org/ghc/>
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