#1496: Newtypes and type families combine to produce inconsistent FC(X) axiom
sets
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Reporter: sorear | Owner: simonpj
Type: bug | Status: new
Priority: high | Milestone: 6.8 branch
Component: Compiler (Type checker) | Version: 6.7
Severity: critical | Resolution:
Keywords: | Difficulty: Unknown
Os: Unknown | Testcase:
Architecture: Unknown |
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Comment (by Isaac Dupree):
Replying to [comment:12 toms]:
> However, GADTs are definitely not functors.
...
> We can attempt to fix the situation by imposing conditions on the
types involved in
> a newtype deriving:
> * generate the fmap code, and hence demand explicit Functor
instances.
> We should be able to expect the compiler to eliminate any runtime
overhead.
From Frisby we have a data type that is a Functor and a GADT, but not a
Functor in the way you and GHC expect:
{{{
data PE a where
-- leaving out most of the constructors, but note especially PMap
Char :: IntSet.IntSet -> PE Char
Failure :: PE a
Not :: PE a -> PE ()
Then :: PE a -> PE b -> PE (a,b)
PMap :: (a -> b) -> PE a -> PE b
instance Functor PE where
fmap = PMap
}}}
Applying the `fmap`-option would make interesting, type-correct code be
generated that could not be optimized away (so the dictionaries couldn't
be shared).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1496#comment:14>
GHC <http://www.haskell.org/ghc/>
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