[Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?
Hello, haskellers. I am trying to write some generic subtyping issue. Here
upcast is always safe operation because of subtype is always behaves like the
parrent type. downcast is not the safe becase of not every parrent type value
can be converted to children type. Rangeable here is the typeclass of values
in some range, so downcasting to Rang1 or Range2 or any other type, having
instance for Rangeable can be done by checking if value is in proper range.
The same for MultipleTo, downcasting can be done with checking if value is
multiple to some value.
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies,
FlexibleContexts, UndecidableInstances, OverlappingInstances,
IncoherentInstances #-}
class SubtypeOf a b | a - b where
upcast :: a - b
downcastSafe :: b - Maybe a
downcast :: b - a
downcast b = case downcastSafe b of
Nothing - error $ can not downcast the value
Just a - a
class (Ord a) = Rangable t a | t - a where
lowLim :: t - a
highLim :: t - a
class Packable t a | t - a where
pack :: a - t
unpack :: t - a
class MultipleTo t a | t - a where
multiple :: t - a
instance (Num a, Ord a, Rangable range a, Packable range a) = SubtypeOf range
a where
upcast = unpack
downcastSafe b | b = (lowLim $ pb) b = (highLim $ pb) = Just $ pb
| otherwise = Nothing
where
pb = pack b
instance (Integral a, Packable range a, MultipleTo range a) = SubtypeOf range
a where
upcast = unpack
downcastSafe b | b `mod` (multiple pb) == 0 = Just pb
| otherwise = Nothing
where
pb = pack b
newtype Range1 a = Range1 {unRange1 :: a}
deriving Show
instance (Num a, Ord a) = Rangable (Range1 a) a where
lowLim _ = 0
highLim _ = 10
instance (Num a, Ord a) = Packable (Range1 a) a where
pack = Range1
unpack = unRange1
newtype Range2 a = Range2 {unRange2 :: a}
deriving Show
instance (Num a, Ord a) = Rangable (Range2 a) a where
lowLim _ = -10
highLim _ = 200
instance (Num a, Ord a) = Packable (Range2 a) a where
pack = Range2
unpack = unRange2
but there is compilation error:
Duplicate instance declarations:
instance [incoherent] (Num a, Ord a, Rangable range a,
Packable range a) =
SubtypeOf range a
-- Defined at ...:22:10
instance [incoherent] (Integral a, Packable range a,
MultipleTo range a) =
SubtypeOf range a
-- Defined at ...:29:10
Failed, modules loaded: none.
If I remove one of instances of SubtypeOf the program is compiling. How to
write this instances properly, or to write proper type casting ?
Thanks
PS. My english is not very good, but I hope this is understandable.___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?
On Fri, Jan 25, 2013 at 3:18 PM, s9gf4...@gmail.com wrote: Duplicate instance declarations: instance [incoherent] (Num a, Ord a, Rangable range a, Packable range a) = SubtypeOf range a -- Defined at ...:22:10 instance [incoherent] (Integral a, Packable range a, MultipleTo range a) = SubtypeOf range a -- Defined at ...:29:10 This would be correct. Constraints on an instance are applied *after* the instance is selected, so when Haskell is looking for an instance, these two are identical. This has the code smell of trying to use typeclasses for OOP. That won't work. (Yes, really.) -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?
This has the code smell of trying to use typeclasses for OOP. That won't work. (Yes, really.) I am not trying to use OOP, I am just writing some typecasting at all. This would be correct. Constraints on an instance are applied *after* the instance is selected, so when Haskell is looking for an instance, these two are identical. I didn't understand why these two instances are identical ? The constraints are different and OverlappingInstances should permit overlapping typeclasses in constraints and select more specific instance clause.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?
http://ideone.com/v2CrAm I has posted to ideone to show what is wrong.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?
On Fri, Jan 25, 2013 at 12:39 PM, s9gf4...@gmail.com wrote: ** This has the code smell of trying to use typeclasses for OOP. That won't work. (Yes, really.) I am not trying to use OOP, I am just writing some typecasting at all. This would be correct. Constraints on an instance are applied *after* the instance is selected, so when Haskell is looking for an instance, these two are identical. I didn't understand why these two instances are identical ? The constraints are different and OverlappingInstances should permit overlapping typeclasses in constraints and select more specific instance clause. They are identical because constraints don't count for deciding that a type is in a class. For the purposes of deciding if a type is in a class, instance Foo (Bar a) instance Fizz a = Foo (Bar a) instance Fuzz a = Foo (Bar a) are exactly the same, and all three are therefore overlapping instances. None is more specific, because they all refer to the same type -- (Bar a). Also, you can just use Typeable instead of that downcasting stuff. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
