Overlapping instances via Nat-kind

Question

This problem actually emerged from attempt to implement few mathematical groups as types.

Cyclic groups have no problem (instance of Data.Group defined elsewhere):

newtype Cyclic (n :: Nat) = Cyclic {cIndex :: Integer} deriving (Eq, Ord)

cyclic :: forall n. KnownNat n => Integer -> Cyclic n
cyclic x = Cyclic $ x `mod` toInteger (natVal (Proxy :: Proxy n))

But symmetric groups have some problem on defining some instances (implementation via factorial number system):

infixr 6 :.

data Symmetric (n :: Nat) where
    S1 :: Symmetric 1
    (:.) :: (KnownNat n, 2 <= n) => Cyclic n -> Symmetric (n-1) -> Symmetric n

instance {-# OVERLAPPING #-} Enum (Symmetric 1) where
    toEnum _ = S1
    fromEnum S1 = 0

instance (KnownNat n, 2 <= n) => Enum (Symmetric n) where
    toEnum n = let
        (q,r) = divMod n (1 + fromEnum (maxBound :: Symmetric (n-1)))
        in toEnum q :. toEnum r
    fromEnum (x :. y) = fromInteger (cIndex x) * (1 + fromEnum (maxBound `asTypeOf` y)) + fromEnum y

instance {-# OVERLAPPING #-} Bounded (Symmetric 1) where
    minBound = S1
    maxBound = S1

instance (KnownNat n, 2 <= n) => Bounded (Symmetric n) where
    minBound = minBound :. minBound
    maxBound = maxBound :. maxBound

Error message from ghci (only briefly):

Overlapping instances for Enum (Symmetric (n - 1))
Overlapping instances for Bounded (Symmetric (n - 1))

So how can GHC know whether n-1 equals to 1 or not? I'd also like to know whether the solution can be written without FlexibleInstances.

score 3 · Accepted Answer · answered Jan 02 '19 at 08:52

Add Bounded (Symmetric (n-1)) and Enum (Symmetric (n-1)) as constraints, because fully resolving those constraints would require knowing the exact value of n.

instance (KnownNat n, 2 <= n, Bounded (Symmetric (n-1)), Enum (Symmetric (n-1))) =>
  Enum (Symmetric n) where
  ...

instance (KnownNat n, 2 <= n, Bounded (Symmetric (n-1))) =>
  Bounded (Symmetric n) where
  ...

To avoid FlexibleInstances (which is not worth it IMO, FlexibleInstances is a benign extension), use Peano numbers data Nat = Z | S Nat instead of GHC's primitive representation. First replace the instance head Bounded (Symmetric n) with Bounded (Symmetric (S (S n'))) (this plays the role of the constraint 2 <= n), and then break up the instance with an auxiliary class (possibly more) to satisfy the standard requirement on instance heads. It might look like this:

instance Bounded_Symmetric n => Bounded (Symmetric n) where ...
instance Bounded_Symmetric O where ...
instance Bounded_Symmetric n => Bounded_Symmetric (S n) where ...

Overlapping instances via Nat-kind

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