通过 Nat-kind 重叠实例

这个问题实际上是由于尝试将少数数学群实现为类型而出现的。

循环群没有问题（实例Data.Group其他地方定义）：

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))

但对称群在定义某些实例时存在一些问题（通过阶乘系统实现）：

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

来自 ghci 的错误消息（仅简要）：

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

那么 GHC 如何知道是否n-1等于1还是不等于1？我还想知道是否可以在不使用的情况下编写解决方案FlexibleInstances.

Add Bounded (Symmetric (n-1)) and Enum (Symmetric (n-1))作为约束，因为完全解决这些约束需要知道 n 的确切值。

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
  ...

避免FlexibleInstances（在我看来，这不值得，FlexibleInstances是良性扩展），使用皮亚诺数data Nat = Z | S Nat而不是 GHC 的原始表示。首先更换实例头Bounded (Symmetric n) with Bounded (Symmetric (S (S n')))（这起到了约束的作用2 <= n），然后用辅助类（可能更多）分解实例，以满足实例头的标准要求。它可能看起来像这样：

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