Haskell 有约束的实例

Question

我正在尝试为有符号数字类型创建一个类型类。这是我的代码：

{-# LANGUAGE TypeFamilies, FlexibleContexts, UndecidableInstances #-}

data Sign = Negative | Zero | Positive
  deriving (Eq, Ord, Read, Show)

class Signed a where
  sign :: a -> Sign

instance Signed Integer where
  sign = undefined

这可以编译，但我想修改此代码以适用于任何 Integral a。

instance (Integral a) => Signed a where
  sign = undefined

此时编译失败。

我检查过 Haskell type family instance with type constraints，但这似乎解决了与我不同的问题。我不认为我的代码中存在语法错误。

Answer 1

尝试编译您的代码会产生以下错误消息：

sign.hs:9:26:
    Illegal instance declaration for ‘Signed a’
      (All instance types must be of the form (T a1 ... an)
       where a1 ... an are *distinct type variables*,
       and each type variable appears at most once in the instance head.
       Use FlexibleInstances if you want to disable this.)
    In the instance declaration for ‘Signed a’
Failed, modules loaded: none.

正如编译器指出的那样，您需要打开 FlexibleInstances 以及 UndecidableInstances。 GHC 的错误消息通常非常具体，尤其是当您忘记打开语言扩展时。以下立即编译：

{-# LANGUAGE UndecidableInstances, FlexibleInstances #-}

data Sign = Negative | Zero | Positive
  deriving (Eq, Ord, Read, Show)

class Signed a where
  sign :: a -> Sign

instance (Integral a) => Signed a where
  sign = undefined

不过，我认为Signed class可能是这个例子中的一个错误。定义一个（非重载的）顶层函数要简单得多，不需要 UndecidableInstances（需要它通常是一种设计味道），并且更能表达 的含义 您的代码："things you can get the sign of" 恰好是实数。

sign :: Real a => a -> Sign
sign x
    | x == 0 = Zero
    | x < 0 = Negative
    | otherwise = Positive

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