Haskell:GHCI中错误的含义

Question

我无法弄清楚msg的这个错误意味着什么.我想在下面定义数据类型的函数距离...我不想使用任何GHC扩展..即使代码很难看,想要在继续​​使用扩展之前更好地理解错误.有人可以让我知道这个错误意味着什么,以及如何摆脱这个.

class Vector v where

         distance :: v -> v -> Double

-- doesn't make sense, but WTH...    
newtype OneD1 a = OD1 a
         deriving (Show)

instance Vector (Maybe m) where
    distance _ _ = 5.6

instance Vector (OneD1 m) where
   distance (OD1 x1) (OD1 x2) = x2-x1

    Prelude> :reload
[1 of 1] Compiling Main             ( VectorTypeClass.hs, interpreted )

VectorTypeClass.hs:33:33:
    Couldn't match expected type `Double' with actual type `m'
      `m' is a rigid type variable bound by
          the instance declaration
          at C:\Users\byamm\Documents\Programming\Haskell\Lab\Expts\VectorTypeClass\VectorTypeClass.hs:32:10
    Relevant bindings include
      x2 :: m
        (bound at C:\Users\byamm\Documents\Programming\Haskell\Lab\Expts\VectorTypeClass\VectorTypeClass.hs:33:27)
      x1 :: m
        (bound at C:\Users\byamm\Documents\Programming\Haskell\Lab\Expts\VectorTypeClass\VectorTypeClass.hs:33:18)
      distance :: OneD1 m -> OneD1 m -> Double
        (bound at C:\Users\byamm\Documents\Programming\Haskell\Lab\Expts\VectorTypeClass\VectorTypeClass.hs:33:4)
    In the first argument of `(-)', namely `x2'
    In the expression: x2 - x1
Failed, modules loaded: none.
Prelude>

Answer 1

instance Vector (OneD1 m) where

这承诺(OdeD1 m)对任何人来说都是一个载体m.包括OneD1 String等

   distance (OD1 x1) (OD1 x2) = x2-x1

在这里,我们尝试应用-两个类型的值m,这可能绝对是任何东西.这里有两个问题:

• m 可能不是数字类型 - 目前GHC没有报告此错误.
• 差异的结果再次是类型m,但距离应该产生一个Double.这是GHC报告的错误.

您需要限制m为某种数字类型,以便您可以使用-,并且该类型必须允许转换Double,否则您无法满足distance签名.

一个简单的方法是:

instance Vector (OneD1 Double) where
   distance (OD1 x1) (OD1 x2) = x2-x1

另一个可能是:

instance Real m => Vector (OneD1 m) where
   distance (OD1 x1) (OD1 x2) = realToFrac (x2-x1)