如何在Haskell中转换(Num a)=> a到Float？

Question

是否可以将以下两个函数组合成Haskell中的一个多态函数？

mutiplyByTwoI :: Int -> Float
mutiplyByTwoI x = fromIntegral x * 2.0

mutiplyByTwoF :: Float -> Float
mutiplyByTwoF x = x * 2.0

我试过Num a,但我找不到转换Num a为Float的方法.

mutiplyByTwo :: (Num a) => a -> Float
mutiplyByTwo = undefined

Answer 1

并非每个实例Num都有明智的转换Float.对于一个简单的(哈哈)示例,考虑复数:

-- represented in rectangular coordinates
data Complex = Complex { real :: Float, imaginary :: Float } deriving (Eq, Ord, Show, Read)

instance Num Complex where
    Complex u v + Complex x y = Complex (u + x) (v + y)
    Complex u v * Complex x y = Complex (u * x - v * y) (v * x + u * y)
    negate (Complex u v) = Complex (negate u) (negate v)
    -- etc

很明显,没有办法将a转换Complex为a Float而不丢弃信息:Floats生活在真实线的(子集)上,而复数则生活在复杂的平面上.没有toFloat :: Num a => a -> Float适用于所有实例的函数Num.