Floating 类型，并且还要求两个操作数和结果都具有相同的类型.

The (**) :: Floating a => a -> a -> a can only be applied to Floating types, and furthermore requires that both operands and the result all have the same type.

您的类型签名则表示 x 是 Int ( Int 是 not Floating 类型类的成员)，并且 x ** 0.5 应该返回 Float ，因此这违反了的类型签名.(**).

Your type signature on the other hand, says that x is an Int (an Int is not a member of the Floating typeclass), and furthermore x ** 0.5 should return a Float, so this violates the type signature of (**).

我们可以转换 Num 类型类与

We can convert any type that is a member of the Integral typeclass to any type that is a member of the Num typeclass with fromIntegral :: (Integral a, Num b) => a -> b . In this case fromIntegral will thus convert an Int to a Float:

sqrt' :: Int -> Float
sqrt' x = fromIntegral x ** 0.5