Haskell 刚性类型？

Question

这个问题已经有答案了:

已关闭 13 年前。

Possible Duplicate:
Return specific type within Haskell

这段代码:

class Alpha a where
    half :: Real f => a -> f

instance Alpha Double where
    half a = a/2.0

产生此错误:

rigid.hs:6:13:
    Couldn't match expected type `f' against inferred type `Double'
      `f' is a rigid type variable bound by
          the type signature for `half' at rigid.hs:3:17
    In the expression: a / 2.0
    In the definition of `half': half a = a / 2.0
    In the instance declaration for `Alpha Double'
Failed, modules loaded: none.

为什么？

编辑:弄清楚刚性类型的业务，我想我已经找到了一个更有针对性的问题。如何将 Double 转换为满足 Ord 和 Num 的值？也就是说，一个让我得到 (Ord a, Num a) => Double -> a 的函数？

Answer 1

您声称 half 可以将类型 a 转换为任何 Real 类型。但你的 half 只能将 a (Double) 转换为 Double。