无法从算术序列`(x - n + 1)... x'推断出(枚举a)

Question

当我尝试编译这段代码时

prod [] = 1
prod (x:xs) = x * prod xs

ff :: (Num a) => a -> a -> a
ff x n = prod [(x - n + 1) .. x]

我收到以下错误:

a.hs:5:15:
    Could not deduce (Enum a)
     arising from the arithmetic sequence `(x - n + 1) .. x'
    from the context (Num a)
      bound by the type signature for ff :: Num a => a -> a -> a
      at a.hs:5:1-32
    Possible fix:
      add (Enum a) to the context of
        the type signature for ff :: Num a => a -> a -> a
    In the first argument of `prod', namely `[(x - n + 1) .. x]'
    In the expression: prod [(x - n + 1) .. x]
    In an equation for `ff': ff x n = prod [(x - n + 1) .. x]

这段代码有什么问题？当我替换Double一切都没事的时候.

Answer 1

[i .. j]是简写enumFromTo i j.enumFromTo是Enum类型类的一部分,而不是Num(你仍然需要Num使用+,-但是)的一部分.

所以你需要说a实现Enum以及实现Num:

ff :: (Num a, Enum a) => a -> a -> a
ff x n = prod [(x - n + 1) .. x]

它适用于Double因为Double实现这两个类型.