Question

在学习Haskell时，我试图编写一个给定数字的函数，将其Collatz sequence的后继者：

next :: (Fractional a, Integral a) => a -> a
next x
    | odd x = x * 3 + 1
    | otherwise = x / 2

当我运行next 7时，我得到：

<interactive>:150:1: error:
* Ambiguous type variable `a0' arising from a use of `print'
  prevents the constraint `(Show a0)' from being solved.
  Probable fix: use a type annotation to specify what `a0' should be.
  These potential instances exist:
    instance Show Ordering -- Defined in `GHC.Show'
    instance Show Integer -- Defined in `GHC.Show'
    instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
    ...plus 22 others
    ...plus 12 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
* In a stmt of an interactive GHCi command: print it

两个问题：

我在签名中使用了最好的类限制吗？
假设我是，我如何show next 7的结果？

Answer 1

我相信Collatz序列是一个整数序列，因此不需要将结果作为分数。

next :: (Integral a) => a -> a

为了能够从除法中获取整数，您应该使用div函数。请注意，除非您只划分偶数，否则除法总是精确的：

next x
    | odd x = x * 3 + 1
    | otherwise = x `div` 2

模糊类型变量`a0＆＃39;使用'print＆＃39;？

1 个答案: