型铸造在Haskell，分数和Int

Question

我写了一个函数，（应该）采取布尔的无限列表和计算时真亦假值在第一n个元素的比例：

prob n list = foldr (+) 0 (map boolToInt (take n list))/n 
    where boolToInt b 
     | b == True = 1 
     | otherwise = 0 

不幸这不是工作：

No instance for (Fractional Int) 
    arising from a use of `/' 
Possible fix: add an instance declaration for (Fractional Int) 
In the expression: foldr (+) 0 (map boolToInt (take n list))/n 
In an equation for `prob': 
    prob n list 
     = foldr (+) 0 (map boolToInt (take n list))/n 
     where 
      boolToInt b 
      | b == True = 1 
      | otherwise = 0 
Failed, modules loaded: none. 

我试图做一个转换，但不工作之一：

prob n list = foldr (+) 0 (map boolToInt (take (fromIntegral (toInteger n)) list))/n 
    where boolToInt b 
     | b == True = 1 
     | otherwise = 0 

它的编制，但只要我尝试调用的函数，我得到一个错误：

*Main> prob 50 toCoin1 
<interactive>:1:6: 
Ambiguous type variable `a0' in the constraints: 
    (Num a0) arising from the literal `50' at <interactive>:1:6-7 
    (Integral a0) arising from a use of `prob' at <interactive>:1:1-4 
    (Fractional a0) arising from a use of `prob' at <interactive>:1:1-4 
Probable fix: add a type signature that fixes these type variable(s) 
In the first argument of `prob', namely `50' 
In the expression: prob 50 toCoin1 
In an equation for `it': it = prob 50 toCoin1 

有什么建议？

Answer 1

您正在转换错误的地方。尝试在整个foldr和n附近附上fromRational。

prob n list = fromIntegral count/fromIntegral n 
    where count = foldr (+) 0 (map boolToInt (take n list)) 
      boolToInt b 
      | b == True = 1 
      | otherwise = 0 

哦，你boolToInt功能是相同的fromEnum专门到Bool秒。

prob n list = fromIntegral count/fromIntegral n 
    where count = foldr (+) 0 (map fromEnum (take n list)) 

---

与你试图做什么的根本问题是你强加给第一个参数prob相互冲突的要求。您使用toInteger约束n为Integral，但其在/中的使用要求它是Fractional，并且没有类型是Integral和Fractional。

Answer 2

Bool是Enum的一个实例，所以boolToInt已由fromEnum提供。此外，foldr做sum，因此整个功能可以简化为：

prob n list = (fromIntegral . sum . map fromEnum . take n) list/fromIntegral n 

我们能抽象出用于计算平均值（尽管这将需要确定列表的长度的代码，我们已经知道）：

mean xs = (fromIntegral . sum) xs/(fromIntegral . length) xs 

prob n = mean . map fromEnum . take n