型鑄造在Haskell，分數和Int

Question

我寫了一個函數，（應該）採取布爾的無限列表和計算時真亦假值在第一n個元素的比例：

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

不幸這不是工作：

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. 

我試圖做一個轉換，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 

它的編制，但只要我嘗試調用的函數，我得到一個錯誤：

*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 

有什麼建議？

Answer 1

您正在轉換錯誤的地方。嘗試在整個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專門到Bool秒。

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

---

與你試圖做什麼的根本問題是你強加給第一個參數prob相互衝突的要求。您使用toInteger約束n爲Integral，但其在/中的使用要求它是Fractional，並且沒有類型是Integral和Fractional。

Answer 2

Bool是Enum的一個實例，所以boolToInt已由fromEnum提供。此外，foldr做sum，因此整個功能可以簡化爲：

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

我們能抽象出用於計算平均值（儘管這將需要確定列表的長度的代碼，我們已經知道）：

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

prob n = mean . map fromEnum . take n