試圖創建Haskell Eq實例

我想爲我創建的列表類型創建一個Eq實例。只有在兩個列表平均值相等時，Eq纔會返回true。試圖創建Haskell Eq實例

average :: (Real a, Fractional b) => [a] -> b 
average xs 
    | xs == [] = 0 
    | otherwise = realToFrac (sum xs)/genericLength xs 

data NumList a = Nlist [a] 

instance Eq (NumList a) where 
    (Nlist x) == (Nlist y) = (average x) == (average y)`

但是當我嘗試編譯此，我得到錯誤：

No instance for (Real a) arising from a use of ‘average’ 
Possible fix: 
    add (Real a) to the context of the instance declaration 
In the first argument of ‘(==)’, namely ‘(average x)’ 
In the expression: (average x) == (average y) 
In an equation for ‘==’: 
(Nlist x) == (Nlist y) = (average x) == (average y)

我不擅長與Haskell和努力學習它，能有人幫我這個錯誤？

來源

2016-11-20 Lukas Karmanovas

如果你有'NumList Banana'？你如何取得一串香蕉？ – MathematicalOrchid

我明白我應該制定某種實例約束，但不知道如何製作它。如何告訴它，它只能是數字列表？ –

你需要同樣類型的約束你的公式例如：

instance (Real a) => Eq (NumList a) where 
    (Nlist x) == (Nlist y) = (average x) == (average y)`

來源

2016-11-20 09:56:50 I3ck

該死的我很愚蠢......我正在嘗試'Eq（NumList a）=>（真實的a）'非常感謝 –

試圖創建Haskell Eq實例

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