哈斯克爾只允許正輸入

Question

我需要定義一個Haskell函數：

func :: Int -> Int 
func 1 = 1 
func 2 = 2 
func x = x+1 

因此，它只允許正數。我已經有一個看一個類似的問題：Non-negative integers

寫了這樣：

newtype Positive a = Positive a 

toPositive :: (Num a, Ord a) => a -> Positive a 
toPositive x 
    | x < 0 = error "number cannot be negative" 
    | otherwise = Positive x 

func :: Positive a -> a 
func (Positive n) = n 

然而這已經引發錯誤。思考？

更新：

錯誤示例：

*Main> func 1 

<interactive>:32:6: 
    No instance for (Num (Positive a0)) arising from the literal `1' 
    Possible fix: add an instance declaration for (Num (Positive a0)) 
    In the first argument of `func', namely `1' 
    In the expression: func 1 
    In an equation for `it': it = func 1 
*Main> 

Answer 1

你忘了打電話給toPositive到Int轉換爲Positive。這樣稱呼：

func $ toPositive 1 

此外，Haskell的一個怪癖是其處理負數文字。爲避免與減法操作符混淆，必須將它們包裝在圓括號中：

func $ toPositive (-1)