功能類型限制

Question

一般情況下，最好是對某個函數使用最嚴格或最寬鬆的類型定義嗎？每種方法的優缺點是什麼？我發現當我使用嚴格雙打重寫my pearson correlation code時，我更容易寫出，跟隨和推理（這可能只是缺乏經驗）。但是我也可以看到，如果有更廣泛的類型定義會使這些功能更普遍適用。更嚴格的類型定義是否被視爲技術債務的一種形式？

隨着類型類：

import Data.List 

mean :: Fractional a => [a] -> a 
mean xs = s/n 
    where 
     (s , n) = foldl' k (0,0) xs 
     k (s, n) x = s `seq` n `seq` (s + x, n + 1) 

covariance :: Fractional a => [a] -> [a] -> a 
covariance xs ys = mean productXY 
    where 
    productXY = zipWith (*) [x - mx | x <- xs] [y - my | y <- ys] 
    mx  = mean xs 
    my  = mean ys 

stddev :: Floating a => [a] -> a 
stddev xs = sqrt (covariance xs xs) 

pearson :: RealFloat a => [a] -> [a] -> a 
pearson x y = fifthRound $ covariance x y/(stddev x * stddev y) 

pearsonMatrix :: RealFloat a => [[a]] -> [[a]] 
pearsonMatrix (x:xs) = [pearson x y | y <- x:xs]:(pearsonMatrix xs) 
pearsonMatrix [] = [] 

fifthRound :: RealFrac a => a -> a 
fifthRound x = (/100000) $ fromIntegral $ round (x * 100000) 

隨着雙打：

import Data.List 

mean :: [Double] -> Double 
mean xs = s/n 
    where 
     (s , n) = foldl' k (0,0) xs 
     k (s, n) x = s `seq` n `seq` (s + x, n + 1) 

covariance :: [Double] -> [Double] -> Double 
covariance xs ys = mean productXY 
    where 
    productXY = zipWith (*) [x - mx | x <- xs] [y - my | y <- ys] 
    mx  = mean xs 
    my  = mean ys 

stddev :: [Double] -> Double 
stddev xs = sqrt (covariance xs xs) 

pearson :: [Double] -> [Double] -> Double 
pearson x y = fifthRound (covariance x y/(stddev x * stddev y)) 

pearsonMatrix :: [[Double]] -> [[Double]] 
pearsonMatrix (x:xs) = [pearson x y | y <- x:xs]:(pearsonMatrix xs) 
pearsonMatrix [] = [] 

fifthRound :: Double -> Double 
fifthRound x = (/100000) $ fromIntegral $ round (x * 100000) 

Answer 1

可讀性是見仁見智。一般來說，我發現更通用的類型簽名更具可讀性，因爲可能的定義更少（有時甚至只有一個非發散定義）。例如，看到mean僅具有Fractional約束立即限制在該函數中執行的操作（與可能對我所知的所有操作執行操作的Double版本相比）。當然，泛化類型is not alwaysmore readable。 （And just for fun）

具有功能更一般的版本的主要缺點是，它們可能在運行時保持未優化，使得的Floating功能Double的字典具有每次調用時要傳遞給mean。

您可以通過添加一個SPECIALIZE pragma來擁有最好的世界。這告訴編譯器基本上覆制你的函數代碼與一些實例化的類型變量。如果你知道你只用Double將被調用你mean功能相當多，那麼這就是我會做什麼

{-# SPECIALIZE mean :: [Double] -> Double #-} 
mean :: Fractional a => [a] -> a 
mean xs = s/n 
    where 
    (s , n) = foldl' k (0,0) xs 
    k (s, n) x = s `seq` n `seq` (s + x, n + 1) 

而且你能看到簽名的專用版本在你的代碼呢！好極了！