Haskell-類型匹配

Question

我無法從輔助函數獲取輸出以匹配我正在使用以下代碼的函數輸出：

getSemiDiag :: [[Maybe Player]] -> Int -> Int -> [Maybe Player]
getSemiDiag [] _ _ = []
getSemiDiag (x:xs) start size = if start > (size -1)
                            then []
                            else (x !! start) : (getSemiDiag xs (start+1) size)



semiRight :: [[Maybe Player]] -> Int -> Int -> [[Maybe Player]]
semiRight [] _ _ = []
semiRight (x:xs) start size = if start > (size -1)
                          then []
                          else (getSemiDiag x start size) : (semiRight xs (start+1) size)

盡管我已經盡力了，功能semiRight還是無法匹配。 有什么想法嗎？

Answer 1

僅根據類型判斷， x會匹配具有[Maybe Player]類型的元素，

您顯然應該將[x]而不是x傳遞給getSemiDiag。 此外，有些括號是多余的。

semiRight :: [[Maybe Player]] -> Int -> Int -> [[Maybe Player]]
semiRight [] _ _ = []
semiRight (x:xs) start size = if start > (size -1)
                          then []
                          else getSemiDiag [x] start size : semiRight xs (start+1) size

為什么不只使用zipWith

semiRight players start size = zipWith (\p i -> [p !! i]) players [start .. size]