Haskell-類型匹配

Question

遇到了麻煩，從一個輔助功能輸出到我的函數的輸出相匹配，我用下面的代碼我：

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) 

功能semiRight不會，儘管我盡了最大努力匹配。有什麼想法嗎？

Answer 1

僅從類型來看，x被匹配到具有類型的[Maybe Player]一個元件，

你應該明顯通過[X]但不x到getSemiDiag。此外，一些括號是多餘的。

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 

爲什麼不直接使用zipWith