有没有办法在列表中间模式匹配值？还是在最后？

Question

就像是

getFirstError :: [Either a b] -> a
getFirstError (x:y:...:Left w:z) = w

wrt Haskell但是知道其他语言模式匹配如何实现这一点可能会很有趣.

Answer 1

尽管有其他答案,您可以使用GHC中的视图模式扩展来执行此操作:

   getFirstError ((msum . map test) -> Just x) = x
        where test (Left x) = Just x
              test (Right x) = Nothing

或者使用图案防护:

   getFirstError (xs) | Just x <- (msum $ map test xs) = x
        where test (Left x) = Just x
              test (Right) x = Nothing