在Haskell中跟踪节点二进制树的路径

Question

我是Haskell的新手，我试图从根跟踪二叉树中节点的路径。 我在SO中关注了这个链接： - Trying to implement path record for Haskell binary tree search

我修改了代码以便更好地理解

inTree:: Eq a => a -> Tree a -> Maybe [Direction]
inTree x End = Nothing
inTree x (Node y l r ) = if (x == y) then  Just [] else (fmap (L:) (inTree x (Node _ l _ )) <|> (fmap (R:) (inTree x (Node _ _ r))))

但是当我尝试编译它时出现以下错误： -

 Not in scope: ‘<|>’
    Perhaps you meant one of these:
      ‘<$>’ (imported from Prelude), ‘<*>’ (imported from Prelude)

任何人都可以帮我解决这个问题

此外，任何人都可以在上面的代码中解释3rd line。 什么是L:，什么是R:？

编辑 -

阅读完评论后，我尝试使用case construct，但我遇到了以下错误： -

inTree:: Eq a => a -> Tree a -> Maybe [Direction] 
inTree x End = Nothing
inTree x (Node y l r ) | x == y = Just [] | l\=Empty = L:(inTree x l)

错误Couldn't match expected type ‘Maybe Direction’ with actual type ‘[Maybe Direction] -> [Maybe Direction]