如何处理 Haskell 中的“可能 [值]”列表？

Question

为了学习 Haskell，我尝试改编康拉德·巴尔斯基 (Conrad Barski) 的优秀著作《Land of Lisp》中的一些练习。我们的想法是制作一个简单的文本游戏引擎。

具体我试过了：

type Clau = String
type Descripcio = String
type Valors = [String]

-- NOTE : Ideas of types http://learnyouahaskell.com/making-our-own-types-and-typeclasses
data Lloc = Lloc String String String deriving (Show) 
llocSituacio :: Lloc -> String  
llocSituacio (Lloc situacio _ _ ) = situacio  
llocDireccio :: Lloc -> String
llocDireccio (Lloc _ direccio _) = direccio
llocPas :: Lloc -> String
llocPas ( Lloc _ _ pas) = pas

nodes :: [(Clau,Descripcio)]
nodes = [("living-room","you are in the living-room. a wizard is snoring loudly on the couch.")
           ,("garden","you are in a beautiful garden. there is a well in front of you.")
           , ("attic", "you are in the attic. there is a giant welding torch in the corner.")]

edges :: [([Char], [Lloc])]
edges = [ ("living-room", [(Lloc "garden"  "west" "door"), ( Lloc "attic" "upstairs" "ladder") ])
        , ("attic", [(Lloc "living-room"  "east"  "door")])
        , ("garden", [(Lloc "living-room" "east"  "door")])]

describePath :: Lloc -> String  
describePath  e = "There is " ++ llocPas e ++ " going " ++ llocDireccio e ++ " from here." 

起初，它似乎运作良好。例如：

*TextGame> describePath (Lloc "living-room"  "east"  "door")
"There is door going east from here."

但是当我尝试将函数应用到列表时，我得到了这个错误：

situacio = "garden"
map (describePath) (lookup situacio edges)



<interactive>:2:22: error:
    • Couldn't match expected **type ‘[Maybe Lloc]’**
                  with actual **type ‘Maybe [Lloc]’**
    • In the second argument of ‘map’, namely ‘(lookup situacio edges)’
      In the expression: map (describePath) (lookup situacio edges)
      In an equation for ‘it’:
          it = map (describePath) (lookup situacio edges)

错误很明显，但我无法解决它。我想解析 Maybe 的值列表并使用运行良好的函数 describePath 打印路径：

有什么想法吗？此外，如果有人想分享替代方案或觉得此代码可能更符合 Haskell 风格，请随时讨论。

Answer 1

可能有一些更高级的库助手，但我认为您应该首先学习如何以最基本（和一般）的方式处理Maybes：使用模式匹配。

case lookup situacio edges of
   Nothing   -> [] -- not found, how do you want to handle this case?
   Just locs -> map describePath locs

通常，人们希望将结果重新包装在另一个 Maybe 中，例如：

case lookup situacio edges of
   Nothing   -> Nothing -- not found
   Just locs -> Just (map describePath locs)

在这种情况下，我们可以使用库辅助函数来缩短代码：

map describePath <$> lookup situacio edges

Answer 2

非常感谢chi，它工作得很好。我刚刚做了一个新功能：

describePaths situacio edges = case lookup situacio edges of
    Nothing -> []
    Just locs -> map describePath locs

.. 效果很好：

*TextGame> situacio = "living-room"
*TextGame> describePaths situacio edges
["There is door going west from here.","There is ladder going upstairs from here."]

但我发现我并不完全理解 运算符。我关注的是：What does <$> mean in Haskell?

所以我遵循建议：https://hoogle.haskell.org/?hoogle=%3C$%3E

这是一个 Functor：

(<$>) :: Functor f => (a->b) -> f a -> f b

其实和fmap一模一样：

fmap :: Functor f => (a -> b) -> f a -> f b

好像是一样的：

*TextGame> (*2) <$> [1..3]
[2,4,6]
*TextGame> fmap (*2) [1..3]
[2,4,6]

但是，实际上它们是不同的：

*TextGame> map describePath <$> lookup situacio edges
Just ["There is door going west from here.","There is ladder going upstairs from here."]
*TextGame> fmap (describePath) (lookup situacio edges)

<interactive>:30:22: error:
    • Couldn't match type ‘[Lloc]’ with ‘Lloc’
      Expected type: Maybe Lloc
        Actual type: Maybe [Lloc]
        ....

有人可以多说一点吗？我不完全明白为什么'fmap（describePath）（查找位置边缘）'不起作用？ （我正在玩 Maybe's 和 Just's 但是......）