Haskell 条件错误

Question

我的 Haskell 代码有问题，我有以下问题:

takeMeHere cs m =
    |(find (==E) cs == Nothing && (checkNextStep pozxCrt+1 pozyCrt m) == True) = (Just E,[(pozxCrt+1,pozyCrt)] ++ m)
    |(find (==S) cs == Nothing && (checkNextStep pozxCrt pozyCrt-1 m) == True) = (Just S,[(pozxCrt,pozyCrt-1)] ++ m)
    |(find (==W) cs == Nothing && (checkNextStep pozxCrt-1 pozyCrt m) == True) = (Just W,[(pozxCrt-1,pozyCrt)] ++ m)
    |(find (==N) cs == Nothing && (checkNextStep pozxCrt pozyCrt+1 m) == True) = (Just N,[(pozxCrt,pozyCrt+1)] ++ m)
    |otherwise = (Nothing,m)
where
    pozxCrt=fst(head m)
    pozyCrt=snd(head m)

checkNextStep x y m = if(find (== (x,y)) m == Nothing) then True
   else False

我在输入“|”时遇到解析错误。如果我用 if then else if then 之类的东西编写代码......它就有效。但我想用 |以获得更紧凑的编码。这里似乎有什么问题？

Answer 1

要修复解析错误，请从第一行删除=。 = 符号放在守卫后面。

接下来，您应该缩进“where”

takeMeHere cs m
    |(find (==E) cs == Nothing && (checkNextStep pozxCrt+1 pozyCrt m) == True) = (Just E,[(pozxCrt+1,pozyCrt)] ++ m)
    |(find (==S) cs == Nothing && (checkNextStep pozxCrt pozyCrt-1 m) == True) = (Just S,[(pozxCrt,pozyCrt-1)] ++ m)
    |(find (==W) cs == Nothing && (checkNextStep pozxCrt-1 pozyCrt m) == True) = (Just W,[(pozxCrt-1,pozyCrt)] ++ m)
    |(find (==N) cs == Nothing && (checkNextStep pozxCrt pozyCrt+1 m) == True) = (Just N,[(pozxCrt,pozyCrt+1)] ++ m)
    |otherwise = (Nothing,m)
  where
    pozxCrt=fst(head m)
    pozyCrt=snd(head m)

这至少会解析，但不会编译。 (checkNextStep pozxCrt pozyCrt+1 m) 应为 (checkNextStep pozxCrt (pozyCrt+1) m)。

让我补充一下，您可以修复很多冗长的代码:

• find (==E) cs == Nothing 可以更改为 E `notElem` x
• 您不需要与 True 进行比较:将 x == True 更改为 x
• if x then True else False 可以更改为 x
• [x]++y 可以更改为 x:y
• 您可以使用如下模式匹配:(pozxCrt, pozyCrt) = head m 或 (pozxCrt, pozyCrt):_ = m

结果是:

takeMeHere cs m                                                                 
    | E `notElem` cs && checkNextStep (pozxCrt+1) pozyCrt m = (Just E,(pozxCrt+1,pozyCrt):m)
    | S `notElem` cs && checkNextStep pozxCrt (pozyCrt-1) m = (Just S,(pozxCrt,pozyCrt-1):m)
    | W `notElem` cs && checkNextStep (pozxCrt-1) pozyCrt m = (Just W,(pozxCrt-1,pozyCrt):m)
    | N `notElem` cs && checkNextStep pozxCrt (pozyCrt+1) m = (Just N,(pozxCrt,pozyCrt+1):m)
    | otherwise = (Nothing,m)                                                   
  where                                                                         
    (pozxCrt, pozyCrt) = head m                                                 

checkNextStep x y m = (x,y) `notElem` m

守卫中有很多重复的地方。大量重复是创造新功能的标志。

move E (x, y) = (x+1, y) 
move S (x, y) = (x, y-1)
move N (x, y) = (x, y+1)
move W (x, y) = (x-1, y)

takeMeHere cs m
    | canGo E = go E
    | canGo S = go S
    | canGo W = go W
    | canGo N = go N
    | otherwise = (Nothing,m)
  where
    pos = head m
    canGo dir = dir `notElem` cs && checkNextStep (move dir pos) m
    go dir = (Just dir, move dir pos:m)

checkNextStep (x, y) m = (x,y) `notElem` m

下一步:使用find canGo [E,S,W,N]摆脱守卫:

 takeMeHere cs m =                                                               
    case find canGo [E,S,W,N] of                                                
      Just dir -> (Just dir, move dir pos:m)                                    
      Nothing -> (Nothing, m) 
    where ...