在存储路径的同时搜索树

Question

type Pattern = [PatternPart]

data PatternPart =
    MatchTuple [PatternPart] |
    Named String |
    MatchAny    

data ArguementIndex =
    InTuple Int ArguementIndex | -- arguement is in tuple at index Int, then at index...
    ArgIndex Int

testPattern = [Any, MatchTuple[Any, Named "hello"]]

getArgIndex :: Pattern -> String -> Maybe ArguementIndex

我需要编写一个函数 getArgIndex 来搜索 testPattern 中的 "hello" 并返回 InTuple 1 (ArgIndex 1)

getArgIndex [Any, Any, MatchTuple [Named "hi"]] "hi" = Just (InTuple 2 (ArgIndex 0))

另一个例子，

我无法想出一种优雅的方法来做到这一点。

请指教。

Answer 1

你怎么知道只有一场比赛？

尝试这样的事情：（

import Data.Maybe (listToMaybe)

getArgIndex :: Pattern -> String -> Maybe ArguementIndex
getArgIndex haystack needle = listToMaybe (getArgIndices haystack needle)

getArgIndices :: Pattern -> String -> [ArguementIndex]
getArgIndices haystack needle
   = concat $ zipWith f [0..] haystack
  where f i (MatchTuple haystack')        = map (InTuple i) $ getArgIndices haystack' needle
        f i (Named item) | item == needle = [ArgIndex i]
        f i _                             = []

未经测试。）这假设您想要第一个匹配项（如果有多个匹配项）。

（您将需要使用“参数”的正确拼写。）

就我个人而言，如果可以的话，我会将参数按相反的顺序排列：

getArgIndices' :: String -> Pattern -> [ArguementIndex]
getArgIndices' needle = g
  where g = concat . zipWith f [0..]
        f i (MatchTuple haystack)         = map (InTuple i) $ g haystack
        f i (Named item) | item == needle = [ArgIndex i]
        f i _                             = []