haskell - 尝试在 Haskell 中构建词法分析程序

Question

我正在开始这个项目，但进展甚微。我刚开始使用haskell，所以我只想使用我实现的功能，以便我将大脑付诸行动。

我想要两个字符串列表，一个代表代码，另一个为给定的语言保留名称。我希望能够从第一个列表中过滤所说的单词。

resNames = ["function","while","for","const","let","this","true","false",";","=", "()", "var"]

findVal :: [String]->String->[String]
findVal z " " = z
findVal [] value = []
findVal (x:y) value = if x == value then findVal y value
                      else x:(findVal y value) 

filterResNames :: [String]->[String]->[String]
filterResNames z [] = z
filterResNames [] z = []
filterResNames (x:y) (u:v) = if x== u then findVal (x:y) (u) else x:(filterResNames y (u:v))```

This obviously doesn't work because the program stops as soon as it finds a match...

Answer 1

如果我正确解释您的想法，这可能是一个可能的解决方案:

resNames = ["function","while","for","const","let","this","true","false",";","=", "()", "var"]

-- Determine whether an element is in a list
findVal :: String->[String]->Bool
findVal _ [] = False
findVal x (namesH:namesT) =
  if x == namesH
  then True
  else (findVal x namesT) 

-- Loop over input list and for each element, if not in reserved list, add to accumulator
filterResNamesAcc :: [String]->[String]->[String]->[String]
filterResNamesAcc _ [] acc = acc
filterResNamesAcc [] _ acc = acc
filterResNamesAcc (inH:inT) names acc =
  if (findVal inH names)
  then (filterResNamesAcc inT names acc)
  else (filterResNamesAcc inT names (inH:acc))

-- Invoke filterResNamesAcc with empty accumulator
filterResNames :: [String]->[String]->[String]
filterResNames inL names = reverse (filterResNamesAcc inL names [])

main = do
   print (filterResNames ["hello", "for", "the", "world"] resNames)

我们在这里使用的是函数式语言中使用的典型累加器模式。 filterResNamesAcc 的“问题”是使用 :前置到累加器会导致结果反转，因此调用 reverse .

留给“读者”:

实现自己的反向或找出不需要它的更复杂的折叠模式。

更改 filterResNames 的签名取一个字符串并进行标记化。