parsing - 缩进解析器示例

Question

有人可以发布一个 IndentParser 用法的小例子吗？我希望解析类似 YAML 的输入，如下所示:

fruits:
    apples: yummy
    watermelons: not so yummy

vegetables:
    carrots: are orange
    celery raw: good for the jaw

我知道有一个 YAML 包。我想学习 IndentParser 的用法。

Answer 1

我在下面勾勒出一个解析器，对于您的问题，您可能只需要 block
来自 IndentParser 的解析器。注意我没有尝试运行它，所以它可能有基本错误。

解析器最大的问题不是缩进，而是只有字符串和冒号作为标记。您可能会发现下面的代码需要进行大量调试，因为它必须对不消耗太多输入非常敏感，尽管我试图小心左因子。因为您只有两个 token ，所以您从 Parsec 的 token 模块中获得的好处并不多。

请注意，解析有一个奇怪的事实，即看起来简单的格式通常不容易解析。对于学习，为简单表达式编写解析器将比或多或少的任意文本格式(这可能只会让你感到沮丧)教给你更多的东西。

data DefinitionTree = Nested String [DefinitionTree]
                    | Def String String
  deriving (Show)


-- Note - this might need some testing.
--
-- This is a tricky one, the parser has to parse trailing 
-- spaces and tabs but not a new line.
--
category :: IndentCharParser st String
category = do 
    { a <- body 
    ; rest 
    ; return a
    } 
  where
    body = manyTill1 (letter <|> space) (char ':') 
    rest = many (oneOf [' ', '\t'])

-- Because the DefinitionTree data type has two quite 
-- different constructors, both sharing the same prefix
-- 'category' this combinator is a bit more complicated
-- than usual, and has to use an Either type to descriminate
-- between the options. 
-- 
definition :: IndentCharParser st DefinitionTree
definition = do 
    { a <- category
    ; b <- (textL <|> definitionsR)
    ; case b of
        Left ss -> return (Def a ss)
        Right ds -> return (Nested a ds)
    }

-- Note this should parse a string *provided* it is on 
-- the same line as the category.
--
-- However you might find this assumption needs verifying...
--
textL :: IndentCharParser st (Either DefinitionTrees a)
textL = do 
    { ss <- manyTill1 anyChar "\n" 
    ; return (Left ss)
    }

-- Finally this one uses an indent parser.
--
definitionsR :: IndentCharParser st (Either a [DefinitionTree]) 
definitionsR = block body 
  where 
    body = do { a <- many1 definition; return (Right a) }