问题
今天遇到一个问题，不知道怎么解决。这对我来说很奇怪，因为我写的代码应该（根据我目前的知识）是正确的。
因此，您可以在下面找到示例解析器组合器。最重要的是pOperator，它以非常简单的方式（仅用于演示目的）构建了一个运算符 AST。它消耗“x”并且可以消耗多个由空格分隔的“x”。
我还有pParens组合器，其定义如下：
pPacked pParenL (pWSpaces *> pParenR)
所以它在关闭括号之前消耗空格。
样本输入/输出
正确的输入/输出应该是：
in: "(x)"
out: Single "x"
in: "(x )"
out: Single "x"
但我得到：
in: "(x)"
out: Single "x"
in: "(x )"
out: Multi (Single "x") (Single "x")
-- Correcting steps:
-- Inserted 'x' at position LineColPos 0 3 3 expecting one of ['\t', ' ', 'x']
但是在第二个示例中，我遇到了错误-解析器的行为就像它贪婪地吃掉了一些令牌（并且没有贪婪的操作）。
我会感谢任何帮助。
示例代码
import Prelude hiding(lex)
import Data.Char hiding (Space)
import qualified Text.ParserCombinators.UU as UU
import Text.ParserCombinators.UU hiding(parse)
import qualified Text.ParserCombinators.UU.Utils as Utils
import Text.ParserCombinators.UU.BasicInstances hiding (Parser)
data El = Multi El El
| Single String
deriving (Show)
---------- Example core grammar ----------
pElement = Single <$> pSyms "x"
pOperator = applyAll <$> pElement <*> pMany (flip <$> (Multi <$ pWSpaces1) <*> pElement)
---------- Basic combinators ----------
applyAll x (f:fs) = applyAll (f x) fs
applyAll x [] = x
pSpace = pSym ' '
pTab = pSym '\t'
pWSpace = pSpace <|> pTab
pWSpaces = pMany pWSpace
pWSpaces1 = pMany1 pWSpace
pMany1 p = (:) <$> p <*> pMany p
pSyms [] = pReturn []
pSyms (x : xs) = (:) <$> pSym x <*> pSyms xs
pParenL = Utils.lexeme $ pSym '('
pParenR = Utils.lexeme $ pSym ')'
pParens = pPacked pParenL (pWSpaces *> pParenR)
---------- Program ----------
pProgram = pParens pOperator
-- if you replace it with following line, it works:
-- pProgram = pParens pElement
-- so it seems like something in pOperator is greedy
tests = [ ("test", "(x)")
, ("test", "(x )")
]
---------- Helpers ----------
type Parser a = P (Str Char String LineColPos) a
parse p s = UU.parse ( (,) <$> p <*> pEnd) (createStr (LineColPos 0 0 0) s)
main :: IO ()
main = do
mapM_ (\(desc, p) -> putStrLn ("\n=== " ++ desc ++ " ===") >> run pProgram p) tests
return ()
run :: Show t => Parser t -> String -> IO ()
run p inp = do let (a, errors) = parse p inp
putStrLn ("-- Result: \n" ++ show a)
if null errors then return ()
else do putStr ("-- Correcting steps: \n")
show_errors errors
putStrLn "-- "
where show_errors :: (Show a) => [a] -> IO ()
show_errors = sequence_ . (map (putStrLn . show))
重要的
pOperator = applyAll <$> pElement <*> pMany (flip <$> (Multi <$ pWSpaces1) <*> pElement)
相当于：
foldr pChainl pElement (Multi <$ pWSpaces1)
根据：组合器解析：简短教程
它用于定义运算符优先级。