我在使用 Megaparsec 6 的makeExprParser
助手时遇到问题。我似乎无法弄清楚如何以我期望的优先级绑定二进制^
和一元。-
使用这个makeExprParser
表达式解析器:
expressionParser :: Parser Expression
expressionParser =
makeExprParser termParser
[
[InfixR $ BinOp BinaryExp <$ symbol "^"],
[
Prefix $ MonOp MonoMinus <$ symbol "-",
Prefix $ MonOp MonoPlus <$ symbol "+"
],
[
InfixL $ BinOp BinaryMult <$ symbol "*",
InfixL $ BinOp BinaryDiv <$ symbol "/"
],
[
InfixL $ BinOp BinaryPlus <$ symbol "+",
InfixL $ BinOp BinaryMinus <$ symbol "-"
]
]
我希望这些测试能够通过:
testEqual expressionParser "1^2" "(1)^(2)"
testEqual expressionParser "-1^2" "-(1^2)"
testEqual expressionParser "1^-2" "1^(-2)"
testEqual expressionParser "-1^-2" "-(1^(-2))"
也就是说,-1^-2
应该解析为与-(1^(-2))
. 这就是例如 Python 解析它的方式:
>>> 2**-2
0.25
>>> -2**-2
-0.25
>>> -2**2
-4
和红宝石:
irb(main):004:0> 2**-2
=> (1/4)
irb(main):005:0> -2**-2
=> (-1/4)
irb(main):006:0> -2**2
=> -4
但是这个 Megaparsec 解析器根本无法解析1^-2
,而是给了我有用的错误:
(TrivialError (SourcePos {sourceName = \"test.txt\", sourceLine = Pos 1, sourceColumn = Pos 3} :| []) (Just (Tokens ('-' :| \"\"))) (fromList [Tokens ('(' :| \"\"),Label ('i' :| \"nteger\")]))")
我读到它说“我本可以在这里拍摄任何这些角色,但这让-
我感到困惑”。
如果我像这样调整运算符表的某些优先级(在一元后移动指数 - ):
expressionParser =
makeExprParser termParser
[
[
Prefix $ MonOp MonoMinus <$ symbol "-",
Prefix $ MonOp MonoPlus <$ symbol "+"
],
[InfixR $ BinOp BinaryExp <$ symbol "^"],
[
InfixL $ BinOp BinaryMult <$ symbol "*",
InfixL $ BinOp BinaryDiv <$ symbol "/"
],
[
InfixL $ BinOp BinaryPlus <$ symbol "+",
InfixL $ BinOp BinaryMinus <$ symbol "-"
]
]
然后我不再遇到解析失败,而是-1^2
错误地解析为(-1)^2
(而不是正确的-(1^2)
)。
这是一个完整的自包含解析器来显示问题(它需要 HUnit,当然还有 megaparsec):
module Hascas.Minimal where
import Data.Void (Void)
import Test.HUnit hiding (test)
import Text.Megaparsec hiding (ParseError)
import Text.Megaparsec.Char
import Text.Megaparsec.Expr
import qualified Text.Megaparsec as MP
import qualified Text.Megaparsec.Char.Lexer as L
data Expression
= Literal Integer
| MonOp MonoOperator Expression
| BinOp BinaryOperator Expression Expression
deriving (Read, Show, Eq, Ord)
data BinaryOperator
= BinaryPlus
| BinaryMinus
| BinaryDiv
| BinaryMult
| BinaryExp
deriving (Read, Show, Eq, Ord)
data MonoOperator
= MonoPlus
| MonoMinus
deriving (Read, Show, Eq, Ord)
type Parser a = Parsec Void String a
type ParseError = MP.ParseError (Token String) Void
spaceConsumer :: Parser ()
spaceConsumer = L.space space1 lineComment blockComment
where
lineComment = L.skipLineComment "//"
blockComment = L.skipBlockComment "/*" "*/"
lexeme :: Parser a -> Parser a
lexeme = L.lexeme spaceConsumer
symbol :: String -> Parser String
symbol = L.symbol spaceConsumer
expressionParser :: Parser Expression
expressionParser =
makeExprParser termParser
[
[InfixR $ BinOp BinaryExp <$ symbol "^"],
[
Prefix $ MonOp MonoMinus <$ symbol "-",
Prefix $ MonOp MonoPlus <$ symbol "+"
],
[
InfixL $ BinOp BinaryMult <$ symbol "*",
InfixL $ BinOp BinaryDiv <$ symbol "/"
],
[
InfixL $ BinOp BinaryPlus <$ symbol "+",
InfixL $ BinOp BinaryMinus <$ symbol "-"
]
]
termParser :: Parser Expression
termParser = (
(try $ Literal <$> L.decimal)
<|> (try $ parens expressionParser))
parens :: Parser a -> Parser a
parens x = between (symbol "(") (symbol ")") x
main :: IO ()
main = do
-- just to show that it does work in the + case:
test expressionParser "1+(-2)" $
BinOp BinaryPlus (Literal 1) (MonOp MonoMinus $ Literal 2)
test expressionParser "1+-2" $
BinOp BinaryPlus (Literal 1 ) (MonOp MonoMinus $ Literal 2)
-- but not in the ^ case
test expressionParser "1^-2" $
BinOp BinaryExp (Literal 1) (MonOp MonoMinus $ Literal 2)
test expressionParser "-1^2" $
MonOp MonoMinus $ BinOp BinaryExp (Literal 1) (Literal 2)
test expressionParser "-1^-2" $
MonOp MonoMinus $ BinOp BinaryExp (Literal 1) (MonOp MonoMinus $ Literal 2)
-- exponent precedence is weird
testEqual expressionParser "1^2" "(1)^(2)"
testEqual expressionParser "-1^2" "-(1^2)"
testEqual expressionParser "1^-2" "1^(-2)"
testEqual expressionParser "-1^-2" "-(1^(-2))"
testEqual expressionParser "1^2^3^4" "1^(2^(3^(4))))"
where
test :: (Eq a, Show a) => Parser a -> String -> a -> IO ()
test parser input expected = do
assertEqual input (Right expected) $ parse (spaceConsumer >> parser <* eof) "test.txt" input
testEqual :: (Eq a, Show a) => Parser a -> String -> String -> IO ()
testEqual parser input expected = do
assertEqual input (p expected) (p input)
where
p i = parse (spaceConsumer >> parser <* eof) "test.txt" i
是否有可能让 Megaparsec 以其他语言的优先级解析这些运算符?