我正在尝试使用Parsec解析Haskell中基于缩进的语言(想想Python,Haskell本身,Boo,YAML)。我已经看过IndentParser库,看起来它是完美的匹配,但我无法弄清楚如何将我的TokenParser
变成缩进解析器。这是我到目前为止的代码:
import qualified Text.ParserCombinators.Parsec.Token as T
import qualified Text.ParserCombinators.Parsec.IndentParser.Token as IT
lexer = T.makeTokenParser mylangDef
ident = IT.identifier lexer
这会引发错误:
parser2.hs:29:28:
Couldn't match expected type `IT.TokenParser st'
against inferred type `T.GenTokenParser s u m'
In the first argument of `IT.identifier', namely `lexer'
In the expression: IT.identifier lexer
In the definition of `ident': ident = IT.identifier lexer
我做错了什么?我该如何创建IT.TokenParser
?或者IndentParser是否被破坏并被避免?
答案 0 :(得分:12)
看起来你在这里使用Parsec 3,而IndentParser期望Parsec 2.你的例子用-package parsec-2.1.0.1
为我编译。
因此IndentParser不一定会被破坏,但是作者应该更加具体地说明依赖列表中的版本。可以安装两个版本的Parsec,因此除非您因为其他原因而决定使用Parsec 3,否则不应该使用IndentParser。
更新:实际上,为了让IdentParser使用Parsec 3,不需要对源进行任何更改。我们双方都遇到的问题似乎是cabal-install
已经导致的问题Parsec 2的"soft preference"。您可以在Parsec版本上使用显式约束重新安装IndentParser:
cabal install IndentParser --reinstall --constraint="parsec >= 3"
答案 1 :(得分:6)
这是我为Parsec 3整理的一组解析器组合器,可用于Haskell样式布局,可能对您有用。关键考虑因素是laidout
启动并运行布局规则,并且您应该使用提供的space
和spaced
组合器而不是库存Parsec
组合器用于相同目的。由于布局和注释的交互,我不得不将注释解析合并到tokenizer中。
{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-}
module Text.Parsec.Layout
( laidout -- repeat a parser in layout, separated by (virtual) semicolons
, space -- consumes one or more spaces, comments, and onside newlines in a layout rule
, maybeFollowedBy
, spaced -- (`maybeFollowedBy` space)
, LayoutEnv -- type needed to describe parsers
, defaultLayoutEnv -- a fresh layout
, semi -- semicolon or virtual semicolon
) where
import Control.Applicative ((<$>))
import Control.Monad (guard)
import Data.Char (isSpace)
import Text.Parsec.Combinator
import Text.Parsec.Pos
import Text.Parsec.Prim hiding (State)
import Text.Parsec.Char hiding (space)
data LayoutContext = NoLayout | Layout Int deriving (Eq,Ord,Show)
data LayoutEnv = Env
{ envLayout :: [LayoutContext]
, envBol :: Bool -- if true, must run offside calculation
}
defaultLayoutEnv :: LayoutEnv
defaultLayoutEnv = Env [] True
pushContext :: Stream s m c => LayoutContext -> ParsecT s LayoutEnv m ()
pushContext ctx = modifyState $ \env -> env { envLayout = ctx:envLayout env }
popContext :: Stream s m c => String -> ParsecT s LayoutEnv m ()
popContext loc = do
(_:xs) <- envLayout <$> getState
modifyState $ \env' -> env' { envLayout = xs }
<|> unexpected ("empty context for " ++ loc)
getIndentation :: Stream s m c => ParsecT s LayoutEnv m Int
getIndentation = depth . envLayout <$> getState where
depth :: [LayoutContext] -> Int
depth (Layout n:_) = n
depth _ = 0
pushCurrentContext :: Stream s m c => ParsecT s LayoutEnv m ()
pushCurrentContext = do
indent <- getIndentation
col <- sourceColumn <$> getPosition
pushContext . Layout $ max (indent+1) col
maybeFollowedBy :: Stream s m c => ParsecT s u m a -> ParsecT s u m b -> ParsecT s u m a
t `maybeFollowedBy` x = do t' <- t; optional x; return t'
spaced :: Stream s m Char => ParsecT s LayoutEnv m a -> ParsecT s LayoutEnv m a
spaced t = t `maybeFollowedBy` space
data Layout = VSemi | VBrace | Other Char deriving (Eq,Ord,Show)
-- TODO: Parse C-style #line pragmas out here
layout :: Stream s m Char => ParsecT s LayoutEnv m Layout
layout = try $ do
bol <- envBol <$> getState
whitespace False (cont bol)
where
cont :: Stream s m Char => Bool -> Bool -> ParsecT s LayoutEnv m Layout
cont True = offside
cont False = onside
-- TODO: Parse nestable {-# LINE ... #-} pragmas in here
whitespace :: Stream s m Char =>
Bool -> (Bool -> ParsecT s LayoutEnv m Layout) -> ParsecT s LayoutEnv m Layout
whitespace x k =
try (string "{-" >> nested k >>= whitespace True)
<|> try comment
<|> do newline; whitespace True offside
<|> do tab; whitespace True k
<|> do (satisfy isSpace <?> "space"); whitespace True k
<|> k x
comment :: Stream s m Char => ParsecT s LayoutEnv m Layout
comment = do
string "--"
many (satisfy ('\n'/=))
newline
whitespace True offside
nested :: Stream s m Char =>
(Bool -> ParsecT s LayoutEnv m Layout) ->
ParsecT s LayoutEnv m (Bool -> ParsecT s LayoutEnv m Layout)
nested k =
try (do string "-}"; return k)
<|> try (do string "{-"; k' <- nested k; nested k')
<|> do newline; nested offside
<|> do anyChar; nested k
offside :: Stream s m Char => Bool -> ParsecT s LayoutEnv m Layout
offside x = do
p <- getPosition
pos <- compare (sourceColumn p) <$> getIndentation
case pos of
LT -> do
popContext "the offside rule"
modifyState $ \env -> env { envBol = True }
return VBrace
EQ -> return VSemi
GT -> onside x
-- we remained onside.
-- If we skipped any comments, or moved to a new line and stayed onside, we return a single a ' ',
-- otherwise we provide the next char
onside :: Stream s m Char => Bool -> ParsecT s LayoutEnv m Layout
onside True = return $ Other ' '
onside False = do
modifyState $ \env -> env { envBol = False }
Other <$> anyChar
layoutSatisfies :: Stream s m Char => (Layout -> Bool) -> ParsecT s LayoutEnv m ()
layoutSatisfies p = guard . p =<< layout
virtual_lbrace :: Stream s m Char => ParsecT s LayoutEnv m ()
virtual_lbrace = pushCurrentContext
virtual_rbrace :: Stream s m Char => ParsecT s LayoutEnv m ()
virtual_rbrace = try (layoutSatisfies (VBrace ==) <?> "outdent")
-- recognize a run of one or more spaces including onside carriage returns in layout
space :: Stream s m Char => ParsecT s LayoutEnv m String
space = do
try $ layoutSatisfies (Other ' ' ==)
return " "
<?> "space"
-- recognize a semicolon including a virtual semicolon in layout
semi :: Stream s m Char => ParsecT s LayoutEnv m String
semi = do
try $ layoutSatisfies p
return ";"
<?> "semi-colon"
where
p VSemi = True
p (Other ';') = True
p _ = False
lbrace :: Stream s m Char => ParsecT s LayoutEnv m String
lbrace = do
char '{'
pushContext NoLayout
return "{"
rbrace :: Stream s m Char => ParsecT s LayoutEnv m String
rbrace = do
char '}'
popContext "a right brace"
return "}"
laidout :: Stream s m Char => ParsecT s LayoutEnv m a -> ParsecT s LayoutEnv m [a]
laidout p = try (braced statements) <|> vbraced statements where
braced = between (spaced lbrace) (spaced rbrace)
vbraced = between (spaced virtual_lbrace) (spaced virtual_rbrace)
statements = p `sepBy` spaced semi