Text.ParserCombinators.Parsec.Prim

Copyright	(c) Paolo Martini 2007
License	BSD-style (see the LICENSE file)
Maintainer	[email protected]
Stability	provisional
Portability	portable
Safe Haskell	Safe
Language	Haskell2010

(<?>) :: ParsecT s u m a -> String -> ParsecT s u m a infix 0 Source

The parser p <?> msg behaves as parser p, but whenever the parser p fails without consuming any input, it replaces expect error messages with the expect error message msg.

This is normally used at the end of a set alternatives where we want to return an error message in terms of a higher level construct rather than returning all possible characters. For example, if the expr parser from the try example would fail, the error message is: '...: expecting expression'. Without the (<?>) combinator, the message would be like '...: expecting "let" or letter', which is less friendly.

(<|>) :: ParsecT s u m a -> ParsecT s u m a -> ParsecT s u m a infixr 1 Source

This combinator implements choice. The parser p <|> q first applies p. If it succeeds, the value of p is returned. If p fails without consuming any input, parser q is tried. This combinator is defined equal to the mplus member of the MonadPlus class and the (<|>) member of Alternative.

The parser is called predictive since q is only tried when parser p didn't consume any input (i.e.. the look ahead is 1). This non-backtracking behaviour allows for both an efficient implementation of the parser combinators and the generation of good error messages.

type Parser = Parsec String () Source

type GenParser tok st = Parsec [tok] st Source

runParser :: GenParser tok st a -> st -> SourceName -> [tok] -> Either ParseError a Source

parse :: Stream s Identity t => Parsec s () a -> SourceName -> s -> Either ParseError a Source

parse p filePath input runs a parser p over Identity without user state. The filePath is only used in error messages and may be the empty string. Returns either a ParseError (Left) or a value of type a (Right).

 main    = case (parse numbers "" "11, 2, 43") of
            Left err  -> print err
            Right xs  -> print (sum xs)

 numbers = commaSep integer

parseFromFile :: Parser a -> FilePath -> IO (Either ParseError a) Source

parseFromFile p filePath runs a string parser p on the input read from filePath using readFile. Returns either a ParseError (Left) or a value of type a (Right).

 main    = do{ result <- parseFromFile numbers "digits.txt"
             ; case result of
                 Left err  -> print err
                 Right xs  -> print (sum xs)
             }

parseTest :: (Stream s Identity t, Show a) => Parsec s () a -> s -> IO () Source

The expression parseTest p input applies a parser p against input input and prints the result to stdout. Used for testing parsers.

token Source

The parser token showTok posFromTok testTok accepts a token t with result x when the function testTok t returns Just x. The source position of the t should be returned by posFromTok t and the token can be shown using showTok t.

This combinator is expressed in terms of tokenPrim. It is used to accept user defined token streams. For example, suppose that we have a stream of basic tokens tupled with source positions. We can then define a parser that accepts single tokens as:

 mytoken x
   = token showTok posFromTok testTok
   where
     showTok (pos,t)     = show t
     posFromTok (pos,t)  = pos
     testTok (pos,t)     = if x == t then Just t else Nothing

tokens :: (Stream s m t, Eq t) => ([t] -> String) -> (SourcePos -> [t] -> SourcePos) -> [t] -> ParsecT s u m [t] Source

tokenPrim Source

The parser tokenPrim showTok nextPos testTok accepts a token t with result x when the function testTok t returns Just x. The token can be shown using showTok t. The position of the next token should be returned when nextPos is called with the current source position pos, the current token t and the rest of the tokens toks, nextPos pos t toks.

This is the most primitive combinator for accepting tokens. For example, the char parser could be implemented as:

 char c
   = tokenPrim showChar nextPos testChar
   where
     showChar x        = "'" ++ x ++ "'"
     testChar x        = if x == c then Just x else Nothing
     nextPos pos x xs  = updatePosChar pos x

tokenPrimEx :: Stream s m t => (t -> String) -> (SourcePos -> t -> s -> SourcePos) -> Maybe (SourcePos -> t -> s -> u -> u) -> (t -> Maybe a) -> ParsecT s u m a Source

try :: GenParser tok st a -> GenParser tok st a Source

label :: ParsecT s u m a -> String -> ParsecT s u m a Source

A synonym for <?>, but as a function instead of an operator.

labels :: ParsecT s u m a -> [String] -> ParsecT s u m a Source

unexpected :: Stream s m t => String -> ParsecT s u m a Source

The parser unexpected msg always fails with an unexpected error message msg without consuming any input.

The parsers fail, (<?>) and unexpected are the three parsers used to generate error messages. Of these, only (<?>) is commonly used. For an example of the use of unexpected, see the definition of notFollowedBy.

pzero :: GenParser tok st a Source

many :: ParsecT s u m a -> ParsecT s u m [a] Source

many p applies the parser p zero or more times. Returns a list of the returned values of p.

 identifier  = do{ c  <- letter
                 ; cs <- many (alphaNum <|> char '_')
                 ; return (c:cs)
                 }

skipMany :: ParsecT s u m a -> ParsecT s u m () Source

skipMany p applies the parser p zero or more times, skipping its result.

 spaces  = skipMany space

getState :: Monad m => ParsecT s u m u Source

Returns the current user state.

setState :: Monad m => u -> ParsecT s u m () Source

An alias for putState for backwards compatibility.

updateState :: Monad m => (u -> u) -> ParsecT s u m () Source

An alias for modifyState for backwards compatibility.

getPosition :: Monad m => ParsecT s u m SourcePos Source

Returns the current source position. See also SourcePos.

setPosition :: Monad m => SourcePos -> ParsecT s u m () Source

setPosition pos sets the current source position to pos.

getInput :: Monad m => ParsecT s u m s Source

Returns the current input

setInput :: Monad m => s -> ParsecT s u m () Source

setInput input continues parsing with input. The getInput and setInput functions can for example be used to deal with #include files.

data State s u Source

getParserState :: Monad m => ParsecT s u m (State s u) Source

Returns the full parser state as a State record.

setParserState :: Monad m => State s u -> ParsecT s u m (State s u) Source

setParserState st set the full parser state to st.