Text.Parser.Combinators

Alternative parser combinators

Parsing Combinators

choice :: Alternative m => [m a] -> m a

choice ps tries to apply the parsers in the list ps in order, until one of them succeeds. Returns the value of the succeeding parser.

option :: Alternative m => a -> m a -> m a

option x p tries to apply parser p. If p fails without consuming input, it returns the value x, otherwise the value returned by p.

 priority = option 0 (digitToInt <$> digit)

optional :: Alternative f => f a -> f (Maybe a)

One or none.

skipOptional :: Alternative m => m a -> m ()

skipOptional p tries to apply parser p. It will parse p or nothing. It only fails if p fails after consuming input. It discards the result of p. (Plays the role of parsec's optional, which conflicts with Applicative's optional)

between :: Applicative m => m bra -> m ket -> m a -> m a

between open close p parses open, followed by p and close. Returns the value returned by p.

 braces  = between (symbol "{") (symbol "}")

some :: Alternative f => forall a. f a -> f [a]

One or more.

many :: Alternative f => forall a. f a -> f [a]

Zero or more.

sepBy :: Alternative m => m a -> m sep -> m [a]

sepBy p sep parses zero or more occurrences of p, separated by sep. Returns a list of values returned by p.

 commaSep p  = p `sepBy` (symbol ",")

sepBy1 :: Alternative m => m a -> m sep -> m [a]

sepBy1 p sep parses one or more occurrences of p, separated by sep. Returns a list of values returned by p.

sepEndBy1 :: Alternative m => m a -> m sep -> m [a]

sepEndBy1 p sep parses one or more occurrences of p, separated and optionally ended by sep. Returns a list of values returned by p.

sepEndBy :: Alternative m => m a -> m sep -> m [a]

sepEndBy p sep parses zero or more occurrences of p, separated and optionally ended by sep, ie. haskell style statements. Returns a list of values returned by p.

 haskellStatements  = haskellStatement `sepEndBy` semi

endBy1 :: Alternative m => m a -> m sep -> m [a]

endBy1 p sep parses one or more occurrences of p, seperated and ended by sep. Returns a list of values returned by p.

endBy :: Alternative m => m a -> m sep -> m [a]

endBy p sep parses zero or more occurrences of p, seperated and ended by sep. Returns a list of values returned by p.

  cStatements  = cStatement `endBy` semi

count :: Applicative m => Int -> m a -> m [a]

count n p parses n occurrences of p. If n is smaller or equal to zero, the parser equals to return []. Returns a list of n values returned by p.

chainl :: Alternative m => m a -> m (a -> a -> a) -> a -> m a

chainl p op x parser zero or more occurrences of p, separated by op. Returns a value obtained by a left associative application of all functions returned by op to the values returned by p. If there are zero occurrences of p, the value x is returned.

chainr :: Alternative m => m a -> m (a -> a -> a) -> a -> m a

chainr p op x parser zero or more occurrences of p, separated by op Returns a value obtained by a right associative application of all functions returned by op to the values returned by p. If there are no occurrences of p, the value x is returned.

chainl1 :: Alternative m => m a -> m (a -> a -> a) -> m a

chainl1 p op x parser one or more occurrences of p, separated by op Returns a value obtained by a left associative application of all functions returned by op to the values returned by p. . This parser can for example be used to eliminate left recursion which typically occurs in expression grammars.

 expr   = term   `chainl1` addop
 term   = factor `chainl1` mulop
 factor = parens expr <|> integer

 mulop  = (*) <$ symbol "*"
      <|> div <$ symbol "/"

 addop  = (+) <$ symbol "+"
      <|> (-) <$ symbol "-"

chainr1 :: Alternative m => m a -> m (a -> a -> a) -> m a

chainr1 p op x parser one or more occurrences of |p|, separated by op Returns a value obtained by a right associative application of all functions returned by op to the values returned by p.

manyTill :: Alternative m => m a -> m end -> m [a]

manyTill p end applies parser p zero or more times until parser end succeeds. Returns the list of values returned by p. This parser can be used to scan comments:

 simpleComment   = do{ string "<!--"
                     ; manyTill anyChar (try (string "-->"))
                     }

Note the overlapping parsers anyChar and string "-->", and therefore the use of the try combinator.

Parsing Class

class Alternative m => Parsing m where

Additional functionality needed to describe parsers independent of input type.

Minimal complete definition

try, (<?>)

Methods

try :: m a -> m a

Take a parser that may consume input, and on failure, go back to where we started and fail as if we didn't consume input.

(<?>) :: m a -> String -> m a infixr 0

Give a parser a name

skipMany :: m a -> m ()

A version of many that discards its input. Specialized because it can often be implemented more cheaply.

skipSome :: m a -> m ()

skipSome p applies the parser p one or more times, skipping its result. (aka skipMany1 in parsec)

unexpected :: String -> m a

Used to emit an error on an unexpected token

eof :: m ()

This parser only succeeds at the end of the input. This is not a primitive parser but it is defined using notFollowedBy.

 eof  = notFollowedBy anyChar <?> "end of input"

notFollowedBy :: (Monad m, Show a) => m a -> m ()

notFollowedBy p only succeeds when parser p fails. This parser does not consume any input. This parser can be used to implement the 'longest match' rule. For example, when recognizing keywords (for example let), we want to make sure that a keyword is not followed by a legal identifier character, in which case the keyword is actually an identifier (for example lets). We can program this behaviour as follows:

 keywordLet  = try $ string "let" <* notFollowedBy alphaNum

Instances

Parsing ReadP

Chunk t => Parsing (Parser t)

(Parsing m, Monad m) => Parsing (IdentityT m)

Parsing m => Parsing (Unlined m)

Parsing m => Parsing (Unspaced m)

Parsing m => Parsing (Unhighlighted m)

(Parsing m, MonadPlus m) => Parsing (ReaderT e m)

(Parsing m, MonadPlus m) => Parsing (StateT s m)

(Parsing m, MonadPlus m) => Parsing (StateT s m)

(Parsing m, MonadPlus m, Monoid w) => Parsing (WriterT w m)

(Parsing m, MonadPlus m, Monoid w) => Parsing (WriterT w m)

(Stream s m t, Show t) => Parsing (ParsecT s u m)

(Parsing m, MonadPlus m, Monoid w) => Parsing (RWST r w s m)

(Parsing m, MonadPlus m, Monoid w) => Parsing (RWST r w s m)