Text.Parser.Combinators

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 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)

One or none.

It is useful for modelling any computation that is allowed to fail.

Examples

Using the Alternative instance of Control.Monad.Except, the following functions:

>>> import Control.Monad.Except

>>> canFail = throwError "it failed" :: Except String Int>>> final = return 42                :: Except String Int

Can be combined by allowing the first function to fail:

>>> runExcept $ canFail *> finalLeft "it failed"

>>> runExcept $ optional canFail *> finalRight 42

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

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

p `surroundedBy` f is p surrounded by f. Shortcut for between f f p. As in between, returns the value returned by p.

One or more.

Examples

>>> some (putStr "la")lalalalalalalalala... * goes on forever *

>>> some Nothingnothing

>>> take 5 <$> some (Just 1)* hangs forever *

Note that this function can be used with Parsers based on Applicatives. In that case some parser will attempt to parse parser one or more times until it fails.

Zero or more.

Examples

>>> many (putStr "la")lalalalalalalalala... * goes on forever *

>>> many NothingJust []

>>> take 5 <$> many (Just 1)* hangs forever *

Note that this function can be used with Parsers based on Applicatives. In that case many parser will attempt to parse parser zero or more times until it fails.

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

sepByNonEmpty p sep parses one or more occurrences of p, separated by sep. Returns a non-empty list of values returned by p.

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

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

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

endByNonEmpty p sep parses one or more occurrences of p, separated and ended by sep. Returns a non-empty list of values returned by p.

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

  cStatements  = cStatement `endBy` semi

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 p op x parses 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 p op x parses 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 p op x parses 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 p op x parses 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 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.

Additional functionality needed to describe parsers independent of input type.