Control.Monad.Combinators

An associative binary operation

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

The identity of <|>

empty <|> a     == a
a     <|> empty == a

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

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

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.

choice = asum

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.

See also: skipCount, count'.

count' m n p parses from m to n occurrences of p. If n is not positive or m > n, the parser equals to return []. Returns a list of parsed values.

Please note that m may be negative, in this case effect is the same as if it were equal to zero.

See also: skipCount, count.

Combine two alternatives.

eitherP a b = (Left <$> a) <|> (Right <$> b)

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` semicolon

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

many p applies the parser p zero or more times and returns a list of the values returned by p.

identifier = (:) <$> letter <*> many (alphaNumChar <|> char '_')

manyTill p end applies parser p zero or more times until parser end succeeds. Returns the list of values returned by p. Note that end result is consumed and lost. Use manyTill_ if you wish to keep it.

See also: skipMany, skipManyTill.

manyTill_ p end applies parser p zero or more times until parser end succeeds. Returns the list of values returned by p and the end result. Use manyTill if you have no need in the result of the end.

See also: skipMany, skipManyTill.

some p applies the parser p one or more times and returns a list of the values returned by p.

word = some letter

someTill p end works similarly to manyTill p end, but p should succeed at least once. Note that end result is consumed and lost. Use someTill_ if you wish to keep it.

someTill p end = liftM2 (:) p (manyTill p end)

See also: skipSome, skipSomeTill.

someTill_ p end works similarly to manyTill_ p end, but p should succeed at least once. Use someTill if you have no need in the result of the end.

See also: skipSome, skipSomeTill.

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

option x p = p <|> pure x

See also: optional.

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` comma

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

sepEndBy p sep parses zero or more occurrences of p, separated and optionally ended by sep. Returns a 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.

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

See also: manyTill, skipManyTill.

skipSome p applies the parser p one or more times, skipping its result.

See also: someTill, skipSomeTill.

skipCount n p parses n occurrences of p, skipping its result. If n is smaller or equal to zero, the parser equals to return ().

See also: count, count'.

skipManyTill p end applies the parser p zero or more times skipping results until parser end succeeds. Result parsed by end is then returned.

See also: manyTill, skipMany.

skipSomeTill p end applies the parser p one or more times skipping results until parser end succeeds. Result parsed by end is then returned.

See also: someTill, skipSome.