Data.Picoparsec

Simple, efficient combinator parsing for LeftGCDMonoid and FactorialMonoid inputs, loosely based on Parsec and derived from Attoparsec.

Differences from Parsec

Compared to Parsec 3, Picoparsec makes several tradeoffs. It is not intended for, or ideal for, all possible uses.

• While Picoparsec can consume input incrementally, Parsec cannot. Incremental input is a huge deal for efficient and secure network and system programming, since it gives much more control to users of the library over matters such as resource usage and the I/O model to use.
• Much of the performance advantage of Picoparsec is gained via high-performance parsers such as takeWhile and string. If you use complicated combinators that return lists of bytes or characters, there is less performance difference between the two libraries.
• Unlike Parsec 3, Picoparsec does not support being used as a monad transformer.
• Parsec parsers can produce more helpful error messages than Picoparsec parsers. This is a matter of focus: Picoparsec avoids the extra book-keeping in favour of higher performance.
• Parsec comes with built-in support for user state. Picoparsec does not maintain any state by default, in order to maximize performance. If your parsing logic needs depends on it, you can track the state by wrapping your input in a Stateful monoid.

Differences from Attoparsec

Compared to Attoparsec, Picoparsec trades away some performance for generality. Attoparsec works only with ByteString and Text inputs. If your input type is one of these two, Attoparsec is the better choice. Use Picoparsec if you want your parser to be applicable to a different input type, especially if you wish to leave the choice of that input type to the end user.

Some Attoparsec primitives like word8 are missing because they are specific to ByteString inputs. Picoparsec is otherwise largely compatible with Attoparsec, having copied from it both the core logic and the full set of parsing combinators.

Incremental input

Picoparsec supports incremental input, meaning that you can feed it a chunk of input that represents only part of the expected total amount of data to parse. If your parser reaches the end of a fragment of input and could consume more input, it will suspend parsing and return a Partial continuation.

Supplying the Partial continuation with another string will resume parsing at the point where it was suspended. You must be prepared for the result of the resumed parse to be another Partial continuation.

To indicate that you have no more input, supply the Partial continuation with an empty string.

Remember that some parsing combinators will not return a result until they reach the end of input. They may thus cause Partial results to be returned.

If you do not need support for incremental input, consider using the parseOnly function to run your parser. It will never prompt for more input.

Performance considerations

A Picoparsec-based parser applied to a strict ByteString or Text input will generally be somewhat slower than Attoparsec, but if properly optimized and specialized the difference should be less than 50%.

To actually achieve high performance, there are a few guidelines that it is useful to follow.

• Use the input-returning parsers whenever possible, e.g. takeWhile1 instead of many1 anyToken. There is a large difference in performance between the two kinds of parsers.
• If you are parsing textual inputs, use the specialized character parsers; e.g. takeCharsWhile1 instead of takeWhile1.
• If the mappend operation is slow for the input monoid type, it may drastically slow down the parsing of large inputs. Try wrapping the input with the Concat newtype to make the mappend time constant.
• Use the INLINE, INLINABLE, and SPECIALIZE pragmas to optimize the more important parts of your parser for the likely input types.
• Make active use of benchmarking and profiling tools to measure, find the problems with, and improve the performance of your parser.

Parser types

data Parser t a

The core parser type. This is parameterised over the type t of string being processed.

This type is an instance of the following classes:

• Monad, where fail throws an exception (i.e. fails) with an error message.
• Functor and Applicative, which follow the usual definitions.
• MonadPlus, where mzero fails (with no error message) and mplus executes the right-hand parser if the left-hand one fails. When the parser on the right executes, the input is reset to the same state as the parser on the left started with. (In other words, Picoparsec is a backtracking parser that supports arbitrary lookahead.)
• Alternative, which follows MonadPlus.

Instances

Monoid i => Alternative (Parser i)

Monad (Parser t)

Functor (Parser t)

Monoid t => MonadPlus (Parser t)

Applicative (Parser i)

(IsString a, LeftGCDMonoid a, MonoidNull a, (~) * a b) => IsString (Parser a b)

type Result = IResult

data IResult i r

The result of a parse. This is parameterised over the type t of string that was processed.

This type is an instance of Functor, where fmap transforms the value in a Done result.

Constructors

Fail i [String] String

The parse failed. The i parameter is the input that had not yet been consumed when the failure occurred. The [String] is a list of contexts in which the error occurred. The String is the message describing the error, if any.

Partial (i -> IResult i r)

Supply this continuation with more input so that the parser can resume. To indicate that no more input is available, pass an empty string to the continuation.

Note: if you get a Partial result, do not call its continuation more than once.

Done i r

The parse succeeded. The i parameter is the input that had not yet been consumed (if any) when the parse succeeded.

Instances

Functor (IResult i)

(Show i, Show r) => Show (IResult i r)

(NFData i, NFData r) => NFData (IResult i r)

compareResults :: (Eq i, Eq r) => IResult i r -> IResult i r -> Maybe Bool

Compare two IResult values for equality.

If both IResults are Partial, the result will be Nothing, as they are incomplete and hence their equality cannot be known. (This is why there is no Eq instance for IResult.)

Running parsers

parse :: Monoid t => Parser t a -> t -> IResult t a

Run a parser.

feed :: Monoid i => IResult i r -> i -> IResult i r

If a parser has returned a Partial result, supply it with more input.

parseOnly :: Monoid t => Parser t a -> t -> Either String a

Run a parser that cannot be resupplied via a Partial result.

parseWith

Arguments

:: (Monoid t, Monad m)

=> m t

An action that will be executed to provide the parser with more input, if necessary. The action must return an mempty string when there is no more input available.

-> Parser t a

-> t

Initial input for the parser.

-> m (Result t a)

Run a parser with an initial input string, and a monadic action that can supply more input if needed.

parseTest :: (Monoid t, Show t, Show a) => Parser t a -> t -> IO ()

Run a parser and print its result to standard output.

Result conversion

maybeResult :: Result t r -> Maybe r

Convert a Result value to a Maybe value. A Partial result is treated as failure.

eitherResult :: Result t r -> Either String r

Convert a Result value to an Either value. A Partial result is treated as failure.

Parsing individual tokens

anyToken :: FactorialMonoid t => Parser t t

Match any prime input token.

notToken :: (Eq t, FactorialMonoid t) => t -> Parser t t

Match any prime input token except the given one.

peekToken :: FactorialMonoid t => Parser t t

Match any prime input token. Returns mempty if end of input has been reached. Does not consume any input.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

satisfy :: FactorialMonoid t => (t -> Bool) -> Parser t t

The parser satisfy p succeeds for any prime input token for which the predicate p returns True. Returns the token that is actually parsed.

digit = satisfy isDigit
    where isDigit w = w >= "0" && w <= "9"

satisfyWith :: FactorialMonoid t => (t -> a) -> (a -> Bool) -> Parser t a

The parser satisfyWith f p transforms an input token, and succeeds if the predicate p returns True on the transformed value. The parser returns the transformed token that was parsed.

skip :: FactorialMonoid t => (t -> Bool) -> Parser t ()

The parser skip p succeeds for any prime input token for which the predicate p returns True.

skipDigit = skip isDigit
    where isDigit w = w >= "0" && w <= "9"

Parsing individual characters

anyChar :: TextualMonoid t => Parser t Char

Match any character.

char :: TextualMonoid t => Char -> Parser t Char

Match a specific character.

notChar :: TextualMonoid t => Char -> Parser t Char

Match any character except the given one.

peekChar :: TextualMonoid t => Parser t (Maybe Char)

Match any input character, if available. Does not consume any input.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

peekChar' :: TextualMonoid t => Parser t Char

Match any input character, failing if the input doesn't start with any. Does not consume any input.

satisfyChar :: TextualMonoid t => (Char -> Bool) -> Parser t Char

The parser satisfy p succeeds for any input character for which the predicate p returns True. Returns the character that is actually parsed.

digit = satisfy isDigit
    where isDigit w = w >= "0" && w <= "9"

Efficient string handling

scan :: FactorialMonoid t => s -> (s -> t -> Maybe s) -> Parser t t

A stateful scanner. The predicate consumes and transforms a state argument, and each transformed state is passed to successive invocations of the predicate on each token of the input until one returns Nothing or the input ends.

This parser does not fail. It will return an empty string if the predicate returns Nothing on the first prime input factor.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

string :: (LeftGCDMonoid t, MonoidNull t) => t -> Parser t t

string s parses a prefix of input that identically matches s. Returns the parsed string (i.e. s). This parser consumes no input if it fails (even if a partial match).

Note: The behaviour of this parser is different to that of the similarly-named parser in Parsec, as this one is all-or-nothing. To illustrate the difference, the following parser will fail under Parsec given an input of "for":

string "foo" <|> string "for"

The reason for its failure is that the first branch is a partial match, and will consume the letters 'f' and 'o' before failing. In Attoparsec, the above parser will succeed on that input, because the failed first branch will consume nothing.

skipWhile :: FactorialMonoid t => (t -> Bool) -> Parser t ()

Skip past input for as long as the predicate returns True.

take :: FactorialMonoid t => Int -> Parser t t

Consume exactly n prime input tokens.

takeWhile :: FactorialMonoid t => (t -> Bool) -> Parser t t

Consume input as long as the predicate returns True, and return the consumed input.

This parser does not fail. It will return an empty string if the predicate returns False on the first input token.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

takeWhile1 :: FactorialMonoid t => (t -> Bool) -> Parser t t

Consume input as long as the predicate returns True, and return the consumed input.

This parser requires the predicate to succeed on at least one input token: it will fail if the predicate never returns True or if there is no input left.

takeTill :: FactorialMonoid t => (t -> Bool) -> Parser t t

Consume input as long as the predicate returns False (i.e. until it returns True), and return the consumed input.

This parser does not fail. It will return an empty string if the predicate returns True on the first input token.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

Efficient character string handling

scanChars :: TextualMonoid t => s -> (s -> Char -> Maybe s) -> Parser t t

A stateful scanner. The predicate consumes and transforms a state argument, and each transformed state is passed to successive invocations of the predicate on each token of the input until one returns Nothing or the input ends.

This parser does not fail. It will return an empty string if the predicate returns Nothing on the first prime input factor.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

skipCharsWhile :: TextualMonoid t => (Char -> Bool) -> Parser t ()

Skip past input characters for as long as the predicate returns True.

takeCharsWhile :: TextualMonoid t => (Char -> Bool) -> Parser t t

Consume input characters as long as the predicate returns True, and return the consumed input.

This parser does not fail. It will return an empty string if the predicate returns False on the first input token.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

takeCharsWhile1 :: TextualMonoid t => (Char -> Bool) -> Parser t t

takeCharsTill :: TextualMonoid t => (Char -> Bool) -> Parser t t

Consume input characters as long as the predicate returns False (i.e. until it returns True), and return the consumed input.

This parser does not fail. It will return an empty string if the predicate returns True on the first input token.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

takeTillChar :: TextualMonoid t => (Char -> Bool) -> Parser t t

Consume all input until the character for which the predicate returns True and return the consumed input.

The only difference between takeCharsTill and takeTillChar is in their handling of non-character data: The former never consumes it, the latter always does.

This parser does not fail. It will return an empty string if the predicate returns True on the first input token.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

takeTillChar1 :: TextualMonoid t => (Char -> Bool) -> Parser t t

Consume all input until the character for which the predicate returns True and return the consumed input.

This parser always consumes at least one token: it will fail if the input starts with a character for which the predicate returns True or if there is no input left.

Consume all remaining input

takeRest :: MonoidNull t => Parser t t

Consume all remaining input and return it as a single string.

Text parsing

endOfLine :: (Eq t, TextualMonoid t) => Parser t ()

Match either a single newline character '\n', or a carriage return followed by a newline character "\r\n".

Combinators

try :: Parser i a -> Parser i a

Attempt a parse, and if it fails, rewind the input so that no input appears to have been consumed.

This combinator is provided for compatibility with Parsec. Picoparsec parsers always backtrack on failure.

(<?>) infix 0

Arguments

:: Parser i a

-> String

the name to use if parsing fails

-> Parser i a

Name the parser, in case failure occurs.

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

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

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

Apply the given action repeatedly, returning every result.

option :: Alternative f => a -> f a -> f a

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

priority  = option 0 (digitToInt <$> digit)

many' :: MonadPlus m => m a -> m [a]

many' p applies the action p zero or more times. Returns a list of the returned values of p. The value returned by p is forced to WHNF.

 word  = many' letter

many1 :: Alternative f => f a -> f [a]

many1 p applies the action p one or more times. Returns a list of the returned values of p.

 word  = many1 letter

many1' :: MonadPlus m => m a -> m [a]

many1' p applies the action p one or more times. Returns a list of the returned values of p. The value returned by p is forced to WHNF.

 word  = many1' letter

manyTill :: Alternative f => f a -> f b -> f [a]

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

 simpleComment   = string "<!--" *> manyTill anyChar (string "-->")

(Note the overlapping parsers anyChar and string "-->". While this will work, it is not very efficient, as it will cause a lot of backtracking.)

manyTill' :: MonadPlus m => m a -> m b -> m [a]

manyTill' p end applies action p zero or more times until action end succeeds, and returns the list of values returned by p. This can be used to scan comments:

 simpleComment   = string "<!--" *> manyTill' anyChar (string "-->")

(Note the overlapping parsers anyChar and string "-->". While this will work, it is not very efficient, as it will cause a lot of backtracking.)

The value returned by p is forced to WHNF.

sepBy :: Alternative f => f a -> f s -> f [a]

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

commaSep p  = p `sepBy` (char ',')

sepBy' :: MonadPlus m => m a -> m s -> m [a]

sepBy' p sep applies zero or more occurrences of p, separated by sep. Returns a list of the values returned by p. The value returned by p is forced to WHNF.

commaSep p  = p `sepBy'` (char ',')

sepBy1 :: Alternative f => f a -> f s -> f [a]

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

commaSep p  = p `sepBy1` (char ',')

sepBy1' :: MonadPlus m => m a -> m s -> m [a]

sepBy1' p sep applies one or more occurrences of p, separated by sep. Returns a list of the values returned by p. The value returned by p is forced to WHNF.

commaSep p  = p `sepBy1'` (char ',')

skipMany :: Alternative f => f a -> f ()

Skip zero or more instances of an action.

skipMany1 :: Alternative f => f a -> f ()

Skip one or more instances of an action.

eitherP :: Alternative f => f a -> f b -> f (Either a b)

Combine two alternatives.

State observation and manipulation functions

endOfInput :: MonoidNull t => Parser t ()

Match only if all input has been consumed.

atEnd :: MonoidNull t => Parser t Bool

Return an indication of whether the end of input has been reached.