Text.Megaparsec

This module includes everything you need to get started writing a parser. If you are new to Megaparsec and don't know where to begin, take a look at the tutorial https://markkarpov.com/tutorial/megaparsec.html.

In addition to the Text.Megaparsec module, which exports and re-exports almost everything that you may need, we advise to import Text.Megaparsec.Char if you plan to work with a stream of Char tokens or Text.Megaparsec.Byte if you intend to parse binary data.

It is common to start working with the library by defining a type synonym like this:

type Parser = Parsec Void Text
                     ^    ^
                     |    |
Custom error component    Input stream type

Then you can write type signatures like Parser Int—for a parser that returns an Int for example.

Similarly (since it's known to cause confusion), you should use ParseErrorBundle type parametrized like this:

ParseErrorBundle Text Void
                 ^    ^
                 |    |
 Input stream type    Custom error component (the same you used in Parser)

Megaparsec uses some type-level machinery to provide flexibility without compromising on type safety. Thus type signatures are sometimes necessary to avoid ambiguous types. If you're seeing an error message that reads like “Type variable e0 is ambiguous …”, you need to give an explicit signature to your parser to resolve the ambiguity. It's a good idea to provide type signatures for all top-level definitions.

Re-exports

Note that we re-export monadic combinators from Control.Monad.Combinators because these are more efficient than Applicative-based ones. Thus many and some may clash with the functions from Control.Applicative. You need to hide the functions like this:

import Control.Applicative hiding (many, some)

Also note that you can import Control.Monad.Combinators.NonEmpty if you wish that combinators like some return NonEmpty lists. The module lives in the parser-combinators package (you need at least version 0.4.0).

This module is intended to be imported qualified:

import qualified Control.Monad.Combinators.NonEmpty as NE

Other modules of interest are:

• Control.Monad.Combinators.Expr for parsing of expressions.
• Control.Applicative.Permutations for parsing of permutations phrases.

module Text.Megaparsec.Pos

module Text.Megaparsec.Error

module Text.Megaparsec.Stream

module Control.Monad.Combinators

Data types

data State s e

This is the Megaparsec's state parametrized over stream type s and custom error component type e.

Constructors

State

Fields

• stateInput :: s

  The rest of input to process

• stateOffset :: !Int

  Number of processed tokens so far

  Since: 7.0.0

• statePosState :: PosState s

  State that is used for line/column calculation

  Since: 7.0.0

• stateParseErrors :: [ParseError s e]

  Collection of “delayed” ParseErrors in reverse order. This means that the last registered error is the first element of the list.

  Since: 8.0.0

Instances

(Data e, Data (ParseError s e), Data s) => Data (State s e)

Defined in Text.Megaparsec.State

Methods

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> State s e -> c (State s e)

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (State s e)

toConstr :: State s e -> Constr

dataTypeOf :: State s e -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (State s e))

dataCast2 :: Typeable t => (forall d e0. (Data d, Data e0) => c (t d e0)) -> Maybe (c (State s e))

gmapT :: (forall b. Data b => b -> b) -> State s e -> State s e

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> State s e -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> State s e -> r

gmapQ :: (forall d. Data d => d -> u) -> State s e -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> State s e -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> State s e -> m (State s e)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> State s e -> m (State s e)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> State s e -> m (State s e)

Generic (State s e)

Defined in Text.Megaparsec.State

Associated Types

type Rep (State s e) :: Type -> Type

Methods

from :: State s e -> Rep (State s e) x

to :: Rep (State s e) x -> State s e

(Show (ParseError s e), Show s) => Show (State s e)

Defined in Text.Megaparsec.State

Methods

showsPrec :: Int -> State s e -> ShowS

show :: State s e -> String

showList :: [State s e] -> ShowS

(NFData s, NFData (ParseError s e)) => NFData (State s e)

Defined in Text.Megaparsec.State

Methods

rnf :: State s e -> ()

(Eq (ParseError s e), Eq s) => Eq (State s e)

Defined in Text.Megaparsec.State

Methods

(==) :: State s e -> State s e -> Bool

(/=) :: State s e -> State s e -> Bool

type Rep (State s e)

Defined in Text.Megaparsec.State

type Rep (State s e) = D1 ('MetaData "State" "Text.Megaparsec.State" "megaparsec-9.5.0-nV6NFlDOdsDHucVPlnquI" 'False) (C1 ('MetaCons "State" 'PrefixI 'True) ((S1 ('MetaSel ('Just "stateInput") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 s) :*: S1 ('MetaSel ('Just "stateOffset") 'SourceUnpack 'SourceStrict 'DecidedStrict) (Rec0 Int)) :*: (S1 ('MetaSel ('Just "statePosState") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (PosState s)) :*: S1 ('MetaSel ('Just "stateParseErrors") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 [ParseError s e]))))

data PosState s

A special kind of state that is used to calculate line/column positions on demand.

Since: 7.0.0

Constructors

PosState

Fields

• pstateInput :: s

  The rest of input to process

• pstateOffset :: !Int

  Offset corresponding to beginning of pstateInput

• pstateSourcePos :: !SourcePos

  Source position corresponding to beginning of pstateInput

• pstateTabWidth :: Pos

  Tab width to use for column calculation

• pstateLinePrefix :: String

  Prefix to prepend to offending line

Instances

Data s => Data (PosState s)

Defined in Text.Megaparsec.State

Methods

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> PosState s -> c (PosState s)

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (PosState s)

toConstr :: PosState s -> Constr

dataTypeOf :: PosState s -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (PosState s))

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (PosState s))

gmapT :: (forall b. Data b => b -> b) -> PosState s -> PosState s

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> PosState s -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> PosState s -> r

gmapQ :: (forall d. Data d => d -> u) -> PosState s -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> PosState s -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> PosState s -> m (PosState s)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> PosState s -> m (PosState s)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> PosState s -> m (PosState s)

Generic (PosState s)

Defined in Text.Megaparsec.State

Associated Types

type Rep (PosState s) :: Type -> Type

Methods

from :: PosState s -> Rep (PosState s) x

to :: Rep (PosState s) x -> PosState s

Show s => Show (PosState s)

Defined in Text.Megaparsec.State

Methods

showsPrec :: Int -> PosState s -> ShowS

show :: PosState s -> String

showList :: [PosState s] -> ShowS

NFData s => NFData (PosState s)

Defined in Text.Megaparsec.State

Methods

rnf :: PosState s -> ()

Eq s => Eq (PosState s)

Defined in Text.Megaparsec.State

Methods

(==) :: PosState s -> PosState s -> Bool

(/=) :: PosState s -> PosState s -> Bool

type Rep (PosState s)

Defined in Text.Megaparsec.State

type Rep (PosState s) = D1 ('MetaData "PosState" "Text.Megaparsec.State" "megaparsec-9.5.0-nV6NFlDOdsDHucVPlnquI" 'False) (C1 ('MetaCons "PosState" 'PrefixI 'True) ((S1 ('MetaSel ('Just "pstateInput") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 s) :*: S1 ('MetaSel ('Just "pstateOffset") 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Int)) :*: (S1 ('MetaSel ('Just "pstateSourcePos") 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 SourcePos) :*: (S1 ('MetaSel ('Just "pstateTabWidth") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 Pos) :*: S1 ('MetaSel ('Just "pstateLinePrefix") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 String)))))

type Parsec e s = ParsecT e s Identity

Parsec is a non-transformer variant of the more general ParsecT monad transformer.

data ParsecT e s m a

ParsecT e s m a is a parser with custom data component of error e, stream type s, underlying monad m and return type a.

Instances

(Ord e, Stream s) => MonadParsec e s (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

parseError :: ParseError s e -> ParsecT e s m a

label :: String -> ParsecT e s m a -> ParsecT e s m a

hidden :: ParsecT e s m a -> ParsecT e s m a

try :: ParsecT e s m a -> ParsecT e s m a

lookAhead :: ParsecT e s m a -> ParsecT e s m a

notFollowedBy :: ParsecT e s m a -> ParsecT e s m ()

withRecovery :: (ParseError s e -> ParsecT e s m a) -> ParsecT e s m a -> ParsecT e s m a

observing :: ParsecT e s m a -> ParsecT e s m (Either (ParseError s e) a)

eof :: ParsecT e s m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> ParsecT e s m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> ParsecT e s m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> ParsecT e s m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> ParsecT e s m (Tokens s)

takeP :: Maybe String -> Int -> ParsecT e s m (Tokens s)

getParserState :: ParsecT e s m (State s e)

updateParserState :: (State s e -> State s e) -> ParsecT e s m ()

mkParsec :: (State s e -> Reply e s a) -> ParsecT e s m a

(VisualStream s, ShowErrorComponent e) => MonadParsecDbg e s (ParsecT e s m)

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> ParsecT e s m a -> ParsecT e s m a

(Stream s, MonadError e' m) => MonadError e' (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

throwError :: e' -> ParsecT e s m a

catchError :: ParsecT e s m a -> (e' -> ParsecT e s m a) -> ParsecT e s m a

(Stream s, MonadReader r m) => MonadReader r (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

ask :: ParsecT e s m r

local :: (r -> r) -> ParsecT e s m a -> ParsecT e s m a

reader :: (r -> a) -> ParsecT e s m a

(Stream s, MonadState st m) => MonadState st (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

get :: ParsecT e s m st

put :: st -> ParsecT e s m ()

state :: (st -> (a, st)) -> ParsecT e s m a

(Stream s, MonadWriter w m) => MonadWriter w (ParsecT e s m)

Since: 9.5.0

Defined in Text.Megaparsec.Internal

Methods

writer :: (a, w) -> ParsecT e s m a

tell :: w -> ParsecT e s m ()

listen :: ParsecT e s m a -> ParsecT e s m (a, w)

pass :: ParsecT e s m (a, w -> w) -> ParsecT e s m a

Stream s => MonadTrans (ParsecT e s)

Defined in Text.Megaparsec.Internal

Methods

lift :: Monad m => m a -> ParsecT e s m a

Stream s => MonadFail (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

fail :: String -> ParsecT e s m a

(Stream s, MonadFix m) => MonadFix (ParsecT e s m)

Since: 6.0.0

Defined in Text.Megaparsec.Internal

Methods

mfix :: (a -> ParsecT e s m a) -> ParsecT e s m a

(Stream s, MonadIO m) => MonadIO (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

liftIO :: IO a -> ParsecT e s m a

(Ord e, Stream s) => Alternative (ParsecT e s m)

empty is a parser that fails without consuming input.

Defined in Text.Megaparsec.Internal

Methods

empty :: ParsecT e s m a

(<|>) :: ParsecT e s m a -> ParsecT e s m a -> ParsecT e s m a

some :: ParsecT e s m a -> ParsecT e s m [a]

many :: ParsecT e s m a -> ParsecT e s m [a]

Stream s => Applicative (ParsecT e s m)

pure returns a parser that succeeds without consuming input.

Defined in Text.Megaparsec.Internal

Methods

pure :: a -> ParsecT e s m a

(<*>) :: ParsecT e s m (a -> b) -> ParsecT e s m a -> ParsecT e s m b

liftA2 :: (a -> b -> c) -> ParsecT e s m a -> ParsecT e s m b -> ParsecT e s m c

(*>) :: ParsecT e s m a -> ParsecT e s m b -> ParsecT e s m b

(<*) :: ParsecT e s m a -> ParsecT e s m b -> ParsecT e s m a

Functor (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

fmap :: (a -> b) -> ParsecT e s m a -> ParsecT e s m b

(<$) :: a -> ParsecT e s m b -> ParsecT e s m a

Stream s => Monad (ParsecT e s m)

return returns a parser that succeeds without consuming input.

Defined in Text.Megaparsec.Internal

Methods

(>>=) :: ParsecT e s m a -> (a -> ParsecT e s m b) -> ParsecT e s m b

(>>) :: ParsecT e s m a -> ParsecT e s m b -> ParsecT e s m b

return :: a -> ParsecT e s m a

(Ord e, Stream s) => MonadPlus (ParsecT e s m)

mzero is a parser that fails without consuming input.

Note: strictly speaking, this instance is unlawful. The right identity law does not hold, e.g. in general this is not true:

v >> mzero = mero

However the following holds:

try v >> mzero = mzero

Defined in Text.Megaparsec.Internal

Methods

mzero :: ParsecT e s m a

mplus :: ParsecT e s m a -> ParsecT e s m a -> ParsecT e s m a

(Stream s, MonadCont m) => MonadCont (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

callCC :: ((a -> ParsecT e s m b) -> ParsecT e s m a) -> ParsecT e s m a

(a ~ Tokens s, IsString a, Eq a, Stream s, Ord e) => IsString (ParsecT e s m a)

Since: 6.3.0

Defined in Text.Megaparsec.Internal

Methods

fromString :: String -> ParsecT e s m a

(Stream s, Monoid a) => Monoid (ParsecT e s m a)

Since: 5.3.0

Defined in Text.Megaparsec.Internal

Methods

mempty :: ParsecT e s m a

mappend :: ParsecT e s m a -> ParsecT e s m a -> ParsecT e s m a

mconcat :: [ParsecT e s m a] -> ParsecT e s m a

(Stream s, Semigroup a) => Semigroup (ParsecT e s m a)

Since: 5.3.0

Defined in Text.Megaparsec.Internal

Methods

(<>) :: ParsecT e s m a -> ParsecT e s m a -> ParsecT e s m a

sconcat :: NonEmpty (ParsecT e s m a) -> ParsecT e s m a

stimes :: Integral b => b -> ParsecT e s m a -> ParsecT e s m a

Running parser

parse

Arguments

:: Parsec e s a

Parser to run

-> String

Name of source file

-> s

Input for parser

-> Either (ParseErrorBundle s e) a

parse p file input runs parser p over Identity (see runParserT if you're using the ParsecT monad transformer; parse itself is just a synonym for runParser). It returns either a ParseErrorBundle (Left) or a value of type a (Right). errorBundlePretty can be used to turn ParseErrorBundle into the string representation of the error message. See Text.Megaparsec.Error if you need to do more advanced error analysis.

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

numbers = decimal `sepBy` char ','

parse is the same as runParser.

parseMaybe :: (Ord e, Stream s) => Parsec e s a -> s -> Maybe a

parseMaybe p input runs the parser p on input and returns the result inside Just on success and Nothing on failure. This function also parses eof, so if the parser doesn't consume all of its input, it will fail.

The function is supposed to be useful for lightweight parsing, where error messages (and thus file names) are not important and entire input should be consumed. For example, it can be used for parsing of a single number according to a specification of its format.

parseTest

Arguments

:: (ShowErrorComponent e, Show a, VisualStream s, TraversableStream s)

=> Parsec e s a

Parser to run

-> s

Input for parser

-> IO ()

The expression parseTest p input applies the parser p on the input input and prints the result to stdout. Useful for testing.

runParser

Arguments

:: Parsec e s a

Parser to run

-> String

Name of source file

-> s

Input for parser

-> Either (ParseErrorBundle s e) a

runParser p file input runs parser p on the input stream of tokens input, obtained from source file. The file is only used in error messages and may be the empty string. Returns either a ParseErrorBundle (Left) or a value of type a (Right).

parseFromFile p file = runParser p file <$> readFile file

runParser is the same as parse.

runParser'

Arguments

:: Parsec e s a

Parser to run

-> State s e

Initial state

-> (State s e, Either (ParseErrorBundle s e) a)

The function is similar to runParser with the difference that it accepts and returns the parser state. This allows us e.g. to specify arbitrary textual position at the beginning of parsing. This is the most general way to run a parser over the Identity monad.

Since: 4.2.0

runParserT

Arguments

:: Monad m

=> ParsecT e s m a

Parser to run

-> String

Name of source file

-> s

Input for parser

-> m (Either (ParseErrorBundle s e) a)

runParserT p file input runs parser p on the input list of tokens input, obtained from source file. The file is only used in error messages and may be the empty string. Returns a computation in the underlying monad m that returns either a ParseErrorBundle (Left) or a value of type a (Right).

runParserT'

Arguments

:: Monad m

=> ParsecT e s m a

Parser to run

-> State s e

Initial state

-> m (State s e, Either (ParseErrorBundle s e) a)

This function is similar to runParserT, but like runParser' it accepts and returns parser state. This is thus the most general way to run a parser.

Since: 4.2.0

Primitive combinators

class (Stream s, MonadPlus m) => MonadParsec e s m | m -> e s where

Type class describing monads that implement the full set of primitive parsers.

Note that the following primitives are “fast” and should be taken advantage of as much as possible if your aim is a fast parser: tokens, takeWhileP, takeWhile1P, and takeP.

Minimal complete definition

parseError, label, try, lookAhead, notFollowedBy, withRecovery, observing, eof, token, tokens, takeWhileP, takeWhile1P, takeP, getParserState, updateParserState, mkParsec

Methods

parseError :: ParseError s e -> m a

Stop parsing and report the ParseError. This is the only way to control position of the error without manipulating the parser state manually.

Since: 8.0.0

label :: String -> m a -> m a

The parser label name p behaves as parser p, but whenever the parser p fails without consuming any input, it replaces names of “expected” tokens with the name name.

hidden :: m a -> m a

hidden p behaves just like parser p, but it doesn't show any “expected” tokens in error message when p fails.

Please use hidden instead of the old label "" idiom.

try :: m a -> m a

The parser try p behaves like the parser p, except that it backtracks the parser state when p fails (either consuming input or not).

This combinator is used whenever arbitrary look ahead is needed. Since it pretends that it hasn't consumed any input when p fails, the (<|>) combinator will try its second alternative even if the first parser failed while consuming input.

For example, here is a parser that is supposed to parse the word “let” or the word “lexical”:

>>> parseTest (string "let" <|> string "lexical") "lexical"
1:1:
unexpected "lex"
expecting "let"

What happens here? The first parser consumes “le” and fails (because it doesn't see a “t”). The second parser, however, isn't tried, since the first parser has already consumed some input! try fixes this behavior and allows backtracking to work:

>>> parseTest (try (string "let") <|> string "lexical") "lexical"
"lexical"

try also improves error messages in case of overlapping alternatives, because Megaparsec's hint system can be used:

>>> parseTest (try (string "let") <|> string "lexical") "le"
1:1:
unexpected "le"
expecting "let" or "lexical"

Note that as of Megaparsec 4.4.0, string backtracks automatically (see tokens), so it does not need try. However, the examples above demonstrate the idea behind try so well that it was decided to keep them. You still need to use try when your alternatives are complex, composite parsers.

lookAhead :: m a -> m a

If p in lookAhead p succeeds (either consuming input or not) the whole parser behaves like p succeeded without consuming anything (parser state is not updated as well). If p fails, lookAhead has no effect, i.e. it will fail consuming input if p fails consuming input. Combine with try if this is undesirable.

notFollowedBy :: m a -> m ()

notFollowedBy p only succeeds when the parser p fails. This parser never consumes any input and never modifies parser state. It can be used to implement the “longest match” rule.

withRecovery

Arguments

:: (ParseError s e -> m a)

How to recover from failure

-> m a

Original parser

-> m a

Parser that can recover from failures

withRecovery r p allows us to continue parsing even if the parser p fails. In this case r is called with the actual ParseError as its argument. Typical usage is to return a value signifying failure to parse this particular object and to consume some part of the input up to the point where the next object starts.

Note that if r fails, the original error message is reported as if without withRecovery. In no way recovering parser r can influence error messages.

Since: 4.4.0

observing

Arguments

:: m a

The parser to run

-> m (Either (ParseError s e) a)

observing p allows us to “observe” failure of the p parser, should it happen, without actually ending parsing but instead getting the ParseError in Left. On success parsed value is returned in Right as usual. Note that this primitive just allows you to observe parse errors as they happen, it does not backtrack or change how the p parser works in any way.

Since: 5.1.0

eof :: m ()

This parser only succeeds at the end of input.

token

Arguments

:: (Token s -> Maybe a)

Matching function for the token to parse

-> Set (ErrorItem (Token s))

Used in the error message to mention the items that were expected

-> m a

The parser token test expected accepts tokens for which the matching function test returns Just results. If Nothing is returned the expected set is used to report the items that were expected.

For example, the satisfy parser is implemented as:

satisfy f = token testToken Set.empty
  where
    testToken x = if f x then Just x else Nothing

Note: type signature of this primitive was changed in the version 7.0.0.

tokens

Arguments

:: (Tokens s -> Tokens s -> Bool)

Predicate to check equality of chunks

-> Tokens s

Chunk of input to match against

-> m (Tokens s)

The parser tokens test chk parses a chunk of input chk and returns it. The supplied predicate test is used to check equality of given and parsed chunks after a candidate chunk of correct length is fetched from the stream.

This can be used for example to write chunk:

chunk = tokens (==)

Note that beginning from Megaparsec 4.4.0, this is an auto-backtracking primitive, which means that if it fails, it never consumes any input. This is done to make its consumption model match how error messages for this primitive are reported (which becomes an important thing as user gets more control with primitives like withRecovery):

>>> parseTest (string "abc") "abd"
1:1:
unexpected "abd"
expecting "abc"

This means, in particular, that it's no longer necessary to use try with tokens-based parsers, such as string and string'. This feature does not affect performance in any way.

takeWhileP

Arguments

:: Maybe String

Name for a single token in the row

-> (Token s -> Bool)

Predicate to use to test tokens

-> m (Tokens s)

A chunk of matching tokens

Parse zero or more tokens for which the supplied predicate holds. Try to use this as much as possible because for many streams this combinator is much faster than parsers built with many and satisfy.

takeWhileP (Just "foo") f = many (satisfy f <?> "foo")
takeWhileP Nothing      f = many (satisfy f)

The combinator never fails, although it may parse the empty chunk.

Since: 6.0.0

takeWhile1P

Arguments

:: Maybe String

Name for a single token in the row

-> (Token s -> Bool)

Predicate to use to test tokens

-> m (Tokens s)

A chunk of matching tokens

Similar to takeWhileP, but fails if it can't parse at least one token. Try to use this as much as possible because for many streams this combinator is much faster than parsers built with some and satisfy.

takeWhile1P (Just "foo") f = some (satisfy f <?> "foo")
takeWhile1P Nothing      f = some (satisfy f)

Note that the combinator either succeeds or fails without consuming any input, so try is not necessary with it.

Since: 6.0.0

takeP

Arguments

:: Maybe String

Name for a single token in the row

-> Int

How many tokens to extract

-> m (Tokens s)

A chunk of matching tokens

Extract the specified number of tokens from the input stream and return them packed as a chunk of stream. If there is not enough tokens in the stream, a parse error will be signaled. It's guaranteed that if the parser succeeds, the requested number of tokens will be returned.

The parser is roughly equivalent to:

takeP (Just "foo") n = count n (anySingle <?> "foo")
takeP Nothing      n = count n anySingle

Note that if the combinator fails due to insufficient number of tokens in the input stream, it backtracks automatically. No try is necessary with takeP.

Since: 6.0.0

getParserState :: m (State s e)

Return the full parser state as a State record.

updateParserState :: (State s e -> State s e) -> m ()

updateParserState f applies the function f to the parser state.

mkParsec :: (State s e -> Reply e s a) -> m a

An escape hatch for defining custom MonadParsec primitives. You will need to import Text.Megaparsec.Internal in order to construct Reply.

Since: 9.4.0

Instances

MonadParsec e s m => MonadParsec e s (IdentityT m)

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> IdentityT m a

label :: String -> IdentityT m a -> IdentityT m a

hidden :: IdentityT m a -> IdentityT m a

try :: IdentityT m a -> IdentityT m a

lookAhead :: IdentityT m a -> IdentityT m a

notFollowedBy :: IdentityT m a -> IdentityT m ()

withRecovery :: (ParseError s e -> IdentityT m a) -> IdentityT m a -> IdentityT m a

observing :: IdentityT m a -> IdentityT m (Either (ParseError s e) a)

eof :: IdentityT m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> IdentityT m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> IdentityT m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> IdentityT m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> IdentityT m (Tokens s)

takeP :: Maybe String -> Int -> IdentityT m (Tokens s)

getParserState :: IdentityT m (State s e)

updateParserState :: (State s e -> State s e) -> IdentityT m ()

mkParsec :: (State s e -> Reply e s a) -> IdentityT m a

MonadParsec e s m => MonadParsec e s (ReaderT r m)

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> ReaderT r m a

label :: String -> ReaderT r m a -> ReaderT r m a

hidden :: ReaderT r m a -> ReaderT r m a

try :: ReaderT r m a -> ReaderT r m a

lookAhead :: ReaderT r m a -> ReaderT r m a

notFollowedBy :: ReaderT r m a -> ReaderT r m ()

withRecovery :: (ParseError s e -> ReaderT r m a) -> ReaderT r m a -> ReaderT r m a

observing :: ReaderT r m a -> ReaderT r m (Either (ParseError s e) a)

eof :: ReaderT r m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> ReaderT r m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> ReaderT r m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> ReaderT r m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> ReaderT r m (Tokens s)

takeP :: Maybe String -> Int -> ReaderT r m (Tokens s)

getParserState :: ReaderT r m (State s e)

updateParserState :: (State s e -> State s e) -> ReaderT r m ()

mkParsec :: (State s e -> Reply e s a) -> ReaderT r m a

MonadParsec e s m => MonadParsec e s (StateT st m)

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> StateT st m a

label :: String -> StateT st m a -> StateT st m a

hidden :: StateT st m a -> StateT st m a

try :: StateT st m a -> StateT st m a

lookAhead :: StateT st m a -> StateT st m a

notFollowedBy :: StateT st m a -> StateT st m ()

withRecovery :: (ParseError s e -> StateT st m a) -> StateT st m a -> StateT st m a

observing :: StateT st m a -> StateT st m (Either (ParseError s e) a)

eof :: StateT st m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> StateT st m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> StateT st m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> StateT st m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> StateT st m (Tokens s)

takeP :: Maybe String -> Int -> StateT st m (Tokens s)

getParserState :: StateT st m (State s e)

updateParserState :: (State s e -> State s e) -> StateT st m ()

mkParsec :: (State s e -> Reply e s a) -> StateT st m a

MonadParsec e s m => MonadParsec e s (StateT st m)

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> StateT st m a

label :: String -> StateT st m a -> StateT st m a

hidden :: StateT st m a -> StateT st m a

try :: StateT st m a -> StateT st m a

lookAhead :: StateT st m a -> StateT st m a

notFollowedBy :: StateT st m a -> StateT st m ()

withRecovery :: (ParseError s e -> StateT st m a) -> StateT st m a -> StateT st m a

observing :: StateT st m a -> StateT st m (Either (ParseError s e) a)

eof :: StateT st m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> StateT st m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> StateT st m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> StateT st m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> StateT st m (Tokens s)

takeP :: Maybe String -> Int -> StateT st m (Tokens s)

getParserState :: StateT st m (State s e)

updateParserState :: (State s e -> State s e) -> StateT st m ()

mkParsec :: (State s e -> Reply e s a) -> StateT st m a

(Monoid w, MonadParsec e s m) => MonadParsec e s (WriterT w m)

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> WriterT w m a

label :: String -> WriterT w m a -> WriterT w m a

hidden :: WriterT w m a -> WriterT w m a

try :: WriterT w m a -> WriterT w m a

lookAhead :: WriterT w m a -> WriterT w m a

notFollowedBy :: WriterT w m a -> WriterT w m ()

withRecovery :: (ParseError s e -> WriterT w m a) -> WriterT w m a -> WriterT w m a

observing :: WriterT w m a -> WriterT w m (Either (ParseError s e) a)

eof :: WriterT w m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> WriterT w m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> WriterT w m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> WriterT w m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> WriterT w m (Tokens s)

takeP :: Maybe String -> Int -> WriterT w m (Tokens s)

getParserState :: WriterT w m (State s e)

updateParserState :: (State s e -> State s e) -> WriterT w m ()

mkParsec :: (State s e -> Reply e s a) -> WriterT w m a

(Monoid w, MonadParsec e s m) => MonadParsec e s (WriterT w m)

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> WriterT w m a

label :: String -> WriterT w m a -> WriterT w m a

hidden :: WriterT w m a -> WriterT w m a

try :: WriterT w m a -> WriterT w m a

lookAhead :: WriterT w m a -> WriterT w m a

notFollowedBy :: WriterT w m a -> WriterT w m ()

withRecovery :: (ParseError s e -> WriterT w m a) -> WriterT w m a -> WriterT w m a

observing :: WriterT w m a -> WriterT w m (Either (ParseError s e) a)

eof :: WriterT w m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> WriterT w m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> WriterT w m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> WriterT w m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> WriterT w m (Tokens s)

takeP :: Maybe String -> Int -> WriterT w m (Tokens s)

getParserState :: WriterT w m (State s e)

updateParserState :: (State s e -> State s e) -> WriterT w m ()

mkParsec :: (State s e -> Reply e s a) -> WriterT w m a

(Ord e, Stream s) => MonadParsec e s (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

parseError :: ParseError s e -> ParsecT e s m a

label :: String -> ParsecT e s m a -> ParsecT e s m a

hidden :: ParsecT e s m a -> ParsecT e s m a

try :: ParsecT e s m a -> ParsecT e s m a

lookAhead :: ParsecT e s m a -> ParsecT e s m a

notFollowedBy :: ParsecT e s m a -> ParsecT e s m ()

withRecovery :: (ParseError s e -> ParsecT e s m a) -> ParsecT e s m a -> ParsecT e s m a

observing :: ParsecT e s m a -> ParsecT e s m (Either (ParseError s e) a)

eof :: ParsecT e s m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> ParsecT e s m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> ParsecT e s m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> ParsecT e s m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> ParsecT e s m (Tokens s)

takeP :: Maybe String -> Int -> ParsecT e s m (Tokens s)

getParserState :: ParsecT e s m (State s e)

updateParserState :: (State s e -> State s e) -> ParsecT e s m ()

mkParsec :: (State s e -> Reply e s a) -> ParsecT e s m a

(Monoid w, MonadParsec e s m) => MonadParsec e s (RWST r w st m)

Since: 5.2.0

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> RWST r w st m a

label :: String -> RWST r w st m a -> RWST r w st m a

hidden :: RWST r w st m a -> RWST r w st m a

try :: RWST r w st m a -> RWST r w st m a

lookAhead :: RWST r w st m a -> RWST r w st m a

notFollowedBy :: RWST r w st m a -> RWST r w st m ()

withRecovery :: (ParseError s e -> RWST r w st m a) -> RWST r w st m a -> RWST r w st m a

observing :: RWST r w st m a -> RWST r w st m (Either (ParseError s e) a)

eof :: RWST r w st m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> RWST r w st m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> RWST r w st m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> RWST r w st m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> RWST r w st m (Tokens s)

takeP :: Maybe String -> Int -> RWST r w st m (Tokens s)

getParserState :: RWST r w st m (State s e)

updateParserState :: (State s e -> State s e) -> RWST r w st m ()

mkParsec :: (State s e -> Reply e s a) -> RWST r w st m a

(Monoid w, MonadParsec e s m) => MonadParsec e s (RWST r w st m)

Since: 5.2.0

Defined in Text.Megaparsec.Class

Methods

parseError :: ParseError s e -> RWST r w st m a

label :: String -> RWST r w st m a -> RWST r w st m a

hidden :: RWST r w st m a -> RWST r w st m a

try :: RWST r w st m a -> RWST r w st m a

lookAhead :: RWST r w st m a -> RWST r w st m a

notFollowedBy :: RWST r w st m a -> RWST r w st m ()

withRecovery :: (ParseError s e -> RWST r w st m a) -> RWST r w st m a -> RWST r w st m a

observing :: RWST r w st m a -> RWST r w st m (Either (ParseError s e) a)

eof :: RWST r w st m ()

token :: (Token s -> Maybe a) -> Set (ErrorItem (Token s)) -> RWST r w st m a

tokens :: (Tokens s -> Tokens s -> Bool) -> Tokens s -> RWST r w st m (Tokens s)

takeWhileP :: Maybe String -> (Token s -> Bool) -> RWST r w st m (Tokens s)

takeWhile1P :: Maybe String -> (Token s -> Bool) -> RWST r w st m (Tokens s)

takeP :: Maybe String -> Int -> RWST r w st m (Tokens s)

getParserState :: RWST r w st m (State s e)

updateParserState :: (State s e -> State s e) -> RWST r w st m ()

mkParsec :: (State s e -> Reply e s a) -> RWST r w st m a

Signaling parse errors

The most general function to fail and end parsing is parseError. These are built on top of it. The section also includes functions starting with the register prefix which allow users to register “delayed” ParseErrors.

failure

Arguments

:: MonadParsec e s m

=> Maybe (ErrorItem (Token s))

Unexpected item (if any)

-> Set (ErrorItem (Token s))

Expected items

-> m a

Stop parsing and report a trivial ParseError.

Since: 6.0.0

fancyFailure

Arguments

:: MonadParsec e s m

=> Set (ErrorFancy e)

Fancy error components

-> m a

Stop parsing and report a fancy ParseError. To report a single custom parse error, see customFailure.

Since: 6.0.0

unexpected :: MonadParsec e s m => ErrorItem (Token s) -> m a

The parser unexpected item fails with an error message telling about unexpected item item without consuming any input.

unexpected item = failure (Just item) Set.empty

customFailure :: MonadParsec e s m => e -> m a

Report a custom parse error. For a more general version, see fancyFailure.

customFailure = fancyFailure . Set.singleton . ErrorCustom

Since: 6.3.0

region

Arguments

:: MonadParsec e s m

=> (ParseError s e -> ParseError s e)

How to process ParseErrors

-> m a

The “region” that the processing applies to

-> m a

Specify how to process ParseErrors that happen inside of this wrapper. This applies to both normal and delayed ParseErrors.

As a side-effect of the implementation the inner computation will start with an empty collection of delayed errors and they will be updated and “restored” on the way out of region.

Since: 5.3.0

registerParseError :: MonadParsec e s m => ParseError s e -> m ()

Register a ParseError for later reporting. This action does not end parsing and has no effect except for adding the given ParseError to the collection of “delayed” ParseErrors which will be taken into consideration at the end of parsing. Only if this collection is empty the parser will succeed. This is the main way to report several parse errors at once.

Since: 8.0.0

registerFailure

Arguments

:: MonadParsec e s m

=> Maybe (ErrorItem (Token s))

Unexpected item (if any)

-> Set (ErrorItem (Token s))

Expected items

-> m ()

Like failure, but for delayed ParseErrors.

Since: 8.0.0

registerFancyFailure

Arguments

:: MonadParsec e s m

=> Set (ErrorFancy e)

Fancy error components

-> m ()

Like fancyFailure, but for delayed ParseErrors.

Since: 8.0.0

Derivatives of primitive combinators

single

Arguments

:: MonadParsec e s m

=> Token s

Token to match

-> m (Token s)

single t only matches the single token t.

semicolon = single ';'

See also: token, anySingle, char, char.

Since: 7.0.0

satisfy

Arguments

:: MonadParsec e s m

=> (Token s -> Bool)

Predicate to apply

-> m (Token s)

The parser satisfy f succeeds for any token for which the supplied function f returns True.

digitChar = satisfy isDigit <?> "digit"
oneOf cs  = satisfy (`elem` cs)

Performance note: when you need to parse a single token, it is often a good idea to use satisfy with the right predicate function instead of creating a complex parser using the combinators.

See also: anySingle, anySingleBut, oneOf, noneOf.

Since: 7.0.0

anySingle :: MonadParsec e s m => m (Token s)

Parse and return a single token. It's a good idea to attach a label to this parser.

anySingle = satisfy (const True)

See also: satisfy, anySingleBut.

Since: 7.0.0

anySingleBut

Arguments

:: MonadParsec e s m

=> Token s

Token we should not match

-> m (Token s)

Match any token but the given one. It's a good idea to attach a label to this parser.

anySingleBut t = satisfy (/= t)

See also: single, anySingle, satisfy.

Since: 7.0.0

oneOf

Arguments

:: (Foldable f, MonadParsec e s m)

=> f (Token s)

Collection of matching tokens

-> m (Token s)

oneOf ts succeeds if the current token is in the supplied collection of tokens ts. Returns the parsed token. Note that this parser cannot automatically generate the “expected” component of error message, so usually you should label it manually with label or (<?>).

oneOf cs = satisfy (`elem` cs)

See also: satisfy.

digit = oneOf ['0'..'9'] <?> "digit"

Performance note: prefer satisfy when you can because it's faster when you have only a couple of tokens to compare to:

quoteFast = satisfy (\x -> x == '\'' || x == '\"')
quoteSlow = oneOf "'\""

Since: 7.0.0

noneOf

Arguments

:: (Foldable f, MonadParsec e s m)

=> f (Token s)

Collection of taken we should not match

-> m (Token s)

As the dual of oneOf, noneOf ts succeeds if the current token not in the supplied list of tokens ts. Returns the parsed character. Note that this parser cannot automatically generate the “expected” component of error message, so usually you should label it manually with label or (<?>).

noneOf cs = satisfy (`notElem` cs)

See also: satisfy.

Performance note: prefer satisfy and anySingleBut when you can because it's faster.

Since: 7.0.0

chunk

Arguments

:: MonadParsec e s m

=> Tokens s

Chunk to match

-> m (Tokens s)

chunk chk only matches the chunk chk.

divOrMod = chunk "div" <|> chunk "mod"

See also: tokens, string, string.

Since: 7.0.0

(<?>) :: MonadParsec e s m => m a -> String -> m a infix 0

A synonym for label in the form of an operator.

match :: MonadParsec e s m => m a -> m (Tokens s, a)

Return both the result of a parse and a chunk of input that was consumed during parsing. This relies on the change of the stateOffset value to evaluate how many tokens were consumed. If you mess with it manually in the argument parser, prepare for troubles.

Since: 5.3.0

takeRest :: MonadParsec e s m => m (Tokens s)

Consume the rest of the input and return it as a chunk. This parser never fails, but may return the empty chunk.

takeRest = takeWhileP Nothing (const True)

Since: 6.0.0

atEnd :: MonadParsec e s m => m Bool

Return True when end of input has been reached.

atEnd = option False (True <$ hidden eof)

Since: 6.0.0

Parser state combinators

getInput :: MonadParsec e s m => m s

Return the current input.

setInput :: MonadParsec e s m => s -> m ()

setInput input continues parsing with input.

getSourcePos :: (TraversableStream s, MonadParsec e s m) => m SourcePos

Return the current source position. This function is not cheap, do not call it e.g. on matching of every token, that's a bad idea. Still you can use it to get SourcePos to attach to things that you parse.

The function works under the assumption that we move in the input stream only forwards and never backwards, which is always true unless the user abuses the library.

Since: 7.0.0

getOffset :: MonadParsec e s m => m Int

Get the number of tokens processed so far.

See also: setOffset.

Since: 7.0.0

setOffset :: MonadParsec e s m => Int -> m ()

Set the number of tokens processed so far.

See also: getOffset.

Since: 7.0.0

setParserState :: MonadParsec e s m => State s e -> m ()

setParserState st sets the parser state to st.

See also: getParserState, updateParserState.