Text.Megaparsec.Prim

The primitive parser combinators.

Used data-types

data State s

This is Megaparsec state, it's parametrized over stream type s.

Constructors

State

Fields

stateInput :: s

statePos :: !SourcePos

stateTabWidth :: !Int

Instances

Eq s => Eq (State s)

Show s => Show (State s)

class (ShowToken t, ShowToken [t]) => Stream s t | s -> t where

An instance of Stream s t has stream type s, and token type t determined by the stream.

Methods

uncons :: s -> Maybe (t, s)

Instances

Stream ByteString Char

Stream ByteString Char

Stream Text Char

Stream Text Char

(ShowToken t, ShowToken [t]) => Stream [t] t

class Stream s t => StorableStream s t where

StorableStream abstracts ability of some streams to be stored in a file. This is used by the polymorphic function parseFromFile.

Methods

fromFile :: FilePath -> IO s

fromFile filename returns action that will try to read contents of file named filename.

Instances

StorableStream String Char

StorableStream ByteString Char

StorableStream ByteString Char

StorableStream Text Char

StorableStream Text Char

type Parsec s = ParsecT s Identity

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

data ParsecT s m a

ParsecT s m a is a parser with stream type s, underlying monad m and return type a.

Instances

MonadError e m => MonadError e (ParsecT s m)

MonadReader r m => MonadReader r (ParsecT s m)

MonadState s m => MonadState s (ParsecT s' m)

Stream s t => MonadParsec s (ParsecT s m) t

MonadTrans (ParsecT s)

Monad (ParsecT s m)

Functor (ParsecT s m)

Applicative (ParsecT s m)

Alternative (ParsecT s m)

MonadPlus (ParsecT s m)

MonadIO m => MonadIO (ParsecT s m)

MonadCont m => MonadCont (ParsecT s m)

Primitive combinators

class (Alternative m, Monad m, Stream s t) => MonadParsec s m t | m -> s t where

Type class describing parsers independent of input type.

Minimal complete definition

failure, label, try, lookAhead, notFollowedBy, eof, token, tokens, getParserState, updateParserState

Methods

failure :: [Message] -> m a

The most general way to stop parsing and report ParseError.

unexpected is defined in terms of the function:

unexpected = failure . pure . Unexpected

Since: 4.2.0

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.

try :: m a -> m a

The parser try p behaves like parser p, except that it pretends that it hasn't consumed any input when an error occurs.

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 when the first parser failed while consuming input.

For example, here is a parser that will try (sorry for the pun) to parse word “let” or “lexical”:

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

What happens here? 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"

lookAhead :: m a -> m a

lookAhead p parses p without consuming any input.

If p fails and consumes some input, so does lookAhead. Combine with try if this is undesirable.

notFollowedBy :: m a -> m ()

notFollowedBy p only succeeds when parser p fails. This parser does not consume any input and can be used to implement the “longest match” rule.

eof :: m ()

This parser only succeeds at the end of the input.

token

Arguments

:: (Int -> SourcePos -> t -> SourcePos)

Next position calculating function

-> (t -> Either [Message] a)

Matching function for the token to parse

-> m a

The parser token nextPos testTok accepts a token t with result x when the function testTok t returns Right x. The position of the next token should be returned when nextPos is called with the tab width, current source position, and the current token.

This is the most primitive combinator for accepting tokens. For example, the char parser could be implemented as:

char c = token updatePosChar testChar
  where testChar x = if x == c
                     then Right x
                     else Left . pure . Unexpected . showToken $ x

tokens

Arguments

:: Eq t

=> (Int -> SourcePos -> [t] -> SourcePos)

Computes position of tokens

-> (t -> t -> Bool)

Predicate to check equality of tokens

-> [t]

List of tokens to parse

-> m [t]

The parser tokens posFromTok test parses list of tokens and returns it. posFromTok is called with three arguments: tab width, initial position, and collection of tokens to parse. The resulting parser will use showToken to pretty-print the collection of tokens in error messages. Supplied predicate test is used to check equality of given and parsed tokens.

This can be used for example to write string:

string = tokens updatePosString (==)

getParserState :: m (State s)

Returns the full parser state as a State record.

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

updateParserState f applies function f to the parser state.

Instances

(Monad m, MonadParsec s m t) => MonadParsec s (IdentityT m) t

(MonadPlus m, Monoid w, MonadParsec s m t) => MonadParsec s (WriterT w m) t

(MonadPlus m, Monoid w, MonadParsec s m t) => MonadParsec s (WriterT w m) t

(MonadPlus m, MonadParsec s m t) => MonadParsec s (ReaderT e m) t

(MonadPlus m, MonadParsec s m t) => MonadParsec s (StateT e m) t

(MonadPlus m, MonadParsec s m t) => MonadParsec s (StateT e m) t

Stream s t => MonadParsec s (ParsecT s m) t

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

A synonym for label in form of an operator.

unexpected :: MonadParsec s m t => String -> m a

The parser unexpected msg always fails with an unexpected error message msg without consuming any input.

The parsers fail, label and unexpected are the three parsers used to generate error messages. Of these, only label is commonly used.

Parser state combinators

getInput :: MonadParsec s m t => m s

Returns the current input.

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

setInput input continues parsing with input. The getInput and setInput functions can for example be used to deal with #include files.

getPosition :: MonadParsec s m t => m SourcePos

Returns the current source position.

See also: SourcePos.

setPosition :: MonadParsec s m t => SourcePos -> m ()

setPosition pos sets the current source position to pos.

getTabWidth :: MonadParsec s m t => m Int

Returns tab width. Default tab width is equal to defaultTabWidth. You can set different tab width with help of setTabWidth.

setTabWidth :: MonadParsec s m t => Int -> m ()

Set tab width. If argument of the function is not positive number, defaultTabWidth will be used.

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

setParserState st set the full parser state to st.

Running parser

runParser

Arguments

:: Stream s t

=> Parsec s a

Parser to run

-> String

Name of source file

-> s

Input for parser

-> Either ParseError a

runParser 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 either a ParseError (Left) or a value of type a (Right).

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

runParser'

Arguments

:: Stream s t

=> Parsec s a

Parser to run

-> State s

Initial state

-> (State s, Either ParseError a)

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

Since: 4.2.0

runParserT :: (Monad m, Stream s t) => ParsecT s m a -> String -> s -> m (Either ParseError 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 ParseError (Left) or a value of type a (Right).

runParserT'

Arguments

:: (Monad m, Stream s t)

=> ParsecT s m a

Parser to run

-> State s

Initial state

-> m (State s, Either ParseError 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

parse

Arguments

:: Stream s t

=> Parsec s a

Parser to run

-> String

Name of source file

-> s

Input for parser

-> Either ParseError a

parse p file input runs parser p over Identity (see runParserT if you're using the ParserT monad transformer; parse itself is just a synonym for runParser). It returns either a ParseError (Left) or a value of type a (Right). show or print can be used to turn ParseError 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 err -> print err
         Right xs -> print (sum xs)

numbers = commaSep integer

parseMaybe :: Stream s t => Parsec s a -> s -> Maybe a

parseMaybe p input runs parser p on input and returns 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 name) are not important and entire input should be parsed. For example it can be used when parsing of single number according to specification of its format is desired.

parseTest :: (Stream s t, Show a) => Parsec s a -> s -> IO ()

The expression parseTest p input applies a parser p against input input and prints the result to stdout. Used for testing.

parseFromFile

Arguments

:: StorableStream s t

=> Parsec s a

Parser to run

-> FilePath

Name of file to parse

-> IO (Either ParseError a)

parseFromFile p filename runs parser p on the input read from filename. Returns either a ParseError (Left) or a value of type a (Right).

main = do
  result <- parseFromFile numbers "digits.txt"
  case result of
    Left err -> print err
    Right xs -> print $ sum xs