Text.Megaparsec.Byte.Lexer

Stripped-down version of Text.Megaparsec.Char.Lexer for streams of bytes.

This module is intended to be imported qualified:

import qualified Text.Megaparsec.Byte.Lexer as L

White space

space

Arguments

:: MonadParsec e s m

=> m ()

A parser for space characters which does not accept empty input (e.g. space1)

-> m ()

A parser for a line comment (e.g. skipLineComment)

-> m ()

A parser for a block comment (e.g. skipBlockComment)

-> m ()

space sc lineComment blockComment produces parser that can parse white space in general. It's expected that you create such a parser once and pass it to other functions in this module as needed (when you see spaceConsumer in documentation, usually it means that something like space is expected there).

sc is used to parse blocks of space characters. You can use space1 from Text.Megaparsec.Char for this purpose as well as your own parser (if you don't want to automatically consume newlines, for example). Make sure the parser does not succeed on empty input though. In earlier version spaceChar was recommended, but now parsers based on takeWhile1P are preferred because of their speed.

lineComment is used to parse line comments. You can use skipLineComment if you don't need anything special.

blockComment is used to parse block (multi-line) comments. You can use skipBlockComment or skipBlockCommentNested if you don't need anything special.

If you don't want to allow a kind of comment, simply pass empty which will fail instantly when parsing of that sort of comment is attempted and space will just move on or finish depending on whether there is more white space for it to consume.

Parsing of white space is an important part of any parser. We propose a convention where every lexeme parser assumes no spaces before the lexeme and consumes all spaces after the lexeme; this is what the lexeme combinator does, and so it's enough to wrap every lexeme parser with lexeme to achieve this. Note that you'll need to call space manually to consume any white space before the first lexeme (i.e. at the beginning of the file).

lexeme

Arguments

:: MonadParsec e s m

=> m ()

How to consume white space after lexeme

-> m a

How to parse actual lexeme

-> m a

This is a wrapper for lexemes. Typical usage is to supply the first argument (parser that consumes white space, probably defined via space) and use the resulting function to wrap parsers for every lexeme.

lexeme  = L.lexeme spaceConsumer
integer = lexeme L.decimal

symbol

Arguments

:: MonadParsec e s m

=> m ()

How to consume white space after lexeme

-> Tokens s

Symbol to parse

-> m (Tokens s)

This is a helper to parse symbols, i.e. verbatim strings. You pass the first argument (parser that consumes white space, probably defined via space) and then you can use the resulting function to parse strings:

symbol    = L.symbol spaceConsumer

parens    = between (symbol "(") (symbol ")")
braces    = between (symbol "{") (symbol "}")
angles    = between (symbol "<") (symbol ">")
brackets  = between (symbol "[") (symbol "]")
semicolon = symbol ";"
comma     = symbol ","
colon     = symbol ":"
dot       = symbol "."

symbol'

Arguments

:: (MonadParsec e s m, FoldCase (Tokens s))

=> m ()

How to consume white space after lexeme

-> Tokens s

Symbol to parse (case-insensitive)

-> m (Tokens s)

Case-insensitive version of symbol. This may be helpful if you're working with case-insensitive languages.

skipLineComment

Arguments

:: (MonadParsec e s m, Token s ~ Word8)

=> Tokens s

Line comment prefix

-> m ()

Given comment prefix this function returns a parser that skips line comments. Note that it stops just before the newline character but doesn't consume the newline. Newline is either supposed to be consumed by space parser or picked up manually.

skipBlockComment

Arguments

:: (MonadParsec e s m, Token s ~ Word8)

=> Tokens s

Start of block comment

-> Tokens s

End of block comment

-> m ()

skipBlockComment start end skips non-nested block comment starting with start and ending with end.

skipBlockCommentNested

Arguments

:: (MonadParsec e s m, Token s ~ Word8)

=> Tokens s

Start of block comment

-> Tokens s

End of block comment

-> m ()

skipBlockCommentNested start end skips possibly nested block comment starting with start and ending with end.

Since: 5.0.0

Numbers

decimal :: forall e s m a. (MonadParsec e s m, Token s ~ Word8, Integral a) => m a

Parse an integer in decimal representation according to the format of integer literals described in the Haskell report.

If you need to parse signed integers, see signed combinator.

octal :: forall e s m a. (MonadParsec e s m, Token s ~ Word8, Integral a) => m a

Parse an integer in octal representation. Representation of octal number is expected to be according to the Haskell report except for the fact that this parser doesn't parse “0o” or “0O” prefix. It is a responsibility of the programmer to parse correct prefix before parsing the number itself.

For example you can make it conform to the Haskell report like this:

octal = char '0' >> char' 'o' >> L.octal

hexadecimal :: forall e s m a. (MonadParsec e s m, Token s ~ Word8, Integral a) => m a

Parse an integer in hexadecimal representation. Representation of hexadecimal number is expected to be according to the Haskell report except for the fact that this parser doesn't parse “0x” or “0X” prefix. It is a responsibility of the programmer to parse correct prefix before parsing the number itself.

For example you can make it conform to the Haskell report like this:

hexadecimal = char '0' >> char' 'x' >> L.hexadecimal

scientific :: forall e s m. (MonadParsec e s m, Token s ~ Word8) => m Scientific

Parse a floating point value as a Scientific number. Scientific is great for parsing of arbitrary precision numbers coming from an untrusted source. See documentation in Data.Scientific for more information.

The parser can be used to parse integers or floating point values. Use functions like floatingOrInteger from Data.Scientific to test and extract integer or real values.

This function does not parse sign, if you need to parse signed numbers, see signed.

float :: (MonadParsec e s m, Token s ~ Word8, RealFloat a) => m a

Parse a floating point number without sign. There are differences between the syntax for floating point literals described in the Haskell report and what this function accepts. In particular, it does not require fractional part and accepts inputs like "3" returning 3.0.

This is a simple short-cut defined as:

float = Sci.toRealFloat <$> scientific <?> "floating point number"

This function does not parse sign, if you need to parse signed numbers, see signed.

signed

Arguments

:: (MonadParsec e s m, Token s ~ Word8, Num a)

=> m ()

How to consume white space after the sign

-> m a

How to parse the number itself

-> m a

Parser for signed numbers

signed space p parser parses an optional sign character (“+” or “-”), then if there is a sign it consumes optional white space (using space parser), then it runs parser p which should return a number. Sign of the number is changed according to the previously parsed sign character.

For example, to parse signed integer you can write:

lexeme        = L.lexeme spaceConsumer
integer       = lexeme L.decimal
signedInteger = L.signed spaceConsumer integer