Text.Megaparsec.Lexer

High-level parsers to help you write your lexer. The module doesn't impose how you should write your parser, but certain approaches may be more elegant than others. Especially important theme is parsing of white space, comments, and indentation.

This module is intended to be imported qualified:

import qualified Text.Megaparsec.Lexer as L

White space

space

Arguments

:: MonadParsec e s m

=> m ()

A parser for a space character (e.g. void spaceChar)

-> m ()

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

-> m ()

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

-> m ()

space spaceChar 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).

spaceChar is used to parse trivial space characters. You can use spaceChar from Text.Megaparsec.Char for this purpose as well as your own parser (if you don't want to automatically consume newlines, for example).

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 if you don't need anything special.

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.integer

symbol

Arguments

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

=> m ()

How to consume white space after lexeme

-> String

String to parse

-> m String

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, Token s ~ Char)

=> m ()

How to consume white space after lexeme

-> String

String to parse (case-insensitive)

-> m String

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 ~ Char)

=> String

Line comment prefix

-> m ()

Given comment prefix this function returns a parser that skips line comments. Note that it stops just before 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 ~ Char)

=> String

Start of block comment

-> String

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 ~ Char)

=> String

Start of block comment

-> String

End of block comment

-> m ()

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

Since: 5.0.0

Indentation

indentLevel :: MonadParsec e s m => m Pos

Return current indentation level.

The function is a simple shortcut defined as:

indentLevel = sourceColumn <$> getPosition

Since: 4.3.0

incorrectIndent

Arguments

:: MonadParsec e s m

=> Ordering

Desired ordering between reference level and actual level

-> Pos

Reference indentation level

-> Pos

Actual indentation level

-> m a

Fail reporting incorrect indentation error. The error has attached information:

• Desired ordering between reference level and actual level
• Reference indentation level
• Actual indentation level

Since: 5.0.0

indentGuard

Arguments

:: MonadParsec e s m

=> m ()

How to consume indentation (white space)

-> Ordering

Desired ordering between reference level and actual level

-> Pos

Reference indentation level

-> m Pos

Current column (indentation level)

indentGuard spaceConsumer ord ref first consumes all white space (indentation) with spaceConsumer parser, then it checks column position. Ordering between current indentation level and the reference indentation level ref should be ord, otherwise the parser fails. On success the current column position is returned.

When you want to parse a block of indentation, first run this parser with arguments like indentGuard spaceConsumer GT (unsafePos 1) — this will make sure you have some indentation. Use returned value to check indentation on every subsequent line according to syntax of your language.

nonIndented

Arguments

:: MonadParsec e s m

=> m ()

How to consume indentation (white space)

-> m a

How to parse actual data

-> m a

Parse a non-indented construction. This ensures that there is no indentation before actual data. Useful, for example, as a wrapper for top-level function definitions.

Since: 4.3.0

data IndentOpt m a b

The data type represents available behaviors for parsing of indented tokens. This is used in indentBlock, which see.

Since: 4.3.0

Constructors

IndentNone a

Parse no indented tokens, just return the value

IndentMany (Maybe Pos) ([b] -> m a) (m b)

Parse many indented tokens (possibly zero), use given indentation level (if Nothing, use level of the first indented token); the second argument tells how to get final result, and third argument describes how to parse an indented token

IndentSome (Maybe Pos) ([b] -> m a) (m b)

Just like IndentMany, but requires at least one indented token to be present

indentBlock

Arguments

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

=> m ()

How to consume indentation (white space)

-> m (IndentOpt m a b)

How to parse “reference” token

-> m a

Parse a “reference” token and a number of other tokens that have greater (but the same) level of indentation than that of “reference” token. Reference token can influence parsing, see IndentOpt for more information.

Tokens must not consume newlines after them. On the other hand, the first argument of this function must consume newlines among other white space characters.

Since: 4.3.0

lineFold

Arguments

:: MonadParsec e s m

=> m ()

How to consume indentation (white space)

-> (m () -> m a)

Callback that uses provided space-consumer

-> m a

Create a parser that supports line-folding. The first argument is used to consume white space between components of line fold, thus it must consume newlines in order to work properly. The second argument is a callback that receives custom space-consuming parser as argument. This parser should be used after separate components of line fold that can be put on different lines.

An example should clarify the usage pattern:

sc = L.space (void spaceChar) empty empty

myFold = L.lineFold sc $ \sc' -> do
  L.symbol sc' "foo"
  L.symbol sc' "bar"
  L.symbol sc  "baz" -- for the last symbol we use normal space consumer

Since: 5.0.0

Character and string literals

charLiteral :: (MonadParsec e s m, Token s ~ Char) => m Char

The lexeme parser parses a single literal character without quotes. Purpose of this parser is to help with parsing of conventional escape sequences. It's your responsibility to take care of character literal syntax in your language (by surrounding it with single quotes or similar).

The literal character is parsed according to the grammar rules defined in the Haskell report.

Note that you can use this parser as a building block to parse various string literals:

stringLiteral = char '"' >> manyTill L.charLiteral (char '"')

If you want to write stringLiteral that adheres to the Haskell report though, you'll need to take care of the \& combination which is not a character, but can be used to separate characters (as in "\291\&4" which is two characters long):

stringLiteral = catMaybes <$> (char '"' >> manyTill ch (char '"'))
  where ch = (Just <$> L.charLiteral) <|> (Nothing <$ string "\\&")

Numbers

integer :: (MonadParsec e s m, Token s ~ Char) => m Integer

Parse an integer without sign 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.

decimal :: (MonadParsec e s m, Token s ~ Char) => m Integer

The same as integer, but integer is labeled with “integer” label, while this parser is labeled with “decimal integer”.

hexadecimal :: (MonadParsec e s m, Token s ~ Char) => m Integer

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 responsibility of the programmer to parse correct prefix before parsing the number itself.

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

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

octal :: (MonadParsec e s m, Token s ~ Char) => m Integer

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 responsibility of the programmer to parse correct prefix before parsing the number itself.

scientific :: (MonadParsec e s m, Token s ~ Char) => m Scientific

Parse a floating point value as Scientific number. Scientific is great for parsing of arbitrary precision numbers coming from an untrusted source. See documentation in Data.Scientific for more information. Representation of the floating point value is expected to be according to the Haskell report.

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

Since: 5.0.0

float :: (MonadParsec e s m, Token s ~ Char) => m Double

Parse a floating point number without sign. This is a simple shortcut defined as:

float = toRealFloat <$> scientific

number :: (MonadParsec e s m, Token s ~ Char) => m Scientific

Parse a number: either integer or floating point. The parser can handle overlapping grammars graciously. Use functions like floatingOrInteger from Data.Scientific to test and extract integer or real values.

signed :: (MonadParsec e s m, Token s ~ Char, Num a) => m () -> m a -> m a

signed space p parser parses an optional sign, then if there is a sign it will consume optional white space (using space parser), then it runs parser p which should return a number. Sign of the number is changed according to previously parsed sign.

For example, to parse signed integer you can write:

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