Documentation

atomic_reader or atomic_readerPrec must be implemented. it can start parsing right away. from the outside, you should call reader which allows enclosing parentheses. to require enclosing parentheses, explicitely use reader_Paren True

read with enclosing parens

Parses a alphabetic or numeric Unicode characters according to isAlphaNum. Returns the parsed character.

Note that numeric digits outside the ASCII range (such as arabic-indic digits like e.g. "٤" or U+0664), as well as numeric characters which aren't digits, are parsed by this function but not by digit.

This parser succeeds for any character. Returns the parsed character.

char c parses a single character c. Returns the parsed character (i.e. c).

 semiColon  = char ';'

Parses an ASCII digit. Returns the parsed character.

Parses a hexadecimal digit (a digit or a letter between 'a' and 'f' or 'A' and 'F'). Returns the parsed character.

Parses an alphabetic Unicode characters (lower-case, upper-case and title-case letters, plus letters of caseless scripts and modifiers letters according to isAlpha). Returns the parsed character.

Parses a lower case character (according to isLower). Returns the parsed character.

Parses a newline character ('\n'). Returns a newline character.

As the dual of oneOf, noneOf cs succeeds if the current character not in the supplied list of characters cs. Returns the parsed character.

 consonant = noneOf "aeiou"

Parses an octal digit (a character between '0' and '7'). Returns the parsed character.

oneOf cs succeeds if the current character is in the supplied list of characters cs. Returns the parsed character. See also satisfy.

  vowel  = oneOf "aeiou"

The parser satisfy f succeeds for any character for which the supplied function f returns True. Returns the character that is actually parsed.

Parses a white space character (any character which satisfies isSpace) Returns the parsed character.

Skips zero or more white space characters. See also skipMany.

string s parses a sequence of characters given by s. Returns the parsed string (i.e. s).

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

Consider using string'.

Parses a tab character ('\t'). Returns a tab character.

Parses an upper case letter (according to isUpper). Returns the parsed character.

The parser anyToken accepts any kind of token. It is for example used to implement eof. Returns the accepted token.

between open close p parses open, followed by p and close. Returns the value returned by p.

 braces  = between (symbol "{") (symbol "}")

chainl p op x parses zero or more occurrences of p, separated by op. Returns a value obtained by a left associative application of all functions returned by op to the values returned by p. If there are zero occurrences of p, the value x is returned.

chainl1 p op parses one or more occurrences of p, separated by op Returns a value obtained by a left associative application of all functions returned by op to the values returned by p. This parser can for example be used to eliminate left recursion which typically occurs in expression grammars.

 expr    = term   `chainl1` addop
 term    = factor `chainl1` mulop
 factor  = parens expr <|> integer

 mulop   =   do{ symbol "*"; return (*)   }
         <|> do{ symbol "/"; return (div) }

 addop   =   do{ symbol "+"; return (+) }
         <|> do{ symbol "-"; return (-) }

chainr p op x parses zero or more occurrences of p, separated by op Returns a value obtained by a right associative application of all functions returned by op to the values returned by p. If there are no occurrences of p, the value x is returned.

chainr1 p op x parses one or more occurrences of |p|, separated by op Returns a value obtained by a right associative application of all functions returned by op to the values returned by p.

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

count n p parses n occurrences of p. If n is smaller or equal to zero, the parser equals to return []. Returns a list of n values returned by p.

endBy p sep parses zero or more occurrences of p, separated and ended by sep. Returns a list of values returned by p.

  cStatements  = cStatement `endBy` semi

endBy1 p sep parses one or more occurrences of p, separated and ended by sep. Returns a list of values returned by p.

This parser only succeeds at the end of the input. This is not a primitive parser but it is defined using notFollowedBy.

 eof  = notFollowedBy anyToken <?> "end of input"

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

 simpleComment   = do{ string "<!--"
                     ; manyTill anyChar (try (string "-->"))
                     }

Note the overlapping parsers anyChar and string "-->", and therefore the use of the try combinator.

notFollowedBy p only succeeds when parser p fails. This parser does not consume any input. This parser can be used to implement the 'longest match' rule. For example, when recognizing keywords (for example let), we want to make sure that a keyword is not followed by a legal identifier character, in which case the keyword is actually an identifier (for example lets). We can program this behaviour as follows:

 keywordLet  = try (do{ string "let"
                      ; notFollowedBy alphaNum
                      })

NOTE: Currently, notFollowedBy exhibits surprising behaviour when applied to a parser p that doesn't consume any input; specifically

• notFollowedBy . notFollowedBy is not equivalent to lookAhead, and
• notFollowedBy eof never fails.

See haskell/parsec#8 for more details.

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

 priority  = option 0 (do{ d <- digit
                         ; return (digitToInt d)
                         })

optionMaybe p tries to apply parser p. If p fails without consuming input, it return Nothing, otherwise it returns Just the value returned by p.

optional p tries to apply parser p. It will parse p or nothing. It only fails if p fails after consuming input. It discards the result of p.

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

 commaSep p  = p `sepBy` (symbol ",")

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

sepEndBy p sep parses zero or more occurrences of p, separated and optionally ended by sep, ie. haskell style statements. Returns a list of values returned by p.

 haskellStatements  = haskellStatement `sepEndBy` semi

sepEndBy1 p sep parses one or more occurrences of p, separated and optionally ended by sep. Returns a list of values returned by p.

skipMany1 p applies the parser p one or more times, skipping its result.

Extracts the source position from the parse error

Increments the column number of a source position.

Increments the line number of a source position.

Set the column number of a source position.

Set the line number of a source position.

Set the name of the source.

Extracts the column number from a source position.

Extracts the line number from a source position.

Extracts the name of the source from a source position.

The parser p <?> msg behaves as parser p, but whenever the parser p fails without consuming any input, it replaces expect error messages with the expect error message msg.

This is normally used at the end of a set alternatives where we want to return an error message in terms of a higher level construct rather than returning all possible characters. For example, if the expr parser from the try example would fail, the error message is: '...: expecting expression'. Without the (<?>) combinator, the message would be like '...: expecting "let" or letter', which is less friendly.

This combinator implements choice. The parser p <|> q first applies p. If it succeeds, the value of p is returned. If p fails without consuming any input, parser q is tried. This combinator is defined equal to the mplus member of the MonadPlus class and the (<|>) member of Alternative.

The parser is called predictive since q is only tried when parser p didn't consume any input (i.e.. the look ahead is 1). This non-backtracking behaviour allows for both an efficient implementation of the parser combinators and the generation of good error messages.

Returns the current input

Returns the full parser state as a State record.

Returns the current source position. See also SourcePos.

Returns the current user state.

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.

many p applies the parser p zero or more times. Returns a list of the returned values of p.

 identifier  = do{ c  <- letter
                 ; cs <- many (alphaNum <|> char '_')
                 ; return (c:cs)
                 }

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

 word  = many1 letter

parse p filePath input runs a parser p over Identity without user state. The filePath is only used in error messages and may be the empty string. Returns either a ParseError (Left) or a value of type a (Right).

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

 numbers = commaSep integer

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

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

setParserState st set the full parser state to st.

setPosition pos sets the current source position to pos.

An alias for putState for backwards compatibility.

skipMany p applies the parser p zero or more times, skipping its result.

 spaces  = skipMany space

The parser token showTok posFromTok testTok accepts a token t with result x when the function testTok t returns Just x. The source position of the t should be returned by posFromTok t and the token can be shown using showTok t.

This combinator is expressed in terms of tokenPrim. It is used to accept user defined token streams. For example, suppose that we have a stream of basic tokens tupled with source positions. We can then define a parser that accepts single tokens as:

 mytoken x
   = token showTok posFromTok testTok
   where
     showTok (pos,t)     = show t
     posFromTok (pos,t)  = pos
     testTok (pos,t)     = if x == t then Just t else Nothing

The parser tokenPrim showTok nextPos testTok accepts a token t with result x when the function testTok t returns Just x. The token can be shown using showTok t. The position of the next token should be returned when nextPos is called with the current source position pos, the current token t and the rest of the tokens toks, nextPos pos t toks.

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

 char c
   = tokenPrim showChar nextPos testChar
   where
     showChar x        = "'" ++ x ++ "'"
     testChar x        = if x == c then Just x else Nothing
     nextPos pos x xs  = updatePosChar pos x

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

The parsers fail, (<?>) and unexpected are the three parsers used to generate error messages. Of these, only (<?>) is commonly used. For an example of the use of unexpected, see the definition of notFollowedBy.

An alias for modifyState for backwards compatibility.

parseFromFile p filePath runs a string parser p on the input read from filePath using readFile. 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)
             }

The abstract data type ParseError represents parse errors. It provides the source position (SourcePos) of the error and a list of error messages (Message). A ParseError can be returned by the function parse. ParseError is an instance of the Show and Eq classes.

The abstract data type SourcePos represents source positions. It contains the name of the source (i.e. file name), a line number and a column number. SourcePos is an instance of the Show, Eq and Ord class.

Conditional failure of Alternative computations. Defined by

guard True  = pure ()
guard False = empty

Examples

>>> safeDiv 4 0
Nothing

>>> safeDiv 4 2
Just 2

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y | y /= 0    = Just (x `div` y)
            | otherwise = Nothing

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y = do
  guard (y /= 0)
  return (x `div` y)

funcall precedence