Documentation

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.

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

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

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 than 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.

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)
                  }

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

  spaces  = skipMany space

Returns the current user state.

An alias for putState for backwards compatibility.

An alias for modifyState for backwards compatibility.

Returns the current source position. See also SourcePos.

setPosition pos sets the current source position to pos.

Returns the current input

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

Returns the full parser state as a State record.

setParserState st set the full parser state to st.