Documentation

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 Text.Parsec.Combinator.notFollowedBy.

ParserT monad transformer and Parser type

ParsecT s u m a is a parser with stream type s, user state type u, underlying monad m and return type a. Parsec is strict in the user state. If this is undesirable, simply used a data type like data Box a = Box a and the state type Box YourStateType to add a level of indirection.

Low-level unpacking of the ParsecT type. To run your parser, please look to runPT, runP, runParserT, runParser and other such functions.

Low-level creation of the ParsecT type. You really shouldn't have to do this.

parserZero always fails without consuming any input. parserZero is defined equal to the mzero member of the MonadPlus class and to the Control.Applicative.empty member of the Control.Applicative.Applicative class.

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 (Control.Applicative.<|>) member of Control.Applicative.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.

An instance of Stream has stream type s, underlying monad m and token type t determined by the stream

Some rough guidelines for a "correct" instance of Stream:

• unfoldM uncons gives the [t] corresponding to the stream
• A Stream instance is responsible for maintaining the "position within the stream" in the stream state s. This is trivial unless you are using the monad in a non-trivial way.

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.

The try combinator can for example be used to distinguish identifiers and reserved words. Both reserved words and identifiers are a sequence of letters. Whenever we expect a certain reserved word where we can also expect an identifier we have to use the try combinator. Suppose we write:

  expr        = letExpr <|> identifier <?> "expression"

  letExpr     = do{ string "let"; ... }
  identifier  = many1 letter

If the user writes "lexical", the parser fails with: unexpected 'x', expecting 't' in "let". Indeed, since the (<|>) combinator only tries alternatives when the first alternative hasn't consumed input, the identifier parser is never tried (because the prefix "le" of the string "let" parser is already consumed). The right behaviour can be obtained by adding the try combinator:

  expr        = letExpr <|> identifier <?> "expression"

  letExpr     = do{ try (string "let"); ... }
  identifier  = many1 letter

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 token 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 Text.Parsec.Char.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

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

The most general way to run a parser. runParserT p state filePath input runs parser p on the input list of tokens input, obtained from source filePath with the initial user state st. The filePath is only used in error messages and may be the empty string. Returns a computation in the underlying monad m that return either a ParseError (Left) or a value of type a (Right).

The most general way to run a parser over the Identity monad. runParser p state filePath input runs parser p on the input list of tokens input, obtained from source filePath with the initial user state st. 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).

  parseFromFile p fname
    = do{ input <- readFile fname
        ; return (runParser p () fname input)
        }

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.

Returns the current source position. See also SourcePos.

Returns the current input

setPosition pos sets the current source position to pos.

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.

updateParserState f applies function f to the parser state.

Returns the current user state.

putState st set the user state to st.

updateState f applies function f to the user state. Suppose that we want to count identifiers in a source, we could use the user state as:

  expr  = do{ x <- identifier
            ; updateState (+1)
            ; return (Id x)
            }

An alias for putState for backwards compatibility.

An alias for modifyState for backwards compatibility.