Replacing the Read Class

Tickets

#61

replace the Read class

Problems with the Haskell 98 class

• the backtracking parser is very inefficient

• writing instances is painful, compared to a monadic interface

• on failure, parsers give no indication of what went wrong

Proposed replacements

• parsers based on  ReadP, from  Parallel Parsing Processes by Koen Classen, JFP 2004.
  • abstract type with monadic interface, invoked with

    readP_to_S :: ReadP a -> String -> [(a, String)]
    

    (The internal representation is more complex.)
  • choice operator supports full nondeterminism by parsing alternatives in parallel. (There is also a committed choice operator.)
  • mostly deterministic parsers are much more efficient than ReadS
  • returns no information on failure, but could be altered to do so, as it knows the position at which all alternatives are exhausted.
  • requires PolymorphicComponents
• GHC has used a new Read class based on these parsers since 5.04 (July 2002).

• Malcolm's experimental  Poly TextParser in the development version of  HaXml.
  • monadic interface, invoked with

    runParser :: TextParser a -> String -> (Either String a, String)
    

    (The internal representation is more general, parameterised on token type.)
  • returns only the first successful alternative, but with internal backtracking, and an explicit commit operator to prevent backtracking where necessary
  • on failure, returns an error string and the unparsed input
  • failure messages can be defined/combined/nested with convenient combinators

• Could also use something based on Daan's  Parsec combinators
  • monadic interface, invoked with (simplifying slightly)

    parse :: Parser a -> SourceName -> String -> Either ParseError a
    

  • mostly deterministic, with explicit 'try' operator for non-determinism
  • on failure, reports an error message including the position at which the error occured.
  • already widely used for other parsing tasks