Re: [Haskell-cafe] OO idioms redux

13 Oct 2004

      In message  John Goerzen
 writes:
...
OK, recently I posed a question about rethinking some OO idioms, and
that spawned some useful discussion.
I now have a followup question.
One of the best features of OO programming is that of inheritance.  It
can be used to slightly alter the behavior of objects without requiring
modification to existing code or breaking compatibility with existing
APIs.
Closures can do this:

// in C++/Java
class ConfigParser {
  void read (String);
  int getFoo (String);
}

-- in Haskell
data ConfigParser = ConfigParser {
    read :: String -> IO ConfigParser
    getFoo :: String -> Int
  }

This is a structure of functions. The difference from C++/Java style objects is
that this gives us object based polymorphism rather than class based
polymorphism. Because they are Haskell closures rather than C-style functions
they can hold private state by using partial application.

configParser :: ConfigParser
configParser = ConfigParser {
    read input = ...
    getFoo = ...
  }

envConfigParser :: ConfigParser
envConfigParser = configParser {
    read input = do env <- getEnvVars
                    read configParser (substEnvVars input)
  }

Duncan

Re: [Haskell-cafe] OO idioms redux

Duncan Coutts