[Haskell-cafe] OO idioms redux

Duncan Coutts duncan.coutts at worcester.oxford.ac.uk
Wed Oct 13 09:15:22 EDT 2004

In message <slrncmo8qa.a82.jgoerzen at christoph.complete.org> John Goerzen
<jgoerzen at complete.org> 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)


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