[Haskell-cafe] [Newbie] Quest for inheritance

[Ralf Lammel]

Re: your enumeration. Let me add one more bullet.
In cases where inheritance is about abstract base classes 
and concrete subclasses ((add interface polymorphism likewise))
you can use a (for unknown reasons) unappreciated pattern for
extensible datatypes in Haskell:
http://homepages.cwi.nl/~ralf/OOHaskell/src/interpreter/extensible.hs
(Note: conceptually unrelated to OOHaskell)

Re: OOHaskell
Thanks for calling this a very good and thorough attempt :-)
I would really like to understand why you think that
a) this could possibly be a "huge hack"
b) "awkward syntax" would be involved.

Regarding a), could it be that you are overwhelmed
by the details of the implementation of OOHaskell'
*typeful* object system? Wouldn't you be similarly concerned
when looking at details of *any* other object system of *any*
other *typed* language?

(( We are currently massaging the OOHaskell paper.
>From what you say, it might seem appropriate to emphasize
the OO programming API more clearly, while moving 
implementation details of OOHaskell out of the face
of a programmer who migrates from C#/Java to Haskell. ))

Regarding b), could it be that you would want to see
special keywords in your face, rather than thinking
in terms of records, monadic functions, open fixpoints,
closing fixpoints, ...? If yes, I guess that's an valid
desire. If there was a mainstream technique for Haskell
syntax extension, we want to use it for OOHaskell.

Ralf


> -----Original Message-----
> From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-
> bounces at haskell.org] On Behalf Of Cédric Paternotte
> Sent: Sunday, June 05, 2005 6:52 AM
> To: haskell-cafe at haskell.org
> Subject: [Haskell-cafe] [Newbie] Quest for inheritance
> 
> Hi. This is my first message here so Hello to everyone.
> 
> I'm just starting to learn Haskell and I really think it's a cool
> language.
> 
> Coming from an OO world, I've been looking for ways to achieve
> inheritance as you get it in Java and the likes.
> 
> I know OO and inheritance is not really the point of Haskell and that
> other mechanisms are provided to somewhat achieve reuse. But it's a
> way of programming I've been so used to that I feel lost without it.
> You might think I'm heading in the wrong direction. My mistake I have
> to agree. Let's take it as a learning exercise then.
> 
> So I've been searching the net for *easy* ways to get it and the least
> I can say is that I'm confused. It soon became apparent that,
> inheritance not being a core feature of the language, many people have
> been hacking Haskell to come up with similar effects (btw I never
> thought there could be so many ways to reach the same goal...Haskell
> programmers are clever bastards). Of the many implementations that
> I've found, few are really simple and most achieve it with various
> levels of success and/or dodgy syntax.
> 
> Here are the various techniques I've come across so far :
> (I'm far from understanding them all)
> 
> 1. Through existential types
> 
> As shown in the Shapes example at
> http://www.angelfire.com/tx4/cus/shapes/haskell.html.
> However, I couldn't spot the advantage over the standard version using
> normal classes at
> http://www.angelfire.com/tx4/cus/shapes/haskell98.html
> 
> The getX function still needs to be defined in both RectangleInstance
> and CircleInstance. This is not what I call inheritance. Inheritance
> would make it possible to define getX only once in ShapeInstance. Or
> maybe the point was only to demonstrate polymorphism. But then what is
> the advantage of using existential types ? It just looks like more
> work compared to the standard version that also makes use of
> polymorphism. Or am I missing something ?
> 
> 2. Through typeful heterogeneous collections
> 
> This technique is described at
> http://homepages.cwi.nl/~ralf/OOHaskell/ and is able to bring most OO
> principles into Haskell. This seems to be a very good and thorough
> attempt. Unfortunately it looks more like a huge hack than a solution.
> The problem I have with it is the awkward syntax it requires, which to
> my newbie eyes doesn't look like Haskell anymore.
> 
> 3. Through phantom types
> 
> I couldn't find any idiot-proof documentation on this topic but this
> technique seems to be used to model inheritance. The few papers I've
> come across (http://research.microsoft.com/Users/simonpj/Papers/oo-
> haskell/overloading_conf.pdf
> ,
> http://www.informatik.uni-bonn.de/~ralf/publications/Phantom.pdf
> ,http://www.informatik.uni-bonn.de/~ralf/publications/With.pdf ) seem
> very interesting but all go way over my head.
> 
> 4. Through O'Haskell (http://www.cs.chalmers.se/~nordland/ohaskell/)
> 
> Which is a non standard extension of Haskell that seems to be dead anyway.
> 
> 5. With this : http://www.cs.utexas.edu/ftp/pub/techreports/tr01-60/tr01-
> 60.pdf
> 
> This is a very interesting paper, at least for a newbie like me who's
> used to Java.
> The aim of this paper was to develop a same application in both Java
> and Haskell and to compare the resulting implementations afterwards.
> 
> What interested me in this was the mechanism they used to model
> inheritance (described on page 16 & 19), based on data types instead
> of classes. The idea being that all constructors of the Employee
> datatype have a same parameter, 'inCommon', of type
> 'CommonOfEmployees'.
> The data type CommonOfEmployees consists of all the fields shared by
> all types of employees. By then defining polymorphic functions that
> only use the 'inCommon' property of any employee, they've managed to
> instantiate these methods only once. My explanation must be horrible
> to you but you'll certainly get it by reading their paper.
> The Haskell source code can be found here
> :http://www.cs.utexas.edu/ftp/pub/techreports/tr01-60/Haskell_Impl/
> 
> The conclusion I drew from my quick & early findings is that the
> latest method is the simplest way to get inheritance in my programs.
> 
> I guess my question now is this : Are there other ways to achieve
> inheritance in Haskell ?
> Simpler techniques ? Or more accurate / comprehensive ? Would you have
> pointers to useful resources ?
> 
> I apology in advance for the errors, false assertions, misconceptions,
> confusions made in this document. I'm still learning Haskell. I'd just
> like to get the most of it.
> 
> 
> Cedric Paternotte
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