I don't think this is publishable research, there is too much other
stuff already out there.
matt
On 06/06/2005, at 5:58 PM, Gracjan Polak wrote:
Matthew Roberts wrote:
I have a project (for an early research student) waiting for an
interested student to take on. It is called "Programming patterns
for OO-like programming in Haskell". The basic idea is to identify
how to achieve OO-like organisation of your code in vanilla haskell.
Hopefully the student can come to a deep understanding of OO and
haskell programming in the process.
If this could count as graduate student project and could spawn
publication then I would be interested. :)
Sounds like exactly what you need. Shame no-one has taken it up yet.
Perhaps next semester :) Anyone out there want to come to Sydney
and take it on?
As I see you did not cc the list, so currently I'm the only one that
received this message. :)
Matt
On 06/06/2005, at 3:55 AM, Gracjan Polak wrote:
Cédric Paternotte wrote:
> 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.
Me too :)
> 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.
Me too :)
> 5. With this :
http://www.cs.utexas.edu/ftp/pub/techreports/tr01-60/tr01-60.pdf
>
I've been thinking about slight generalization of this lately. Here
are my semi-backed thoughts as of now.
First of all, in Haskell there will be strict separation between
interfaces and data, so almost every method will be declared twice.
This is not so strange to anybody programing in Java, but for C++
programmers can be. Inheritance relation is specified after data.
There is also separation between two concepts: what interfaces each
piece of data implements and which intefaces given interface
inherits. So:
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
module Main where
-- general inheritance relation
class Inherits b x where
get_super :: x -> b
-- declare interface with one method
class IA a where
get_a :: a -> Int
-- define data with one field
data DA = DA { da_field :: Int }
-- say how data DA conforms to interface IA
instance IA DA where
get_a x = da_field x
-- declare some other interface IB
-- note: IB is unrelated to IA
class IB a where
get_b :: a -> String
-- data that inherits fields of DA and adds one another field
data DB = DB { db_super :: DA, db_field :: String }
-- DB inherits fields and methods of DA
instance Inherits DA DB where
get_super x = db_super x
-- data DB implements interface IB
instance IB DB where
get_b x = db_field x
-- some other random data
data DC = DC { dc_super :: DA }
-- DC implements interface IB
instance IB DC where
get_b x = show (get_a x)
-- and inherits DA
instance Inherits DA DC where
get_super x = dc_super x
-- now the tricky part: state that every data x inheriting DA
-- implements all interfaces of DA (repeat for each interface)
instance (Inherits DA x) => IA x where
get_a w = da_field (get_super w)
main = do
let db = DB (DA 123) "zzz"
let dc = DC (DA 123)
putStrLn $ show (get_a db)
putStrLn $ show (get_a dc)
putStrLn $ show (get_b db)
putStrLn $ show (get_b dc)
As you see there is much more writting as in Java. But this gives
better control over inheritance and subsumption because everything
must be stated explicitly. Multiple inheritance is allowed :) Also
it is "private inheritance" (as in C++) by default.
There are some problems left: how to update a field? or how to make
inheritance transitive. I don't know it yet :)
>
> I guess my question now is this : Are there other ways to achieve
> inheritance in Haskell ?
Me too:)
My proposal (above) is about the level of 'OO' things done in
procedural languages (example: C with GTK+ library). There must be a
better way. Any comments?
--
Gracjan
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