At 07:46 25-9-00 -0300, Prof. José Romildo Malaquias wrote:
>Hello.
>
>Is there any Haskell implementation that supports
>extensible data types, in which new value constructors
>can be added to a previously declared data type,
>like
>
> data Fn = Sum | Pro | Pow
> ...
> extend data Fn = Sin | Cos | Tan
>
>where first the Fn datatype had only three values,
>(Sum, Pro and Pow) but later it was extended with
>three new values (Sin, Cos and Tan)?
>
>What are the pointers to documentation on such
>kind of extensions to Haskell?
In the Clean Object I/O library we encountered a similar challenge and
solved it using type constructor classes. The solution can also be used in
Haskell. The basic idea is as follows:
(1) For each type constructor that has to be extensible you introduce a
type constructor class;
(2) For each of the data constructors of such type constructors you
introduce a new type constructor (reusing their name is very convenient);
(3) For each such new type constructor define it as an instance of the type
constructor class that was created from its parent type constructor.
Example:
(1) From type constructor Fn you create:
class FnExt where
... -- Define your class member functions here
(2) From the data constructors Sum | Pro | Pow you create:
data Sum = Sum
data Pro = Pro
data Pow = Pow
(3) For each newly defined type constructor in (2) you define an instance:
instance FnExt Sum
instance FnExt Pro
instance FnExt Pow
All of your functions that previously were defined on Fn are now overloaded
functions:
fun :: ... Fn ... -> ... Fn ...
becomes:
fun :: (FnExt fn) => ... fn ... -> ... fn ...
I hope you find this scheme useful. I guess its applicability depends on
your particular class member functions and program.
Regards,
Peter Achten
=========================
References: although the perspective in the Clean Object I/O library is on
the data structures with generic interpretation functions, the technique is
basically the same as described here.
In my PhD Thesis you can find an early reference:
* The reference:
Achten, P.M. Interactive Functional Programs - Models, Methods, and
Implementation. PhD thesis, University of Nijmegen, 1996.
* The abstract at:
http://www.cs.kun.nl/~peter88/PeterThesisCh6.html
* The chapter at:
ftp://ftp.cs.kun.nl/pub/CSI/SoftwEng.FunctLang/papers/achp96-thesis6.ps.gz
A more modern version can be found here:
* The reference:
Achten, P.M. and Plasmeijer, M.J. Interactive Functional Objects in Clean.
In Clack, C., Hammond, K., and Davie, T. eds., Proceedings 9th
International Workshop Implementation of Functional Languages, IFL'97,
St.Andrews, Scotland, UK, September 1997, selected papers, LNCS 1467,
Springer, pp. 304-321.
* The abstract at:
ftp://ftp.cs.kun.nl/pub/Clean/papers/1998/achp97-InteractFuncObjects.abs
* The paper at:
ftp://ftp.cs.kun.nl/pub/Clean/papers/1998/achp97-InteractFuncObjects.ps.gz
