On Mon, Sep 25, 2000 at 11:37:24AM +0100, Chris Angus wrote:
> I've not seen this before,
>
> out of interest, why would you want/need such a thing?
> >
> > 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)?
I want this to make a system I am workin on flexible.
Without it, my system is going to be too rigid.
I am working on an Computer Algebra system to transform
mathematic expressions, possibly simplifing them. There
is a data type to represent the expressions:
data Expr = Int Integer
| Cte String
| Var String
| App Fn [Expr]
An expression may be an integer, a named constante (to
represent "known" contantes like pi and e), a variable
or an application. An application has a functor (function
name) and a list of arguments. The functor is introduced
with
data Fn = Sum | Pro | Pow
meaning the application may be a sum, a product or a
power.
The project should be modular. So there are modules to
deal with the basic arithmetic and algebraic transformations
involving expressions of the type above. But there are
optional modules to deal with trigonometric expressions,
logarithms, vectors, matrices, derivatives, integrals,
equation solving, and so on. These should be available as
a library where the programmer will choose what he
needs in his application, and he should be able to
define new ones to extend the library. So these modules
will certainly need to extend the bassic types above.
If it is really impossible (theoriticaly or not implemented)
then I will have to try other ways to implement the idea.
The functions manipulating the expressions also should be
extensible to accomodate new algorithms. The extensions
is in a form of hooks (based on the Fn component of an
application function) in the main algorithms.
Romildo
--
Prof. José Romildo Malaquias <[EMAIL PROTECTED]>
Departamento de Computação
Universidade Federal de Ouro Preto
Brasil
