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February 10, 2005
Re: Multimethod library - mml.d
> For multi-dispatch, I cannot see any way to do that. If you have any
> pointers about it, I would personally be very interested.

I am currently developing a simulation engine for DoD modeling and 
simulations.  The simulation engine uses a template for two dimensional 
multimethods.  The multimethods are used to describe relationships between 
different kinds of objects in the simulation.  It requires that each type be 
enumerated.  This is difficult to do using C++ RTTI, so I use my own type 
system.  Once each type is enumerated, O(1) algorithms are possible.  The 
simplest way to do this is with an N-Dimensional array of methods.  Since 
each type's enumerated value will be its index into the array.  Each 
multimethod must have its own array.  The number of dimensions will 
correspond to the number of virtual parameters.

It is easy to see why this method is so fast.  It is simply indexing a 
lookup table.  The problem is that these arrays can become considerably 
large, especially as the number of virtual parameters increases.  For this 
reason, my currently implementation is limited to two dimensions.  However, 
there are algorithms that compress the lookup tables and still retain the 
O(1) performance.  The drawback is that this approach is considerably more 
complex and difficult to implement.

> As for the design of D: it clearly is a difficult decision which features 
> a
> language should support. Every programmer has some favorite programming
> style that depends on certain features in the language. Supporting
> everything is impossible and would lead to a cluttered language. Whether
> multimethods are important enough to include them is a matter of
> investigation. In any case, I would really want to see some real-world
> example that inherently depends on multimethods.


Well my vote is to include multimethods natively.

I suppose that a first step to providing high-performance multimethods in D 
is providing enumeration values for the types.  They would have to be 
enumerated from each base class.  Thus if B inherits A and C inherits B, C 
would have two enumerations:  one with base A and another with base B.  (If 
I'm confusing you just let me know and I will expound some more.)  This 
would allow for high-performance multimethod libraries to be written.  The 
next step would be to add the keywords and syntax sugar that replaces those 
libraries.

-Craig
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