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Re: simple method dispatch question

"Jeff Dubrule" <igor@pobox.com> wrote in message
20001111005047.P26044@secret-lab.dartmouth.edu">news:20001111005047.P26044@secret-lab.dartmouth.edu...
>> This looks interesting, but it seems to me to obfuscate
>> semi-trivial (and very conventional) conditional logic using
>> the dispatch mechanism.  Doesn't this tend to make write-only
>> code outside of trivial examples?

> Not at all.  Here's a not-quite-as-trivial example: consider
> writing a virtual machine...

OK, please keep in mind that I've already withdrawn my first-glance
objection to Bruce's example and then allow me to play devil's advocate.

> define function run(program)
>   for (opcode in program)
>     run-instruction(opcode);
>   end for;
> end function;

> define method run-instruction(instruction == #add)
>   push(*stack*, pop(*stack*) + pop(*stack*));
> end method;

> define method run-instruction(instruction == #multiply)
>   push(*stack*, pop(*stack*) * pop(*stack*));
> end method;

> ..
> ..
> ..

> This is certainly tons cleaner than a traditional switch statement,
> just about as fast,  you can split opcode implementations up into
> multiple files, and you can even add opcode extensions later just by
> defining new methods.

Cleaner I'm not so sure I agree with.  At a glance it seems to require more
lines of code -- much of which is boilerplate code -- than a ladder of if
statements would.  On the other hand each instruction is set out in a
fashion which is much more clearly separate, so this may cancel out.
Speed-wise I'm sure it's almost as fast in any half-decent compiler.  But
the last two points are what make me shiver.

Consider a VM written the way you've just advocated -- opcode
implementations scattered across files and opcode extensions added later,
probably in a different module entirely.  Now consider someone new being
brought into the project.  Throw the source code at the newbie and watch him
founder.

I withdraw my objection to the trick, as I said.  I can see its advantages
now that I've tried it out for a bit.  But it is the kind of trick which
*CAN* create write-only code and use of it would have to be policed against
this.
References: