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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Dynamic functions
Newsgroups: comp.lang.ada
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Date: Thu, 9 Aug 2007 19:58:27 +0200
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Date: 2007-08-09T19:58:25+02:00
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On Thu, 09 Aug 2007 18:06:23 +0200, Poul-Erik Andreasen wrote:

> How is the best way to make a representation of a
> function/algorithm wich can bee setup at runtime in Ada.
> 
> Let say i have for instans a function/algorithm looking like this
> 
> F(x,y,z) = (3x+25**y)/z
> 
> This will ofcource require a parser to do the setup, that I can handle.
> But what will bee the best structure for such generalised function.

You have to decide whether you want native or interpretable code. The
former could be a little bit complex. The latter is much simpler, it could
be some sort of reverse polish notation.

> I am thinking about some base functions and a tree struture with some
> access to function variables. Anyone with a better idea?

Tree is only needed if you wanted a full blown compiler with some complex
semantic analysis phase. In your case simple stack would suffice:

3, x, *, 25, y, **, +, z, /

Operands can be kept in a separate storage, organized, again as a stack,
maybe split into several stacks one per operand's type/size. If you know
the element size, stack could be implemented more efficiently. Access to
function for operators is OK, if you don't need to store or else marshal
the code. Otherwise you would need OP-codes instead.

> Due to peformance consideration i cannot let the parser calculate the 
> result imidialty.

Yes, you parse into interpretable code as most interpreters do. You can
take a look at examples:

http://www.dmitry-kazakov.de/ada/components.htm#Parsers_etc

Where they evaluate an expression or generate a parsing tree for it, you
just generate the code as above by pushing the operand or operation on the
stack(s). Interpretation is trivial: if an operand then push it onto the
working stack (or a reference to the operand), if an operation, call to it,
pop all its n operands from the working stack,  push the result back,
repeat ad infinitum.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de