[GiNaC-devel] Custom power evaluation

[Sheplyakov Alexei]

Hello,

On Fri, Oct 27, 2006 at 12:37:03PM +0100, Vladimir Kisil wrote:
> 		Following my previous message on the subject
>   (http://www.ginac.de/pipermail/ginac-devel/2006-October/001071.html) I
>   am including the full patch. Its functionality is now described in
>   ginac.info.
[snipped]

While I certainly like the idea, I dislike the proposed implementation
very much. Keep on reading to find out why.

> +It is not unusual to use some objects which have special properties with
> +respect to exponentiation. For example, if a symbol @code{s} may take
> +only values @math{-1} or @math{1} then @math{s^2=1}. Other examples are

[snipped]

GiNaC offers ex::map() method for implementation of such custom
evaluation rules. Frankly, I don't understand why it is not enough...

> + at example
> +	virtual ex basic::eval_power_basis(const ex & exp) const;
> +	virtual ex basic::eval_power_exponent(const ex & basis) const;
> + at end example

Adding more methods to the `basic' class does not sound like good
idea to me.  What if someone implements custom evaluation rules, say,
for the `mul' class? E.g. foosymbol times barfunction is always zero.
Add even more methods to basic?

[snip]

> + at example
> +    your_class::info(info_flags::is_power_basis);
> +    your_class::info(info_flags::is_power_exponent);
> + at end example

Likewise, I think adding even more stuff to info_flags is bad idea.

[huge snip]
  
> +ex basic::eval_power_basis(const ex & exp) const
> +{
> +	std::cerr << "WARNING: virtual method  basic::eval_power_basis is called," << std::endl
> +			  << "   did you forget to redefine it in the derived class?" << std::endl;
> +	return power(*this, exp).hold();
> +}

Spamming to std{out,err} is a MORTAL SIN. NEVER, EVER do this.
Either return the expression unevaluated or throw an exception.

> +	// user defined classes may provide their own virtual methods for power evaluation
> +	if (ebasis.info(info_flags::is_power_basis))
> +		return ebasis.eval_power_basis(eexponent);
> +
> +	if (eexponent.info(info_flags::is_power_exponent))
> +		return eexponent.eval_power_exponent(ebasis);

This slows down a bit evaluation of *every* power expression, including 
those which do not provide such a method. GiNaC's performance is not 
exactly brilliant, why reduce it even more?

Best regards,
 Alexei

-- 
All science is either physics or stamp collecting.

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