[GiNaC-devel] Custom power evaluation

[Vladimir Kisil]

		Dear Alexei,

>>>>> "SA" == Sheplyakov Alexei <varg at theor.jinr.ru> writes:

    SA> GiNaC offers ex::map() method for implementation of such custom
    SA> evaluation rules. 

	Unfortunately I cannot see how to use map() for simplification of
  automatic evaluations for derived classes, could you give me a hint?

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

	There is a limited number of evaluations (add(), mul(), ncmul()) so
  only three additional virtual methods will cover any desire in
  automatic evaluation.

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

	However this makes a check for existence of an additional evaluation method much
  quicker. Are there a better way to check this?

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

	Alright, I am not insisting on the warning message. Throwing an
  exception is not appropriate since nothing dangerous happens. 

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

	However it make much-much more faster evaluation of expressions
  which do contain such object. For example, calculations in two
  dimensions can be done now without usage Clifford algebras and *very*
  time-expensive non-commutativity. 

  In my opinion it is a fair exchange for slowing down *a bit* for
  other calls. Everything in this world come at a price.

  Best wishes,
  Vladimir
-- 
Vladimir V. Kisil     email: kisilv at maths.leeds.ac.uk
--                      www: http://maths.leeds.ac.uk/~kisilv/