Re: pari-2.2.1.alpha released

[Karim BELABAS <Karim.Belabas@math.u-psud.fr>]

On Mon, 5 Nov 2001, Ilya Zakharevich wrote:
> On Sun, Nov 04, 2001 at 02:39:00PM +0100, Karim BELABAS wrote:
> > > BTW, is there a "better" semantic than doing substitutions
> > > left-to-right?  Here "better" meaning one or more of "more
> > > convenient", "more functionality", "less surprising".
> >
> > There should be a way to choose between 'left-to-right' (sequential) and
> > 'parallel' (simultaneous) substitution. Both are relatively easy to
> > implement.
>
> But what does 'parallel' substitution means mathematically?  E.g.
>
>   x  ->  y
>   y  ->  x
>  x+y -> x-y

Indeed, 'parallel' only makes sense for variable substitution, not for a
general algebraic one (as per recent extension and proposal).

Hum. In fact, this is a strong point for advocating the sequential default...
Then the 'parallel' flag could be documented to make sense on a restricted
set of intputs (substitute variables only), but the default behaviour would
be consistent.

Unfortunately, sequential substitution is probably more confusing for
standard use (I would expect to substitute variables most of the time, esp.
many at the same time):

  subst([x,y], [x,y], [y,x], DEFAULT)  -->  [x,x]  \\ maybe surprising

  subst([x,y], [x,y], [y,x], PARALLEL) -->  [y,x]  \\ "expected"

    Karim.
-- 
Karim Belabas                    email: Karim.Belabas@math.u-psud.fr
Dep. de Mathematiques, Bat. 425
Universite Paris-Sud             Tel: (00 33) 1 69 15 57 48
F-91405 Orsay (France)           Fax: (00 33) 1 69 15 60 19
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