> < ^ Date: Fri, 20 Oct 1995 19:30:00 +0100
> < ^ From: Andrew Mathas <mathas@maths.su.oz.au >
> < ^ Subject: Re: Workshop on GAP internals?

#I talked briefly about this with prof. Neubuser during the DAG
#day in Amiens, and he also thought it a good idea, and that
#possibly next spring could be a good time for such a workshop.
#What do other people think of this?

Given the rate at which GAP is currently growing such a meeting would seem
to be not only a good idea but also necessary.

Another aspect of GAP programming which I think needs to be discussed is
the implementation of 'utility functions' which would be useful additions
to many of the existing GAP functions. In particular, I think that functions
SaveTo() and TeX() described below are needed. Currently such functions are
either non-existent, implemented on an ad hoc basis, or written by
individual users and so not publically available.

1. SaveTo(): At present there is no uniform way to save
a GAP data structure for future retrieval. The most common way around this
is to print the structure to a file; however, this is often very
inefficient to read back in (eg. polynomials, sparse matrices). The SaveTo()
handler might look like this:

SaveTo:=function(arg)local save;
  save:=function(args)local a;
    for a in args do
      if IsRec(a) and IsBound(a.operations) and
      then a.operations.SaveTo(a);
      else Print(a);
 if IsFunc(arg[1]) then arg[1](arg[2],  save(arg{[3..Length(arg)]}));
 else PrintTo(arg[1], save(arg{[2..Length(arg)]}));

Thus, SaveTo("fred", x) and SaveTo(AppendTo, "fred", x) are both valid
function calls (each having the obvious effect), and x.operations.SaveTo()
would use the Print() statement throughout.

2. TeX(): Obviously useful. I have written TeX functions for printing
polynomials, elements of the Hecke algebras (from the Weyl package), and
some other things. I am happy to make these available; however in order to
implement these functions properly there should be a TeX() handling function
which calls x.operations.TeX(); it would look something like this:

TeX:=function(arg) local a,i;
  for a in arg do
    if IsInt(a) or IsString(a) then Print(a);
    elif IsList(a) then for i in a do TeX(i); od;
    elif IsRec(a) and IsBound(a.operations) and IsBound(a.operations.TeX)
    then a.operations.TeX(a);
    else Print("*error*", "TeX(<a>), don't know how to TeX <a>\n");

3. Other candidates are a Maple() function and any functions which are common
to many areas of mathematics, such as InnerProduct(), Induce() and so on.

Andrew Mathas

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