Guile Plug-In for Gnumeric

[Mark Probst <schani@unix.cslab.tuwien.ac.at>]

hi all

i've written a Guile plug-in for Gnumeric (the GNOME spreadsheet). i'd
like to invite you to check out the CVS version of Gnumeric and take a
look at it. since i'm pretty new to Guile, i'd really appreciate if
you could browse through the code (plugins/guile) and tell me what
i did wrong :-).

one of the highlights of the plug-in is that it is able to operate
not only on the values residing in the cells but also on the expressions.
as examples of what is possible i have written a little expression
simplifier (constant folding and simplification of things like x*1, a+0)
and a symbolic differentiator. they are not neatly integrated with
Gnumeric yet, however.

the plug-in exports two functions to Gnumeric: scm_eval and scm_apply.
both do pretty much the obious. e.g.:

  =scm_eval("(+ 1 1)")
  =scm_apply("(lambda (a b) (if (> a b) a b))",1,2)

one can of course write new Gnumeric functions in Scheme. examples
can be found in plugins/guile/gnumeric_startup.scm.

my main concern is currently (apart from the trivial tasks) a smob
wrapper for Gnumeric's ExprTrees (ExprTree is the data structure
Gnumeric uses for internally storing expressions). the problems are:

  * ExprTrees are reference counted. Can be solved by increasing the
    refcount if an ExprTree is smobified. In order to avoid increasing
    the refcount more than once, a list of all smobified ExprTrees
    would need to be kept.

  * ExprTrees are recursive. This means that if the user traverses the
    whole ExprTree from out of Guile, a smob for every node would need
    to be created. Is this ok?

  * CellRefs can be (and usually are) relative. This means that in
    order to evaluate an ExprTree its cell must be known in order to
    resolve the relative CellRefs. Thus, most functions dealing with
    ExprTrees would need to be passed the original CellRef. Another
    possibility would be to store the CellRef in the smob. Yet another
    solution would be to have a 'current CellRef' variable. Somehow,
    none of these solutions appeal to me. Suggestions, anyone?

i hoped maybe someone could give me some advice on these topics.

bye
schani

-- 
Mark Probst
Student, Programmer
http://www.unix.cslab.tuwien.ac.at/~schani/