[Chicken-users] I need some help to edit a tree

[minh thu]

Hi,

I'd like to do something that I thought would be very easy to do with
Scheme but I'm getting lost.

I want to have a global *TREE* symbol to hold nested list. (Each node
is either a string or a list of node, but in this mail I make the description
with only lists) Initially it is (define *tree* '()), the empty tree.
If I push a node, it becomes '(()).

Now the fun part. I want to keep track of the current (or selected)
node. Initially there's no node, after a first push, the current node
is the inner list.

I want to be able to add a node to the left or to the right of the
current node. Also,
I want to be able to swap two nodes, or push a node outside of its parent....

Now, I take the fisrt case : adding a node to the left of the current one.
To do this, I need to SET! its parent. But of course I've no pointer
to its parent.
So what I've done is to use a *ACCESS-LIST* which contains a list of
'CAR, 'CDR, ... to have the path from the root to the selected node.

Then I can access the current node with
(define (current-node)
 ((apply compose (map eval *access-list*)) *tree*)

(I use a list of symbol because it's niser when printed than <procedures>)
Similarly, I've defined a PARENT-NODE procedure (just drop-right the
last element of the acces list).
The problem is I can't SET! (PARENT-NODE).

Do you have an idea about how can I manage this ? I guess there's an
elegant way to write tree editing code but I don't see.

Thanks,
thu