If the value is something easy to compute, like a backreference in a tree,
and you only need it while walking, why bother to store it at all?
Especially if you're going to need to use mutable state, which will make
your whole tree less useful for all kinds of things. Here's an example, in
which I imagine that your walk wants to update each node so that its value
is the sum of its old value, its parent's old value, and its children's old
values - you can adjust what goes where if you want one or the other of
these directions to see new values instead of old values.
(defn transform [parent node]
{:value (apply + (get parent :value 0) ;; the parent's value, or
zero
(:value node) ;; the node's old value
(map :value (:children node))) ;; the children's sum
:children (map (partial transform node)
(:children node))})
(transform nil {:value 1
:children [{:value 2}
{:value 3
:children [{:value 4}]}]})
{:value 6, ;; 1 for old value, 2+3 for children
:children ({:value 3, ;; 2 for old value, 1 for parent
:children ()}
{:value 8, ;; 3 for old value, 1 for parent, 4 for children
:children ({:value 7, ;; 4 for old value, 3 for parent
:children ()})})}
On Friday, July 6, 2012 4:56:22 PM UTC-7, Warren Lynn wrote:
>
> Hi, Phil:
>
> Thanks for your detailed analysis.
>
> My use-case is I have a tree and want to walk over the tree and apply a
> function on the nodes. The operation of the function also depends on the
> parent and children of the node, so it will be nice if each node already
> contains the references to its parent and children. Something like that.
>
> I guess I will just use atoms to hold the reference. I am not in pursuit
> of pure functional programming, so if it is too much trouble to do it with
> FP, I will take whatever works.
>
>
>
>
>
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