Hi, everybody,
I'm just throwing this out to see if the Clojure core team agrees, and if
they would like me to code something up and send a PR.
clojure.zip works great if you want to walk a tree and add nodes here and
there -- append-child, insert-child, insert-left, and insert-right cover
all the important use cases. If you want to walk a tree and filter nodes
out, remove has you covered. I've discovered, though, that trying to *move*
nodes is very cumbersome, far more than it needs to be.
The crux of the problem is here (to quote the API docs):
removefunction
Usage: (remove loc)
Removes the node at loc, returning the loc that would have preceded
it in a depth-first walk.
The problem is that remove "returns the loc that would have preceded" the
removed node "in a depth-first walk". If I want to push a node down below its
left/right sibling, or pull it up to become a sibling of its parent, or make it
swap places with its left/right sibling, etc. etc., going to "the loc that
would have preceded it in a depth-first walk" is useless. Once the node is
removed, it's very hard to find my way back to the loc where I want to insert
it.
I'm not saying that the API for remove is faulty -- it's perfect if your goal
is just to filter nodes out. But functions are needed which can remove a node
*and* return the loc above/to the left/to the right.
When I ran into this problem, I cracked open the source for clojure.zip, looked
at the source for "remove", and coded this variant up:
(defn remove-and-up [loc]
(let [[node {l :l, ppath :ppath, pnodes :pnodes, rs :r, :as path}] loc]
(if (nil? path)
(throw (new Exception "Remove at top"))
(if (pos? (count l))
(up (with-meta [(peek l)
(assoc path :l (pop l) :changed? true)]
(meta loc)))
(with-meta [(make-node loc (peek pnodes) rs)
(and ppath (assoc ppath :changed? true))]
(meta loc))))))
That removes a node, and returns the loc of its parent. My problem is now
solved. But to a newbie programmer, that problem would have probably been
insurmountable.
What do you think? Is adding a function or two to clojure.zip warranted?
Thanks, AD
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