On Sun, Oct 3, 2010 at 3:32 AM, Alan <[email protected]> wrote:
> I've got a collection of unique objects, and I need to partition them
> into sets. That part's easy enough, but I need to have both of the
> following be efficient, and preferably easy:
> - Given an object, determine what set it's in
> - List all the objects in a given set
>
> In an imperative language this would be fairly simple: create some
> empty sets, then iterate over all objects; at each step add the object
> to a set, and adjust the object's "parent" pointer to point at the set
> it's in.
>
> But in a functional, immutable context the circularity involved seems
> to make this really inconvenient: if I create the set first then it
> can't point to the object, and if I create the object first it can't
> point at its set.
>
> I could construct all the objects and have a single "global" map, with
> mappings for both set-id=>[objects] and object=>set-id, but this seems
> kinda gross and obscures what is actually meant (objects belong to
> sets) with implementation (integers/keywords mapping to groups of
> objects, and objects mapping to integers).
>
> Likewise, I could use some kind of mutable refs in part of the
> process, but (a) that feels like giving up, and (b) since I'll be
> modifying these sets recursively and backtracking to "past" states it
> would add a tremendous amount of bookkeeping to undo changes.
>
> This seems like a pretty common problem, so I'm sure some clever
> Clojure coder out there has already solved it; what am I missing?
>
This is a topic that comes up periodically. I've wondered this myself and
recently realized that clojure.set does deal with some of these issues.
(ns db
(:use clojure.set))
(def animals #{{:name "penguin" :type "bird"}
{:name "dolphin" :type "mammal"}
{:name "lion" :type "mammal"}
{:name "ant" :type "insect"}})
;; group the animals into sets
(def by-type (index animals [:type]))
;; get all birds who's first name start with the character 'p'
(select (fn [{[c] :name}] (= c \p))
(by-type {:type "bird"}))
Is something about this that doesn't work for your problem?
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