Hi,
As I was trying to wrap my head around the reducers library[1], I thought
implementing group-by would be a good exercise to gain some insight.
After spending a few hours with it, I'm still pretty much clueless, so hope
to find someone here to help me out:
So if I understood the reducer lingo introduced in [2],[3] and group-by
correctly, it reduces the following reducing function on a collection
(fn group-by-reducef [keyfn ret x]
(let [k (keyfn x)]
(assoc ret k (conj (get ret k []) x))))
where keyfn is provided by a partial function application.
fold needs a combining function that takes two result maps that have
already been grouped and merges them.
A naive implementation could look like
(defn group-by-combinef
([] {})
([g1 g2]
(persistent!
(reduce (fn [res k v]
(assoc! res k (into (get res k []) v)))
(transient g1) g2))))
(defn group-by [f coll]
(fold (partial gr-by-reducef f) gr-by-combinef coll))
Now couple of questions:
1) I expected fold to actually perform the operation, how can I force it to
give me the result?
2) Can somehow the actual reducing at the leaf nodes still take advantage
of transient collections?
3) I took a look at flatten as it seems the "closest" match. Again, if I
call (flatten [[1 2] [2 4]]), I don't actually get the result. How do I get
to the result?
Thanks!
[1]
https://github.com/clojure/clojure/blob/master/src/clj/clojure/core/reducers.clj
[2]
http://clojure.com/blog/2012/05/08/reducers-a-library-and-model-for-collection-processing.html
[3] http://clojure.com/blog/2012/05/15/anatomy-of-reducer.html
--
László Török
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