> I've noticed that there is group-by in clojure 1.2. However it uses
> reduce and conj.
> Doesn't it consume all sequence at once?
Yes. But then, it would have to:
-------------------------
clojure.contrib.seq/group-by
([f coll])
Returns a sorted map of the elements of coll keyed by the result of
f on each element. The value at each key will be a vector of the
corresponding elements, in the order they appeared in coll.
So let's say you did
(group-by first [[1 :a] [2 :a] [3 :a] [2 :b] [1 :b]])
You'd get
{1 [[1 :a] [1 :b]], 2 [[2 :a] [2 :b]], 3 [[3 :a]]}
And there's no way it would know about the second element in the first
entry unless it looked through the whole input collection.
> Here I have lazy version I have written once:
>
> (defn group-by
> [input]
> (lazy-seq
> (when-let [s (seq input)]
> (let [k (ffirst s) [vs r] (split-with #(-> % first (= k)) s)]
> (cons [k vs] (group-by r))))))
That doesn't do the same thing as group-by. For one thing, it doesn't
take a function to produce the keys. So you might want to consider
choosing a different name.
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