Laurens,
I don't think I've encountered an identical function I can point you to,
but here's an alternative implementation:
(defn partition-when [pred coll]
(lazy-seq
(when-let [[h & t] (seq coll)]
(let [[run remains] (split-with (complement pred) t)]
(cons (cons h run) (partition-when pred remains))))))
Do you like that any better?
P.S: Say Hi to Chris for me!
Take care,
Moe
On Thu, Aug 20, 2015 at 1:34 AM, Laurens Van Houtven <[email protected]> wrote:
> Hi,
>
>
> I needed a function that partitions (coll a) => (coll (coll a)), starting
> a new subcoll whenever a given pred is true. Use case: I have a sequence of
> rows; these rows form groups; I want to group them. A group is started by a
> header; so I want a new coll whenever my is-header? pred returns true.
>
> I found an implementation that used partition-by on the mailing list, but
> this had some issues: if multiple elems next to each other were true under
> the pred, it wouldn’t create a new pred for each one.
>
> I came up with the following implementation.
>
> (defn ^:private partition-when
> "Partitions the coll whenever (f elem) is true."
> [pred coll]
> (-> (reduce (fn [[first & rest :as groups] elem]
> (if (pred elem)
> (conj groups [elem])
> (conj rest (conj first elem))))
> '()
> coll)
> reverse))
>
> It feels like this could be written a *lot* better. With Haskell’s
> Data.List.Split, this becomes:
>
> partitionWith p xs = filter (/= []) (split (whenElt p) xs)
>
> (thanks to my colleague Christopher Armstrong for the help with the
> Haskell version)
>
> It would *almost* be partitionWith p = split (whenElt p), but for some
> reason split sometimes returns empty lists. Granted, this is a specialized
> library; I’d be more than happy to get partition-when from a 3rd party.
> It’d be even nicer if it was in Clojure though :)
>
>
> thanks
> lvh
>
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