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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