That implementation of partitions feels really low level. If you implement the monadic version of partition-when (which I call partition-where in my own code), it looks as simple as this:
(defn partitions [xs] (run-seq (m-partition-where (const [false true]) xs))) -Per On Wed, Mar 17, 2010 at 10:12 AM, Greg Fodor <gfo...@gmail.com> wrote: > Just saw that I need to sign the contributor agreement. Will do > promptly. I additionally have implemented two new functions that I > believe could fit in clojure-contrib.seq and clojure- > contrib.combinatorics. The seq function partition-at-fenceposts allows > you to partition a seq based upon bits flipped in an integer value > passed in, where 1-bits correspond to partition boundaries, the least > significant bit being earlier fenceposts in the seq. As such, this > makes it possible to easily implement a function named partitions for > clojure-contrib.combinatorics, which yields a lazy seq of all possible > partitions of a seq. > > user=> (partition-at-fenceposts 1 '(1 2 3 4 5)) > ((0) (1 2 3 4)) > > user=> (partition-at-fenceposts 2 '(1 2 3 4 5)) > ((0 1) (2 3 4)) > > user=> (partition-at-fenceposts 3 '(1 2 3 4 5)) > ((0) (1) (2 3 4)) > > user=> (partitions '(1 2 3)) > (((0 1 2)) ((0) (1 2)) ((0 1) (2)) ((0) (1) (2))) > > user=> (partitions '(1 2 3 4 5)) > (((0 1 2 3 4)) ((0) (1 2 3 4)) ((0 1) (2 3 4)) ((0) (1) (2 3 4)) ((0 1 > 2) (3 4)) ((0) (1 2) (3 4)) ((0 1) (2) (3 4)) ((0) (1) (2) (3 4)) ((0 > 1 2 3) (4)) ((0) (1 2 3) (4)) ((0 1) (2 3) (4)) ((0) (1) (2 3) (4)) > ((0 1 2) (3) (4)) ((0) (1 2) (3) (4)) ((0 1) (2) (3) (4)) ((0) (1) (2) > (3) (4))) > > implementation: > > (defn take-to-first > "Returns a lazy sequence of successive items from coll up to > and including the point at which it (pred item) returns true. > pred must be free of side-effects." > [pred coll] > (lazy-seq > (when-let [s (seq coll)] > (if-not (pred (first s)) > (cons (first s) (take-to-first pred (rest s))) > (list (first s)))))) > > (defn partition-when > "Applies f to each value in coll, splitting it each time f returns > true. Returns a lazy seq of lazy seqs." > [f coll] > (when-let [s (seq coll)] > (lazy-seq > (let [run (take-to-first f s) > res (drop (count run) s)] > (cons run (partition-when f res)))))) > > (defn partition-at-fenceposts [b coll] > "Partitions coll at fenceposts corresponding to bits in b that are > set to 1. Returns a lazy seq of lazy seqs." > (let [bits b] > (map #(map first %) > (partition-when > (fn [[i v]] (not (zero? (bit-and (bit-shift-left 1 i) > bits)))) > (indexed coll))))) > > (defn partitions [coll] > "Returns a lazy seq of possible partitions of coll." > (map #(partition-at-fenceposts % coll) (range (expt 2 (- (count > coll) 1))))) > > > On Mar 15, 1:24 pm, Greg Fodor <gfo...@gmail.com> wrote: >> Hi there, I am just learning Clojure and am processing some BER >> encoded integer values. Basically, the most significant bit of the >> integer in the stream indicates the split point between integers, and >> so I was looking into partition-by to see if that would help. Turns >> out, what I really need are two complementary functions: take-to-first >> and partition-when. Take-to-first is similar to take-while, but is >> *inclusive* and also inverts the boolean. For example: >> >> Clojure=> (take-to-first even? [1 1 1 1]) >> (1 1 1 1) >> Clojure=> (take-to-first even? [1 1 1 1 2 3 3 3]) >> (1 1 1 1 2) >> Clojure=> (take-to-first even? [2 2 2 ]) >> (2) >> >> Additionally, partition-when runs through the seq and partitions it on >> demand when a predicate is true. (Leaving the entry where it is seen >> to be true in the current partition: >> >> Clojure=> (partition-when even? [1 1 1 2 3 3 3 4 3 3 3 4 3 3 3]) >> ((1 1 1 2) (3 3 3 4) (3 3 3 4) (3 3 3)) >> Clojure=> (partition-when even? [1 1 1]) >> ((1 1 1)) >> Clojure=> (partition-when even? [1 1 1 2 3 3 3]) >> ((1 1 1 2) (3 3 3)) >> Clojure=> (partition-when even? [2 2 2 2]) >> ((2) (2) (2) (2)) >> >> These seem to sit aside the current take and partitioning functions >> since they are basically looking at an truth value that indicates a >> partition or stopping event that we want to capture and cease moving >> forward. Here is the source: >> >> (defn take-to-first >> "Returns a lazy sequence of successive items from coll up to >> and including the point at which it (pred item) returns true. >> pred must be free of side-effects." >> [pred coll] >> (lazy-seq >> (when-let [s (seq coll)] >> (if-not (pred (first s)) >> (cons (first s) (take-to pred (rest s))) >> (list (first s)))))) >> >> (defn partition-when >> "Applies f to each value in coll, splitting it each time f returns >> true. Returns a lazy seq of lazy seqs." >> [f coll] >> (when-let [s (seq coll)] >> (let [run (take-to-first #(f %) s) >> res (drop (count run) s)] >> (lazy-seq >> (cons run (partition-when f res)))))) >> >> I think these could make a good addition to clojure.contrib.seq. >> Please let me know if there is an easier way to get this in if you >> agree. Also, please let me know if these are the best ways to write >> these functions, since I am still a newbie! > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en -- You received this message because you are subscribed to the Google Groups "Clojure" group. 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