Here is partition-before and partition-after -- partition-after is simpler.
(defn partition-before-internal [pred coll]
(let [first-item (first coll)]
(if (nil? first-item) ()
(letfn [(get-partial-partition-with-item []
(let [partial-partition (partition-before-internal pred
(rest coll))]
(cons
(cons
first-item
(first partial-partition))
(rest partial-partition))))]
(if (pred first-item)
(cons () (lazy-seq (get-partial-partition-with-item)))
(get-partial-partition-with-item))))))
(defn partition-before [pred coll]
(lazy-seq
(let [partition-result (partition-before-internal pred coll)]
(if (empty? (first partition-result))
(rest partition-result)
partition-result))))
(deftest partitioning-before
(are [input output] (= ',output (partition-before true? ',input))
() ()
(true) ((true))
(false) ((false))
(false false false true false true true true)
((false false false) (true false) (true) (true) (true))
(true false false false true false true true true)
((true false false false) (true false) (true) (true) (true))))
(defn partition-after [pred coll]
(lazy-seq
(let [first-item (first coll)]
(if (nil? first-item) ()
(let [partial-partition (lazy-seq (partition-after pred (rest coll)))]
(if (pred first-item)
(cons (list first-item) partial-partition)
(cons
(cons
first-item
(first partial-partition))
(rest partial-partition))))))))
(deftest partitioning-after
(are [input output] (= ',output (partition-after true? ',input))
() ()
(true) ((true))
(false) ((false))
(false false false true false true true true)
((false false false true) (false true) (true) (true))
(true false false false true false true true true)
((true) (false false false true) (false true) (true) (true))))
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