You can also solve this using `lazy-gen` and `yield-all` from the Tupelo
library
<https://github.com/cloojure/tupelo#generator-functions-for-lazy-sequences-a-la-python>.
It allows you to make
a lazy generator function (a la Python):
(let [seq-of-seqs [(range 0 5)
(range 10 15)
(range 20 25)]
flat-seq (lazy-gen
(doseq [curr-seq seq-of-seqs]
(yield-all curr-seq)))]
(is= flat-seq [0 1 2 3 4 10 11 12 13 14 20 21 22 23 24]))
Alan
On Tue, Nov 21, 2017 at 4:47 PM, Jason Wolfe <ja...@w01fe.com> wrote:
> I think this
> <https://github.com/plumatic/plumbing/blob/318af7798bb701607aaae8639d12829014941184/src/plumbing/core.cljx#L182>
> will do it:
>
> (lazy-cat (first coll) (when-let [n (next coll)] (lazy-flatten n))))
>
>
> On Tuesday, November 21, 2017 at 2:34:15 PM UTC-8, Matt Anderson wrote:
>>
>> I have a function that is returning a lazy-seq of lazy-seq's.
>>
>>
>> (seq (seq [1 2 3]) (seq [4 5 6]) ...)
>>
>>
>> The end-user API, however, should be able to get back a lazy-seq of the
>> individual items across all lazy-seq's--essentially a flattening of the
>> output of my function--instead of the "top level" seqs.
>>
>>
>> (seq [1 2 3 4 5 6 ...])
>>
>> Each of the internal lazy-seq's is expensive in terms of memory usage so
>> I'd like to only "load" one at a time. I've been trying to find a way to
>> only calculate one of the top-level lazy-seq's at a time and then not
>> "take" the next until the user gets to the end of the first and needs the
>> first item from the next (eg: (seq [4 5 6]) doesn't get calculated until we
>> have consumed 3 and are asking for the next), but haven't found a way to do
>> it. Best I've gotten is "loading" 2 "top level" seqs at a time and I'm
>> afraid that may be the best I get, but I thought it might be an exercise
>> worth presenting in case anyone had ideas. Here's a contrived example:
>>
>>
>> (defn lazy-flatten
>>
>> [coll]
>>
>> (when-let [s (seq coll)]
>>
>> (lazy-seq
>>
>> (if (seq? (first s))
>>
>> (concat (lazy-flatten (first s)) (lazy-flatten (rest s)))
>>
>> (cons (first s) (lazy-flatten (rest s)))))))
>>
>> (->> (repeatedly
>> (fn []
>> (println "New seq...")
>> (map (fn [x] + x (rand-int 10)) (range 4))))
>> lazy-flatten
>> (take 1))
>>
>>
>>
>> Prints:
>>
>>
>> New seq...
>>
>> New seq...
>>
>> => (8)
>>
>>
>> I realize this is because 2 items must be taken for "concat", so there
>> would need to be another approach (this was just my best shot
>> implementation).
>>
>>
>> Any ideas on how to get the bottom form to only print 1 "New seq..."?
>>
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