How to flatten and lazily concat a list of lists

Question

I am playing around with lazy lists and cannot seem to figure this out. I think I can solve my problem by writing this as one big recursive function that does all that is necessary, but I would like to compose it of simpler functions.

I will try to write a simple example that should translate to my problem easily:

(defn numbers 
  ([] (numbers 1))
  ([n] (cons n (lazy-seq (numbers (inc n))))))

(defn repeat-n [n]
  (take n (repeat n)))

So, we have two functions. One returns a lazy sequence of numbers. The other returns its number parameter n times (hopefully it is also lazy; if not it seems easy to write one that would be).

I would like to map the repeat-n to numbers in such a way as to return lazy sequence of results. I have played around a bit with lazy-seq, lazy-cat, concat, and recursive functions, but I still have problems with it.

The function should be something like this

(lazy-concat-map [f items] ...)

And (hopefully) the result of calling

(take 11 (lazy-concat-map repeat-n numbers)) 

would be

12233344445

Any ideas?

Answer 1

(take 11 (mapcat #(repeat % %) (range)))
;=> (1 2 2 3 3 3 4 4 4 4 5)

The functions map, concat (and combination mapcat), as well as repeat and range are all lazy.

List comprehension, for, is also lazy

(take 11 (apply concat (for [x (range)] (repeat x x))))
;=> (1 2 2 3 3 3 4 4 4 4 5)