C[omp]ute

From: andrew cooke <andrew@...>

Date: Wed, 18 Jul 2012 23:41:56 -0400

(defn stream [seed]
  (defn helper
    ([slow] (helper (first slow) seed (rest slow)))
    ([c fast slow]
      (let [fast (seq fast)]
        (if fast
          (lazy-seq (cons (str c (first fast)) (helper c (rest fast) slow)))
          (helper slow)))))
  (declare delayed)
  (let [slow (cons "" (lazy-seq delayed))]
    (def delayed (helper slow))
    delayed))

(take 5 (stream ["a" "b" "c"]))
("a" "b" "c" "aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc" "aaa" 
"aab" "aac" "abb" "abc" "aca" "acb" "acc" "baa" "bab" "bac" "bba")

From http://stackoverflow.com/a/11553140/181772

The idea is pretty simple: I take the output from the sequence and feed it
back on itself. For each value in the output (which is also the input), I
generate a new output by appending each of the letters in the seed
sequence. Since this is circular it just keeps on going.

The process of feeding the output into the input is called "tying the knot" in
a fairly famous paper for Haskell. But it's harder to do in Clojure because
it's an eager language (and even lazy sequences aren't "lazy enough") - the
only solution I could find was that mess with def (I suspect someone might do
better with delay and force, but I had no luck).

https://github.com/andrewcooke/clojure-lab/blob/master/src/org/acooke/lab/perms.clj

Andrew