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   "outputs": [
    {
     "data": {
      "text/plain": [
       "[0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181]"
      ]
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     "execution_count": 1,
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   "source": [
    "; lazy infinite sequence with recursive definition\n",
    "(def fib-seq-lazy \n",
    "  ((fn rfib [a b] \n",
    "     (lazy-seq (cons a (rfib b (+ a b)))))\n",
    "   0 1))\n",
    "(take 20 fib-seq-lazy)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false,
    "deletable": true,
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   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Will print\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "null"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "; 'unless' cannot be defined with a function because\n",
    "; it does not always evaluate both its arguments.\n",
    "(defmacro unless [pred a b]\n",
    "  `(if (not ~pred) ~a ~b))\n",
    "(unless false (println \"Will print\") (println \"Will not print\"))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "([0 1 2 3 4 5] [6 7 8 9 10 11] [12 13 14 15 16 17] [18 19 20 21 22 23] [24 25 26 27 28 29] [30 31 32 33 34 35])\n",
      "Distinct: 36\n",
      "([17 2 16 26 24 25] [11 28 27 3 23 35] [34 21 8 31 7 19] [20 0 30 15 1 12] [29 22 32 13 6 14] [5 10 18 4 9 33])\n",
      "Distinct: 36\n"
     ]
    },
    {
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      "text/plain": [
       "null"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
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   ],
   "source": [
    "(defn run [nvecs nitems nthreads niters]\n",
    "  (let [vec-refs (vec (map (comp ref vec)\n",
    "                           (partition nitems (range (* nvecs nitems)))))\n",
    "        swap #(let [v1 (rand-int nvecs)\n",
    "                    v2 (rand-int nvecs)\n",
    "                    i1 (rand-int nitems)\n",
    "                    i2 (rand-int nitems)]\n",
    "                (dosync\n",
    "                 (let [temp (nth @(vec-refs v1) i1)]\n",
    "                   (alter (vec-refs v1) assoc i1 (nth @(vec-refs v2) i2))\n",
    "                   (alter (vec-refs v2) assoc i2 temp))))\n",
    "        report #(do\n",
    "                 (prn (map deref vec-refs))\n",
    "                 (println \"Distinct:\"\n",
    "                          (count (distinct (apply concat (map deref vec-refs))))))]\n",
    "    (report)\n",
    "    (dorun (apply pcalls (repeat nthreads #(dotimes [_ niters] (swap)))))\n",
    "    (report)))\n",
    "; When run, we see no values get lost or duplicated in the shuffle.\n",
    "; There are 36 distinct numbers before and after. \n",
    "(run 6 6 6 100000)"
   ]
  },
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