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functional.clj
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167 lines (140 loc) · 4.76 KB
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(ns examples.functional)
; START: stack-consuming-fibo
; bad idea
(defn stack-consuming-fibo [n]
(cond
(= n 0) 0 ; <label id="code.stack-consuming-fibo.0"/>
(= n 1) 1 ; <label id="code.stack-consuming-fibo.1"/>
:else (+ (stack-consuming-fibo (- n 1)) ; <label id="code.stack-consuming-fibo.n"/>
(stack-consuming-fibo (- n 2))))) ; <label id="code.stack-consuming-fibo.n2"/>
; END: stack-consuming-fibo
; START: tail-fibo
(defn tail-fibo [n]
(letfn [(fib ; <label id="code.tail-fibo.letfn"/>
[current next n] ; <label id="code.tail-fibo.args"/>
(if (zero? n)
current ; <label id="code.tail-fibo.terminate"/>
(fib next (+ current next) (dec n))))] ; <label id="code.tail-fibo.recur"/>
(fib 0 1 n))) ; <label id="code.tail-fibo.call"/>
; END: tail-fibo
; START: recur-fibo
; better but not great
(defn recur-fibo [n]
(letfn [(fib
[current next n]
(if (zero? n)
current
(recur next (+ current next) (dec n))))] ; <label id="code.recur-fibo.recur"/>
(fib 0 1 n)))
; END: recur-fibo
; START: lazy-seq-fibo
(defn lazy-seq-fibo
([]
(concat [0 1] (lazy-seq-fibo 0 1))) ; <label id="code.lazy-seq-fibo.basis"/>
([a b]
(let [n (+ a b)] ; <label id="code.lazy-seq-fibo.n"/>
(lazy-seq ; <label id="code.lazy-seq-fibo.lazy-seq"/>
(cons n (lazy-seq-fibo b n)))))) ; <label id="code.lazy-seq-fibo.cons"/>
; END: lazy-seq-fibo
; START: fibo
(defn fibo []
(map first (iterate (fn [[a b]] [b (+ a b)]) [0 1])))
; END: fibo
; START: head-fibo
; holds the head (avoid!)
(def head-fibo (lazy-cat [0 1] (map + head-fibo (rest head-fibo))))
; END: head-fibo
; START: count-heads-pairs
(defn count-heads-pairs [coll]
(loop [cnt 0 coll coll] ; <label id="code.count-heads-loop.loop"/>
(if (empty? coll) ; <label id="code.count-heads-loop.basis"/>
cnt
(recur (if (= :h (first coll) (second coll)) ; <label id="code.count-heads-loop.filter"/>
(inc cnt)
cnt)
(rest coll)))))
; END: count-heads-pairs
(def count-heads-loop count-heads-pairs)
; START: by-pairs
; overly complex, better approaches follow...
(defn by-pairs [coll]
(let [take-pair (fn [c] ; <label id="code.by-pairs.take"/>
(when (next c) (take 2 c)))]
(lazy-seq ; <label id="code.by-pairs.lazy-seq"/>
(when-let [pair (seq (take-pair coll))] ; <label id="code.by-pairs.when-let"/>
(cons pair (by-pairs (rest coll))))))) ; <label id="code.by-pairs.cons"/>
; END: by-pairs
; START: count-heads-by-pairs
(defn count-heads-pairs [coll]
(count (filter (fn [pair] (every? #(= :h %) pair))
(by-pairs coll))))
; END: count-heads-by-pairs
(def count-heads-by-pairs count-heads-pairs)
; START: count-if
(use '[clojure.contrib.def :only (defvar)])
(defvar count-if (comp count filter) "Count items matching a filter")
; END: count-if
; START: count-runs
(defn
count-runs
"Count runs of length n where pred is true in coll."
[n pred coll]
(count-if #(every? pred %) (partition n 1 coll)))
; END: count-runs
; START: count-heads-by-runs
(defvar count-heads-pairs (partial count-runs 2 #(= % :h))
"Count runs of length two that are both heads")
; END: count-heads-by-runs
(def count-heads-by-runs count-heads-pairs)
; START: my-odd-even
(declare my-odd? my-even?)
(defn my-odd? [n]
(if (= n 0)
false
(my-even? (dec n))))
(defn my-even? [n]
(if (= n 0)
true
(my-odd? (dec n))))
; END: my-odd-even
; START: parity
(defn parity [n]
(loop [n n par 0]
(if (= n 0)
par
(recur (dec n) (- 1 par)))))
; END: parity
; START: my-odd-even-parity
(defn my-even? [n] (= 0 (parity n)))
(defn my-odd? [n] (= 1 (parity n)))
; END: my-odd-even-parity
; START: curry
; almost a curry
(defn faux-curry [& args] (apply partial partial args))
; END: curry
; --------------------------------------------------------------------------------------
; -- See www.cs.brown.edu/~sk/Publications/Papers/Published/sk-automata-macros/paper.pdf
; --------------------------------------------------------------------------------------
(defn machine [stream]
(let [step {[:init 'c] :more
[:more 'a] :more
[:more 'd] :more
[:more 'r] :end
[:end nil] true}]
(loop [state :init
stream stream]
(let [next (step [state (first stream)])]
(when next
(if (= next true)
true
(recur next (rest stream))))))))
(declare init more end)
(defn init [stream]
(if (#{'c} (first stream))
(more (rest stream))))
(defn more [stream]
(cond
(#{'a 'd} (first stream)) (more (rest stream))
(#{'r} (first stream)) (end (rest stream))))
(defn end [stream]
(when-not (seq stream) true))