├── .gitignore ├── .travis.yml ├── LICENSE ├── README.md ├── project.clj ├── src └── xodarap │ ├── core.clj │ └── macro_utils.clj └── test └── xodarap ├── core_test.clj ├── macro_utils_test.clj └── test_utils.clj /.gitignore: -------------------------------------------------------------------------------- 1 | /target 2 | /classes 3 | /checkouts 4 | pom.xml 5 | pom.xml.asc 6 | *.jar 7 | *.class 8 | /.lein-* 9 | /.nrepl-port 10 | .hgignore 11 | .hg/ 12 | -------------------------------------------------------------------------------- /.travis.yml: -------------------------------------------------------------------------------- 1 | language: clojure -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | THE ACCOMPANYING PROGRAM IS PROVIDED UNDER THE TERMS OF THIS ECLIPSE PUBLIC 2 | LICENSE ("AGREEMENT"). 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No party to this 212 | Agreement will bring a legal action under this Agreement more than one year 213 | after the cause of action arose. Each party waives its rights to a jury trial 214 | in any resulting litigation. 215 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # xodarap 2 | [![Clojars Project](https://img.shields.io/clojars/v/xodarap.svg)](https://clojars.org/xodarap) 3 | [![Build Status](https://travis-ci.com/divs1210/xodarap.svg?branch=master)](https://travis-ci.com/divs1210/xodarap) 4 | 5 | Fearless recursion in Clojure! 6 | 7 | ## Why 8 | 9 | The JVM puts a hard limit on how deep our functions can recurse without blowing the stack: 10 | 11 | ```clojure 12 | ;; Ex. 1 13 | ;; ===== 14 | (defn unsafe-factorial [n] 15 | (if (< n 2) 16 | 1 17 | (*' n (unsafe-factorial (dec n))))) 18 | 19 | (unsafe-factorial 5000) 20 | ;; => StackOverflowError clojure.lang.Numbers.lt (Numbers.java:3816) 21 | ``` 22 | 23 | This is a very simple example, and the fn can be defined in a tail-recursive manner to avoid this issue, 24 | but we are not always this lucky! Consider: 25 | 26 | ```clojure 27 | ;; Ex. 2 28 | ;; ===== 29 | (defn unsafe-ackermann [m n] 30 | (cond (zero? m) (inc n) 31 | (zero? n) (recur (dec m) 1) 32 | :else (recur (dec m) (unsafe-ackermann m (dec n))))) 33 | 34 | (unsafe-ackermann 3 10) 35 | ;; => StackOverflowError clojure.lang.Numbers$LongOps.inc (Numbers.java:545) 36 | ``` 37 | Try converting the above to use tail recursion - possible, but not trivial. 38 | 39 | ... and then there are mutually-recursive functions: 40 | 41 | ```clojure 42 | ;; Ex. 3 43 | ;; ===== 44 | (letfn [(is-odd? [n] 45 | (if (zero? n) 46 | false 47 | (is-even? (dec n)))) 48 | (is-even? [n] 49 | (if (zero? n) 50 | true 51 | (is-odd? (dec n))))] 52 | (is-even? 10000)) 53 | ;; => StackOverflowError clojure.lang.Numbers$LongOps.isZero (Numbers.java:443) 54 | ``` 55 | Clojure's `recur` form doesn't help us here, so we generally end up using a [trampoline](https://clojuredocs.org/clojure.core/trampoline). 56 | 57 | Is there a general way to recurse safely (ie without blowing up the stack) 58 | in all these cases, and without changing the structure of our code? 59 | 60 | ## Usage 61 | 62 | ```clojure 63 | (use 'xodarap.core) 64 | 65 | ;; Ex. 1 66 | ;; ===== 67 | (defrec factorial [n] 68 | (if (< n 2) 69 | 1 70 | (*' n (rec (factorial (dec n)))))) 71 | 72 | (factorial 5000) 73 | ;; => 4228577926605543522201064200233584405390... 74 | ``` 75 | 76 | Here, we define a safe recursive factorial fn using the `defrec` form. Note that the 77 | recursive call to itself is wrapped in a `rec` form. 78 | 79 | ```clojure 80 | ;; Ex. 2 81 | ;; ===== 82 | (defrec ackermann [m n] 83 | (cond (zero? m) (inc n) 84 | (zero? n) (recur (dec m) 1) 85 | :else (recur (dec m) (rec (ackermann m (dec n)))))) 86 | 87 | (ackermann 3 10) 88 | ;; => 8189 (after a long pause) 89 | ``` 90 | 91 | We can crack tougher nuts using the same method. 92 | 93 | ```clojure 94 | ;; Ex. 3 95 | ;; ===== 96 | (letrec [(is-odd? [n] 97 | (if (zero? n) 98 | false 99 | (rec (is-even? (dec n))))) 100 | (is-even? [n] 101 | (if (zero? n) 102 | true 103 | (rec (is-odd? (dec n)))))] 104 | (is-even? 10000)) 105 | ;; => true 106 | ``` 107 | 108 | Here we define mutually recursive functions using `letrec`. 109 | 110 | ```clojure 111 | ;; like fn 112 | (recfn fact [n] 113 | (if (< n 2) 114 | 1 115 | (*' n (rec (fact (dec n)))))) 116 | ``` 117 | 118 | Similarly, `recfn` is an alternative to `fn`. 119 | 120 | ## License 121 | 122 | Copyright © 2018 Divyansh Prakash 123 | 124 | Distributed under the Eclipse Public License either version 1.0 or (at 125 | your option) any later version. 126 | -------------------------------------------------------------------------------- /project.clj: -------------------------------------------------------------------------------- 1 | (defproject xodarap "0.2.1" 2 | :description "Fearless recursion in Clojure!" 3 | :url "http://github.com/divs1210/xodarap" 4 | :license {:name "Eclipse Public License" 5 | :url "http://www.eclipse.org/legal/epl-v10.html"} 6 | :dependencies [[org.clojure/clojure "1.8.0"] 7 | [org.clojure/core.async "0.4.474"]]) 8 | -------------------------------------------------------------------------------- /src/xodarap/core.clj: -------------------------------------------------------------------------------- 1 | (ns xodarap.core 2 | (:require [clojure.core.async :refer [ #'u/fact meta :private false?)) 49 | (is (-> #'u/private-fact meta :private true?)))) 50 | -------------------------------------------------------------------------------- /test/xodarap/macro_utils_test.clj: -------------------------------------------------------------------------------- 1 | (ns xodarap.macro-utils-test 2 | (:require [clojure.test :refer :all] 3 | [xodarap.macro-utils :as u])) 4 | 5 | (deftest parse-params-test 6 | (is (= {:name 'foo 7 | :doc "Docstring" 8 | :argv ['a 'b] 9 | :body (list + 'a 'b)} 10 | (u/parse-params ['foo "Docstring" ['a 'b] + 'a 'b]))) 11 | (is (= {:name 'foo 12 | :doc "" 13 | :argv ['a 'b] 14 | :body (list + 'a 'b)} 15 | (u/parse-params ['foo ['a 'b] + 'a 'b])))) 16 | -------------------------------------------------------------------------------- /test/xodarap/test_utils.clj: -------------------------------------------------------------------------------- 1 | (ns xodarap.test-utils 2 | (:require [xodarap.core :refer [defrec defrec- rec]])) 3 | 4 | (defrec fact 5 | "test fn" 6 | [n] 7 | (if (< n 2) 8 | 1 9 | (*' n (rec (fact (dec n)))))) 10 | 11 | (defrec- private-fact [n] 12 | (fact n)) 13 | --------------------------------------------------------------------------------