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https://github.com/divs1210/recursor

Better recursion for Clojure
https://github.com/divs1210/recursor

clojure optimize recursion stack stackoverflow

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Better recursion for Clojure

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# recursor

[![Clojars Project](https://img.shields.io/clojars/v/recursor.svg)](https://clojars.org/recursor)

[![CircleCI](https://circleci.com/gh/divs1210/recursor/tree/master.svg?style=svg)](https://circleci.com/gh/divs1210/recursor/tree/master)

[![codecov](https://codecov.io/gh/divs1210/recursor/branch/master/graph/badge.svg)](https://codecov.io/gh/divs1210/recursor)



Better recursion for Clojure.



## Usage



Here's the Ackermann function:



```

A(0, n) = n+1

A(m, 0) = A(m-1, 1)

A(m, n) = A(m-1, A(m, n-1))

```



Here's a straightforward implementation in Clojure:



```clojure

(defn A [m n]

  (cond

    (zero? m) (inc n)

    (zero? n) (recur (dec m) 1)

    :else (recur (dec m) (A m (dec n)))))

```



Great! Let's try it out!



```clojure

user> (time (A 3 8))

"Elapsed time: 85.518103 msecs"

;; => 2045

user> (time (A 4 1))

;; StackOverflowError !!!

```



Now, while you:



1. look for a mathematician, and

2. plead them to derive an iterative version of the function, and

3. refuse to budge till they optimize it



I will do this:



```clojure

(require ' [recursor.core :as r :refer [defrec recurse return]])



(defrec ^{::r/cache-size 100000}

  A

  [m n]

  (cond

    (zero? m) (return (inc n))

    (zero? n) (recurse (A (dec m) 1))

    :else (recurse (A m (dec n))

            :then #(A (dec m) %))))

```



Let's see what it can do:



```clojure

user> (time (A 3 8))

"Elapsed time: 95.970201 msecs"

;; => 2045

user> (time (A 4 1))

"Elapsed time: 2790.475629 msecs"

;; => 65533

```



It achieves this by:



1. Using a custom **unbounded** stack

2. **Optionally** using an [LU cache](https://github.com/clojure/core.cache/wiki/LU) to remember past results if specified in the metadata



It also provides replacements for some other Clojure core forms, all of which can be memoized by setting `^{::r/cache-size }` before their names:



| clojure.core  | recursor.core |

|:-------------:|:-------------:|

| `fn`          | `recfn`       |

| `defn`        | `defrec`      |

| `defn-`       | `defrec-`     |

| `letfn`       | `letrec`      |



All of these work in the way you would expect them to, with the following caveats:



1. No varargs in `recfn`s - use a `fn` with varargs to dispatch to fixed-arity `recfn`

2. No multiple-arity `recfn`s - use a multi-arity `fn` to dispatch to fixed-arity `recfn`

3. No destructuring maps in arguments vector - `(recfn f [{:keys [a b]}])`

4. No mutual recursion support - use [trampoline](https://clojuredocs.org/clojure.core/trampoline) as usual



All this is subject to change **without** breaking the API.



## License



Copyright © 2020 Divyansh Prakash



This program and the accompanying materials are made available under the

terms of the Eclipse Public License 2.0 which is available at

http://www.eclipse.org/legal/epl-2.0.



This Source Code may also be made available under the following Secondary

Licenses when the conditions for such availability set forth in the Eclipse

Public License, v. 2.0 are satisfied: GNU General Public License as published by

the Free Software Foundation, either version 2 of the License, or (at your

option) any later version, with the GNU Classpath Exception which is available

at https://www.gnu.org/software/classpath/license.html.