https://github.com/Wilfred/tco.el

Tail call optimisation in Emacs lisp
https://github.com/Wilfred/tco.el

emacs-lisp tail-call-optimisation

Last synced: about 2 months ago
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Tail call optimisation in Emacs lisp

• Host: GitHub
• URL: https://github.com/Wilfred/tco.el
• Owner: Wilfred
• Created: 2013-05-25T14:59:47.000Z (over 11 years ago)
• Default Branch: master
• Last Pushed: 2019-11-29T20:40:38.000Z (almost 5 years ago)
• Last Synced: 2024-06-15T14:33:46.800Z (3 months ago)
• Topics: emacs-lisp, tail-call-optimisation
• Language: Emacs Lisp
• Homepage:
• Size: 19.5 KB
• Stars: 76
• Watchers: 5
• Forks: 7
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGELOG.md

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README

        
# tco.el

**Tail call optimisation for Emacs lisp**

[![Build Status](https://travis-ci.org/Wilfred/tco.el.svg?branch=master)](https://travis-ci.org/Wilfred/tco.el)

tco.el provides tail-call optimisation for functions in elisp that

call themselves in tail-position. Mutually recursive functions are

unchanged.

It works by replacing each self-call with a thunk, and wrapping the

function body in a loop that repeatedly evaluates the thunk. Roughly

speaking, a function `foo`:

```lisp

(defun-tco foo (...)

  (...)

  (foo (...)))

```

Is rewritten as follows:

```lisp

(defun foo (...)

   (flet (foo-thunk (...)

               (...)

               (lambda () (foo-thunk (...))))

     (let ((result (apply foo-thunk (...))))

       (while (functionp result)

         (setq result (funcall result)))

       result)))

```

## Example

```lisp

;; -*- lexical-binding: t -*-

(require 'tco)

(defun-tco sum (n &optional accum)

  (setq accum (or accum 0))

  (if (zerop n)

      accum

    (sum (1- n) (+ accum n))))

;; Without TCO, values greater than `max-lisp-eval-depth' (usually

;; 600) would cause stack overflow here:

(sum 700)

```

## Known issues

Due to

[a bug in cl-letf in Emacs 24.3.1](http://emacs.stackexchange.com/questions/3450/whats-the-correct-replacement-for-flet-on-new-emacsen#comment5015_3452),

this package will not work on Emacs 24.3.1.

## Other Projects

[recur](https://github.com/VincentToups/recur) also offers TCO for

self-tail recursion.