Mutual recursion: Difference between revisions

Revision as of 13:21, 26 May 2011

Revision as of 14:57, 12 March 2012

Line 19:

Mutual recursion is very common in the [[functional programming]] style, and is often used for programs written in [[Lisp programming language|LISP]], [[Scheme (programming language)|Scheme]], [[ML programming language|ML]], and similar [[programming language|languages]]. In languages such as [[Prolog programming language|Prolog]], mutual recursion is almost unavoidable.

Some programming styles discourage mutual recursion, claiming that it can be confusing to distinguish the conditions which will return an answer from the conditions that would allow the code to run forever without producing an answer. It is usually possible to turn two mutually recursive functions into a single recursive function by inlining the code of one into the other, possibly at the expense of legibility. [[Peter Norvig]] points to a [[design pattern]] which discourages the use entirely, stating

{{quote|text=If you have two mutually-recursive functions that both alter the state of an object, try to move almost all the functionality into just one of the functions. Otherwise you will probably end up duplicating code.|sign=Peter Norvig <ref>[http://norvig.com/sudoku.html Solving Every Sudoku Puzzle]</ref>}}

Any mutual recursion can be converted to direct recursion using proceduralby inlining the code of one procedure into the other.<ref>[http://delivery.acm.org/10.1145/180000/176510/p151-kaser.pdf?key1=176510&key2=1857140721&coll=GUIDE&dl=GUIDE&CFID=82873082&CFTOKEN=54657523 On the Conversion of Indirect to Direct Recursion] by Owen Kaser, C. R. Ramakrishnan, and Shaunak Pawagi at [[State University of New York, Stony Brook]] (1993)</ref>

In mathematics, the [[Hofstadter sequence#Hofstadter Female and Male sequences|Hofstadter Female and Male sequences]] are an example of a pair of integer sequences defined in a mutually recursive manner.