{"id":122,"date":"2016-01-06T05:04:06","date_gmt":"2016-01-06T05:04:06","guid":{"rendered":"http:\/\/www.rangakrish.com\/?p=122"},"modified":"2016-01-06T05:04:06","modified_gmt":"2016-01-06T05:04:06","slug":"lisp-returning-multiple-values-from-a-function","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2016\/01\/06\/lisp-returning-multiple-values-from-a-function\/","title":{"rendered":"Lisp: Returning Multiple Values From A Function"},"content":{"rendered":"

One of the interesting features of Lisp is its support for returning multiple values from a function, without bundling the values in a special container. Languages such as Python and Ruby support multi-value return (although there are subtle differences from Lisp). In C++11, we can use std::make_tuple()<\/strong><\/em> and std::tie()<\/strong><\/em>.<\/p>\n

To take an example, in Lisp, the built-in function floor<\/i><\/b> returns both the quotient and reminder. However, the caller may use only the first value (quotient) and ignore the second value:<\/p>\n

(setf quotient (floor 7 3))<\/strong><\/span><\/p>\n

Here the call to floor<\/i><\/b> looks exactly like a call to a regular function that returns a single value, and as expected, the variable quotient<\/i><\/b> gets the value 2 (the reminder is discarded).<\/p>\n

Let us first see how a function can return multiple values. The following function computes both the sum and difference of two numbers passed as arguments:<\/p>\n

(defun sum-diff (num1 num2)<\/strong><\/span><\/p>\n

(values (+ num1 num2) (- num1 num2)))<\/strong><\/span><\/p>\n

The values<\/i><\/strong> function bundles all its arguments (which are evaluated) into a value list to be used by the caller. It is important to keep in mind that returning multiple values is not the same as returning a list of many values. The former is more efficient and flexible.<\/p>\n

OK. How do we consume multiple values from a function? We use the multiple-value-bind <\/i><\/b>construct.<\/p>\n

(multiple-value-bind (s d) (sum-diff)<\/strong><\/span><\/p>\n

(format t \u201cSum = ~D, Difference = ~D~%\u201d s d))<\/strong><\/span><\/p>\n

multiple-value-bind<\/i><\/b> is a macro, whose first argument is a list of variables, the second argument is a multi-valued expression, and the rest are forms that are evaluated in the context of the variable bindings.<\/p>\n

Going back to the floor<\/i><\/b> example, we can use it thus:<\/p>\n

(multiple-value-bind (quot rem) (floor 7 3)<\/strong><\/span><\/p>\n

(format t \u201cQuotient = ~D, Reminder = ~D~%\u201d quot rem))<\/strong><\/span><\/p>\n

if there are more variables than values returned by the multi-valued function, the extra variables get nil<\/strong><\/em>. If there are fewer variables than returned values, then only the corresponding values are bound.<\/p>\n

When do we write such multi-valued functions? There are two scenarios:<\/p>\n

* The values computed are logically related. In the case of floor<\/b>, the quotient and reminder are logically related<\/em><\/p>\n

* It is more efficient to compute the values together because most of the steps are common<\/em><\/p>\n

(Please note that the sum-diff function falls into\u00a0neither of the two scenarios, and hence is not a good example of a multi-valued function.)<\/p>\n

As an interesting use of the values<\/i><\/b> function, the following swaps the state of the two variables a<\/strong><\/em> and b<\/strong><\/em>:<\/p>\n

(setf (values a b) (values b a))<\/strong><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

One of the interesting features of Lisp is its support for returning multiple values from a function, without bundling the values in a special container. Languages such as Python and Ruby support multi-value return (although there are subtle differences from Lisp). In C++11, we can use std::make_tuple() and std::tie(). To take an example, in Lisp, […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[18,17],"tags":[19,36],"class_list":["post-122","post","type-post","status-publish","format-standard","hentry","category-lisp","category-programming","tag-lisp","tag-multiple-value-bind"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-1Y","jetpack-related-posts":[{"id":476,"url":"https:\/\/www.rangakrish.com\/index.php\/2017\/02\/03\/multi-value-return-in-c-7-0\/","url_meta":{"origin":122,"position":0},"title":"Multi-value Return in C# 7.0","author":"admin","date":"February 3, 2017","format":false,"excerpt":"One of the nice language enhancements to C# in the latest release (7.0) is the ability to return multiple values from a function. Although one could use Tuples for this purpose, it is not an elegant or efficient approach. For more details, see this article. 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