{"id":2071,"date":"2020-08-01T19:23:24","date_gmt":"2020-08-01T13:53:24","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=2071"},"modified":"2020-08-01T19:23:24","modified_gmt":"2020-08-01T13:53:24","slug":"elixir-wrapping-lisp-calls-in-a-macro","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2020\/08\/01\/elixir-wrapping-lisp-calls-in-a-macro\/","title":{"rendered":"Elixir: Wrapping Lisp Calls in a Macro"},"content":{"rendered":"

In the last article<\/strong><\/em><\/a>, I showed how to make calls to Lisp functions from Elixir<\/strong><\/em>. We followed this pattern:<\/p>\n

1) Initialize the Lisp environment by calling Lisp.init<\/strong><\/em><\/span><\/p>\n

2) Call Lisp functions as needed<\/span><\/p>\n

3) Free the Lisp environment by calling Lisp.quit<\/strong><\/em><\/span><\/p><\/blockquote>\n

Using the powerful macro programming capabilities of Elixir<\/strong><\/em>, we can write a simple macro to wrap calls to Lisp<\/strong><\/em> functions, by adding Lisp.init<\/strong><\/em> and Lisp.quit<\/strong><\/em> automatically. This way, we do not have to worry about “forgetting” to handle initialization and termination. Such an approach is quite common when we program in Lisp<\/strong><\/em> itself.<\/p>\n

I have made a minor change in the Lisp NIF<\/strong><\/em> layer, corresponding to the “quit”<\/strong><\/em> function, to ensure that “init”<\/strong><\/em> and “quit”<\/strong><\/em> can be called multiple times as a pair. Here is the modified version:<\/p>\n

\"Updated<\/a>
Updated Quit Function<\/strong><\/figcaption><\/figure>\n

Elixir<\/strong><\/em> macros are a major topic and we are not getting into the details here. There are several nice tutorials available on the internet. For example, see this<\/strong><\/em><\/a>\u00a0and this<\/strong><\/em><\/a>.<\/p>\n

Here is our macro:<\/p>\n

\"Our<\/a>
Our Macro<\/strong><\/figcaption><\/figure>\n

Macros can be difficult to understand and debug. So it helps if we have a utility function that shows how a macro call is actually expanded by Elixir<\/strong><\/em>. Writing such a function (a macro again!) is straightforward. I took the following code snippet from this article<\/strong><\/em><\/a>:<\/p>\n

\"Understanding<\/a>
Understanding Macro Expansion<\/strong><\/figcaption><\/figure>\n

Now, it is time to actually put our macro to use. Here is a sample session:<\/p>\n

\"Sample<\/a>
Sample Session<\/strong><\/figcaption><\/figure>\n

Since we are calling a macro, we have to “require Lisp”<\/strong><\/em> first. Additionally, for convenience, I am importing<\/strong><\/em> the same package. This allows us to use the functions in the package without explicitly using the package prefix.<\/p>\n

You can also see how “mex”<\/strong><\/em>\u00a0dumps the expanded macro call.\u00a0Lisp.init<\/strong><\/em>\u00a0and Lisp.quit <\/strong><\/em>are automatically inserted at the beginning and end respectively by our “with_lisp”<\/strong><\/em> macro.<\/p>\n

The updated NIF “C”<\/strong><\/em> source and the Elixir<\/strong><\/em> code are available here<\/strong><\/em><\/a>.<\/p>\n

Have a nice weekend!<\/p>\n","protected":false},"excerpt":{"rendered":"

In the last article, I showed how to make calls to Lisp functions from Elixir. We followed this pattern: 1) Initialize the Lisp environment by calling Lisp.init 2) Call Lisp functions as needed 3) Free the Lisp environment by calling Lisp.quit Using the powerful macro programming capabilities of Elixir, we can write a simple macro […]<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[242,18,17],"tags":[243,19,246,245],"class_list":["post-2071","post","type-post","status-publish","format-standard","hentry","category-elixir","category-lisp","category-programming","tag-elixir","tag-lisp","tag-macro","tag-nif"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-xp","jetpack-related-posts":[{"id":2057,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/07\/19\/calling-lisp-functions-from-elixir\/","url_meta":{"origin":2071,"position":0},"title":"Calling Lisp Functions from Elixir","author":"admin","date":"July 19, 2020","format":false,"excerpt":"In the last article\u00a0I showed how we can simulate the idea of Lisp's \"closure\" in Elixir. Today, I would like to demonstrate how we can call Lisp functions from Elixir using the NIF interface. What is the need to integrate Elixir with another language? 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