API for a Mobile App in Common Lisp

Not too long ago, I released a mobile app called Lil’ Baby Names, for iOS and Android. I had a few friends help me. Mark Edelsberg designed the UI, Liu Xinjun built the Android front end, and Pritesh Shah built the iOS front end. I built the API that the devices use. What’s interesting about the API is that it’s written in Common Lisp. Why? Most of all because I love Lisp code. To my eye, the same algorithm written in Lisp or any other language always looks prettier in Lisp. But, I’ll get to that in another post. Here are some other reasons why I chose Lisp:

  • The code compilation process is fast and transparent
  • The code compiles directly into machine code
  • I can recompile functions in a running program, so I don’t have to restart the production server if I change any code
  • The REPL allows me to test pieces of my code as I go, providing continuous feedback on the work that I’m doing
  • Lisp macros and other lisp features make the code concise and easy to follow
  • The resulting service is really fast

For example, the Lil’ Baby Names API has a single endpoint and here’s how I define that endpoint:

(map-routes
  (get "/api/names" api-names))

Here’s another example. This code reads a small map of genders to gender IDs from the database:

(flatten (db-cmd *db* :query
                 (:select 'gender 'id :from 'genders)))

Note how I write my SQL in Common Lisp. That’s possible thanks to Lisp macros. This is not just beautiful, but also convenient: now, my editor can properly highlight, indent, and detect syntax errors in my SQL code.

As a final example, take a look at code that formats the results into JSON or plain text, depending on the value of the format variable:

(case format
  (:json
   (ds-to-json
    (ds `(:map :time ,(format nil "~ss" (read-time event))
               :total-matches ,results-length
               :page ,page
               :page-size ,page-size
               :regex ,regex
               :starts-with ,starts-with
               :ends-with ,ends-with
               :contains-string ,(get-parameter "contains-string")
               :contains-letters ,(get-parameter "contains-letters")
               :min-length ,min-length
               :max-length ,max-length
               :format ,format
               :sort ,sort
               :results ,(ds-list sorted-paged-results)))))
  (otherwise
   (format nil "~{~a~^~%~}"
           (mapcar (λ (x) (gethash :name x))
                   sorted-paged-results))))

Note the use of the Common Lisp format function, and how concise it can make the code. Also, notice how easy it is to transform a Lisp object or nested list into JSON.

Sure, there are some negative aspects of the language. For one, Common Lisp is a big language with a number of fairly powerful functions and macros (think format and loop). It can take a little time to get the hang of those. Furthermore, the best Common Lisp editor is probably Emacs, and who wants to learn that? However, for those brave programmers who choose to overcome those obstacles, Common Lisp provides a real glimpse into the future of programming.

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