Under the Hood of App Inventor for Android

Posted by Bill Magnuson, Hal Abelson, and Mark Friedman

We recently announced our App Inventor for Android project on the Google Research Blog. That blog entry was long on vision but short on technological details--details which we think would be of interest to our readers.

Of particular interest is our use of Scheme. Part of our development environment is a visual programming language similar to Scratch. The visual language provides a drag-and-drop interface for assembling procedures and event handlers that manipulate high-level components of Android-based phones. The components are similar to the ones in the recently announced Simple; in fact, the code bases share an ancestor.

We parse the visual programming language into an S-expression intermediate language, which is a domain-specific language expressed as a set of Scheme macros, along with a Scheme runtime library. We did this for a few reasons:

• S-expressions are easy to generate and read for both humans and machines.
• Scheme macros are a convenient (albeit sometimes arcane) way to express S-expression based syntax.
• Scheme is a small, powerful and elegant language well suited to describe and evaluate a large set of programming semantics. Additionally, it provides the flexibility that we require as our language and its semantics grow and develop.
• Scheme expertise was readily available among our team.
• A pre-existing tool (Kawa by Per Bothner) to create Android compatible output from scheme code was already available.

For now the project is just an experiment we're performing with a dozen colleges and universities, but we hope to eventually open up the development environment to wider use and to open-source parts of the code.