λGTK: a Portable Graphics Layer for Common Music

λGTK (LAMBDA GTK) is a new, portable graphics layer for implementing graphical user interface (GUI) tools in Common Music [1] (CM), an algorithmic music composition environment that runs in Common Lisp [2] and Scheme [3]. λGTK provides a complete interface to the popular GTK2 [4] graphics libraries and currently supports the development of graphical displays in three different public domain Lisp implementations on two different operating systems. Two full-featured GUI tools for Common Music -a music data visualization tool called Plotter and an executive “front end” window called CMIO -are briefly described as examples of λGTK functionality. A key feature of λGTK is that it is generated programmatically from information parsed from GTK header declarations rather than coded directly by hand. One benefit of this implementation method is that larger (or less inclusive) interfaces can be generated on demand so Lisp applications only need to load bindings for entry points in the GTK libraries that they actually use. Porting λGTK to new Lisp implementations is also much less work than if the layer had been coded by hand, and resolving errors and other porting issues is now a global operation (regenerating the layer) rather than a local one (finding each affected entry and editing it by hand in a text editor). To port λGTK to a new Lisp the developer simply provides “back end” functions to the generating program that translates the system’s intermediate Lisp representation of GTK’s data structures into the appropriate foreign function interface (FFI) code for the new target host. The λGTK layer itself does not depend on anything in Common Music and is available as a separate, free software package released under the Lisp Lesser General Public License (LLGPL). 1. BACKGROUND AND MOTAVATION Common Music is an object-oriented music composition environment. It generates sound by transforming a high-level Lisp representation of musical structure (algorithms, musical event descriptions and other compositional representations) into a variety of protocols for controlling sound synthesis and music score generation. CM has bindings for both Common Lisp and Scheme dialects of Lisp and runs in a variety of Lisp implementations and operating systems. Its highly portable nature, coupled with the fact that neither Common Lisp nor Scheme requires a graphics standard, has made Common Music an essentially text-based system until now. For a certain class of user – someone trained in musical synthesis languages or having prior programming experience – the lack of GUI tools has not presented an undue hardship or problem. But with the publication of Notes from the Metalevel [5] (the author’s book on algorithmic composition) in 2004, new users who have no background in sound synthesis or programming have begun to experiment with algorithmic composition and Common Music. The author decided to use this recent circumstance as the impetus to implement a graphic layer. Since CM is both portable and free, two central requirements of the new graphics code are that (1) the graphics libraries must also be free and (2) the graphical tools that are developed must be usable in more than one public domain Lisp implementation and on more than one operating system. After considering a number of different strategies and options the author decided to base CM’s graphics on Version 2 of the Gimp Tool Kit (GTK2). GTK is a popular, public domain graphics package that runs on multiple platforms, including Linux, OS X and Windows. Its powerful windowing features, free distribution, and stable release cycles make it a good choice to implement source portable graphical interfaces for Common Music. After first attempting to use and adopt a few existing Lisp/GTK interfaces it became apparent that there was no existing software that offered a complete Lisp binding for the complete GTK2 API or that would run on multiple operating systems and Lisp implementations. Mario Momar’s LGTK [6] package was the best candidate system, but, because it is written by hand, it implements only a tiny fraction of the thousands of possible GTK entry points. After considering the size of GTK and the fact that multiple bindings would be required for a portable layer, the author decided that generating Common Music’s graphics layer was the appropriate way to solve both problems.