Shelf is a lightweight implementation of prototypes for Guile Scheme.
Shelf lets you define simple objects containing key/value pairs. These objects can use another object as a prototype. Objects have all the properties of their prototype, plus whatever key/value pairs they choose to add or override.
Shelf lets you create methods as object properties with 'this' and 'super' bindings similar to object-oriented languages. Think something like Javascript with the clean syntax and power of of Scheme.
Saving and loading with Shelf is easy—Shelf objects can be written out as Scheme syntax and then run back through the interpreter.
Shelf also lets you compile your objects to object code and then back to Scheme whenever you like.
I was writing a roguelike game that involved lots of monsters, items, and map features that each have a lot of properties. I wanted to be able to add new properties to objects without the trouble of modifying class definitions all the time.
First I tried to write the game in C and add a little config language with a Flex/Bison lexer/parser for defining and saving objects. That worked all right but I got frustrated that I couldn't easily include functions in it. I didn't want to write a whole programming language from scratch!
I had some experience with Javascript and I liked first-class functions and prototype object models. But I didn't like much anything else about Javascript. There are lots of nice languages that do have prototypes, like Self, Lua, and Tcl, but I decided it would be fun to actually create a prototype system of my own instead.
With its powerful language-extending macros, Scheme is the perfect language for that sort of thing. I picked Guile as my implementation because I already had some experience writing scripts with it to extend the musical score editor Lilypond.
Loading a really big Shelf object with lots of Scheme objects as properties is slow. That's partly because defining all of an object's properties inside of a closure created at run-time seems to defeat many of Guile's attempts at optimization. There is some exciting work going on right now to bring a faster VM and JIT compilation to Guile. I think that will help speed Shelf up a lot. I should probably look for some optimizations on my own, too.
Guile ships with a really neat and powerful object system called GOOPS. It's so powerful, in fact, I'm fairly sure you can make it do what Shelf does. But I wanted to do this from scratch. Maybe I'll look for some ways to integrate the two in my next version.
Though this release contains several Guile-specific features, the basic behavior of Shelf is Scheme-implementation independent. In other words, you could pretty easily take out all the Guile stuff and port this to another Scheme. The only extra features absolutely required are hashtables and procedures with setters. I made a Chicken Scheme port of Shelf a while back, though it's not very up to date at the moment.
Oh yeah, maybe some time I'll finish and release the game that inspired all this.