Making a small Common Lisp Project (in Cusp)

In honor of my recent addition to Planet Lisp, and as a special thank you to Xach, I figured I'd counter-blog his old post on how to make a lisp project (in Slime). You can go read that if you want. Here's the Cusp way:

Make the project. Go to New > Lisp Project. Enter the name of your project. You're done.

Write the code. You'll notice that a file named "main.lisp" has been opened for you. Put your lisp code there. You can compile the definitions as you write them (with Alt+C), but that's mostly a vestigial holdover from Slime. Cusp is smart. Save your file and it will automatically compile any new or changed top-level forms. (It'll even undefine functions that you've deleted.)

Later on... If you close Cusp, when you come back you'll need to reload the project. To do this, right-click on the .asd file in the project, and choose "Load Project." You may also need to change the package in the REPL to match the one for this project.

So what is all this stuff?

You've got all you need to start hacking, but you probably want to know what's going on behind the scenes. The New Project wizard creates a project folder with three files in it: main.lisp, defpackage.lisp, and [project name].asd. Here's what they do:

main.lisp is the main code file. Put the guts of your program here. You can, of course, create additional files as your project grows. More on that in a moment.

defpackage.lisp defines the package for this lisp project, which is basically the lisp equivalent of a namespace. You could pollute cl-user with your project internals, but that's just asking for trouble down the line. You'll note that main.lisp started out with (in-package :[project-name]). You'll want to make sure any new files you add start that way, too.

If you want to import symbols from another package into your project (so you can say split rather than cl-ppcre:split) put the name of that package into the :use section. This is similar to using in C++ or C#, or import in Java. If your project turns into a library to be used by other projects, you would put any exported symbols into the :export section. Notice that by default, you are already importing :cl.

[project-name].asd is best thought of as your project file. This tells Lisp what libraries to load up for your project, and what files are part of your project. You'll notice that defpackage and main are already listed in the :components section. If you add more files to your project, make sure to tack them on to the end of that list. Files will be loaded in the order they are listed.

Dependencies are also handled by the asd file. If, as in the earlier example, you want to use cl-ppcre, make sure to add it to the :depends-on section.

Asd files can get more complicated if you need, but this should be enough to take care of you in 95% of all cases. Happy hacking!