## Setting up Lisp Systems

### What is a Lisp System?

The common definition of a Lisp System is actually an implementation, or dialect, of Lisp itself. SBCL for example is a Lisp system based on the ANSI standard. For our purposes however, we will be using it in the same way that it is used in Zach Beane's post on Quickproject, which forms the basis of this article. Actually this is just a paraphrasing of Zach's post, which I recommend that every beginner (and maybe not-so-beginners) read.

The terminology is slightly vague, it's probably best described as being akin to a library, as it is designed to be called by ASDF, but it does not have to be a set of user tools (which in my mind is what a library is, a set of reusable functions and/or class objects). Project would be a good way to define it, and that's actually the term the original article uses, but since ASDF itself uses the command defsystem to define it, we will use that term.

Essentially it is code that can be loaded by ASDF (along with any other dependencies that are described in the project files) and therefore used in other projects.

### Enough with the Etymology

So let's get started... we will assume that you have already loaded and installed Quicklisp, if not go ahead and do so.

So ASDF2, the latest incarnation of ASDF, will search for systems in the default directory ~/.local/share/common-lisp/source. If this is where your lisp code lives or will be living then you don't need to do any further setup. If however, you want to put your code in another directory (mine for example lives in ~/dev/lisp/) then create the following file in the following directory:

~/.config/common-lisp/source-registry.conf.d/projects.conf

Add this to the file (in this example we will be using my toplevel lisp directory):

This tells ASDF that all your "systems" are located here.

Load Quickproject through Quicklisp and create a project:

The last directory in the argument passed to make-project is created, if not already existing, and is also used as the project name. make-project then populates the directory with four files:

The project can now be called by ASDF and Quicklisp:

Of course the source files are empty so nothing's going to happen, but at least we have now understood an easy way of creating Lisp systems.

### Where does the Code go?

So now that we have a system that can be called and loaded by ASDF, and therefore also Quicklisp, we should start writing down some code... but where do we put it? And what are all the other files for?

So the functions and internal workings of the project will be put in my-tools.lisp

This is some old code that I wrote that lists all possible permutations of a list. It's incredibly clunky and inefficient, but it will serve for our demo purposes.

So now that we have the code in my-tools.lisp, what about the other files? README.txt is pretty self-explanatory, but what does package.lisp contain?

(quickproject:make-project) automatically creates a namespace for our project and puts that in package.lisp. These are the default contents:

This is also where you would include any external libraries or projects if you want to call them without package prefixes. As Zach's original post has an example of this, I won't be including it here (why then am I writing this if one can read the original article? good question...).

Now on to my-tools.asd. This is the file that describes to ASDF what the project consists of and what external projects have to loaded upon loading our project.

If we want to call an external library, for example Drakma, then we would add :depends-on (#:drakma) like so:

So let's try loading (or reloading) our new system. Remember, my-tools.lisp now contains our code so we should be able to call the functions in it.

Now to test the functions found in my-tools.lisp:

It works! We can now load independent systems (and all their dependencies) through Quicklisp, making them ridiculously easy to use in other projects.

### Namespace

There is one glaring problem with the way we have organized the code however... if we try to call the function without the namespace prefix like so...

... the REPL will complain. Of course we can just do what we did earlier and call it with double colons my-tools::permute but this is generally considered bad form, read this post on packages for the why. Instead we can add this to package.lisp.

Now the function is publicly available, we can call it with a single colon. First we reload the whole system, then try the newly available function.

For larger projects you'll probably split the code between several files, e.g. when I was experimenting with writing a simple CMS, I split the code into four files:

• models.lisp which was where the content layout, etc. was described
• urls.lisp which dealt with creating the individual urls of each page
• views.lisp which contained code for viewing the posts
• init.lisp, the part that called the dependencies and the individual source files

To load your individual files at the time that the system is called, you update the system definition and add in the files with the :components parameter.

One thing to keep in mind is that since the :serial parameter is set to t, the files will load sequentially, so you have to order them in such a way that the files that are not dependent on each other are called first.

### Bundling your Systems with Newly-Created Systems

You might create a whole new project but are dependent on some previous system's code, e.g. again using my CMS as an example, I had to load the system aserve, which is a webserver called AllegroServe by the guys over at Franz.

You can do this by specifying the wanted library/project/system when you create a new project.

As you can see, once the system aserve is loaded, the packages that are bundled with it, like net.aserve, become available.

### From here...

There are a few other things that are found on Zach's page that I haven't discussed, again I highly recommend going over his post on the subject.

Happy Hacking!