example of using external libraries or packages in Common Lisp - linux

Question

In Common Lisp, quicklisp is a popular library management tool. I'm going to use that tool and I'm going to try and use CL-WHO. I use the SBCL 1.0.57 implementation. I'm going to answer my own question below.
As a beginner, it's not clear how ASDF and quicklisp actually work together. And so it's not clear how to actually use packages or libraries that you've downloaded through quicklisp in an external source file. The quicklisp FAQ, at least at this moment, does not help. In python, it's incredibly simple: you can just put 'import somemodule' and life is great. Is there an equivalent for CL+quicklisp?
If you search, you find many results. Here are some of the most relevant ones I've found:
Lisp importing/loading file
How to use packages installed by quicklisp?
When I was reading through these originally, at least one question came to mind: do I actually have to care about ASDF if I'm using quicklisp? Quicklisp seems to be a higher level management tool. Other people suggest using quickproject. But is that really necessary?

Answer 1

The analogy to Python's imports is the system definition... well, this is a very loose analogy, but, it's the way to go. You declare dependencies in the system definition and then in source code you expect it to be there, so that if you later refer to the bits of the foreign code, you just do it.
Eg. in the system definition you might have: (usually it would be in my-program.asd file)
(defsystem :my-program
:version "0.0.1"
:serial t
:description "My program"
:components ((:file "some-source-file"))
;; `some-external-package' here is the "import", i.e. here you
;; declared that you will be using code from this package.
;; ASDF will generally "know" how to get the code of that package
;; from here on. But if it doesn't, then there are ways to "help it"
;; similar to how in Python there's a procedure to prepare your local
;; files to be used by easy_install
:depends-on (:some-external-package))
Later on in your code you just assume that the some-external-package is available to your program, e.g.:
(some-external-package:exported-symbol)
should just work. ("Your code" is the some-source-file.lisp, you've specified in the components).
This is the ASDF documentation on how to define systems
After you have this file in the place where ASDF might find it*, assuming you have ASDF installed (available to your Lisp, SBCL comes bundled with it), you'd load this system using (asdf:load-system :my-program) Explained here.
* - A quick way to test it would be to do
(push "/path/to/your/system/definition/" asdf:*central-registry*)

Answer 2

Download cl-who through the instructions on the quicklisp page and run this:
#!/usr/bin/sbcl --script
(load "~/quicklisp/setup.lisp")
(ql:quickload "asdf")
(asdf:load-system 'cl-who)
(with-open-file (*standard-output* "out.html" :direction :output)
(cl-who:with-html-output (*standard-output* nil :indent t)
(:html
(:head
(:title "Test page"))
(:body
(:p "CL-WHO is really easy to use")))))
To the beginner, or someone who's really lazy, there's no reason why you should have to write 3 lines at the top instead of just one (like in python).