Exercise 1.2.14

Pathnames

Lisp has a special way of handling files from your computer: pathname objects. They allow Lisp to handle file-names, for the most part, in a platform-agnostic manner. If they seem like overkill, just keep in mind how many different platforms and file-systems Lisp supports.

Pathname objects are represented using the reader macro syntax #P"..."; they look like strings preceded by a Sharpsign-P, but they have a lot of internal structure you have to know about. Let's take a look at what you can get out of a seemingly simple pathname:

(truename ".")

(pathname-directory (truename "."))

(pathname-host (truename "."))

(pathname-name (truename "."))

(pathname-type (truename "."))

You'll get nil back for pathname-name and pathname-type. But now let's create a new file with your text editor named "llthw-ex-1-2-14.lisp" and save it in the folder on your computer you got back from (truename "."). If you changed the default folder to open SBCL in, like you were instructed to, this should be your home folder, or a folder dedicated to your Lisp code.

In "llthw-ex-1-2-14.lisp" enter the following code and save it again:

;; an empty Common Lisp file

Back at the REPL, try this now:

(truename "llthw-ex-1-2-14.lisp")

(pathname-name (truename "llthw-ex-1-2-14.lisp"))

(pathname-type (truename "llthw-ex-1-2-14.lisp"))

(file-namestring (truename "llthw-ex-1-2-14.lisp"))

What You Should See

Obviously, the exact pathnames you see will be personalized for your system. Mine below are just for sake of example, so don't panic if you don't see exactly the same pathname objects that I have.

* (truename ".")
#P"/home/colin/"
* (pathname-directory (truename "."))
(:ABSOLUTE "home" "colin")
* (pathname-host (truename "."))
#<SB-IMPL::UNIX-HOST {1000020BE3}>
* (pathname-name (truename "."))
NIL
* (pathname-type (truename "."))
NIL

After creating the file "llthw-ex-1-2-14.lisp" in the directory you got from (truename "."):

* (truename "llthw-ex-1-2-14.lisp")
#P"/home/colin/llthw-ex-1-2-14.lisp"
* (pathname-name (truename "llthw-ex-1-2-14.lisp"))
"llthw-ex-1-2-14"
* (pathname-type (truename "llthw-ex-1-2-14.lisp"))
"lisp"
* (file-namestring (truename "llthw-ex-1-2-14.lisp"))
"llthw-ex-1-2-14.lisp"

Do you understand what's going on here? Take a moment to review to see if you can guess what everything is doing, and that all the code is returning what you think should be returned.

Let's start with the function truename. You can see that it takes a regular string that represents, but isn't, a pathname object. In fact, truename accepts any pathname designator, which can be a relative or full pathname namestring, a file stream, or an actual pathname object.

The rest of the new functions you've seen this exercise will only accept a pathname object, proper. That is why you need to first call truename on the pathname designator strings above, to get a pathname object to pass to the pathname functions.

Notice the last function in the examples above, file-namestring? It gives you back just the file's namestring for a pathname object to a file. If you give it a pathname object to a directory, it just returns an empty string, "". There are some other, similar functions related to it: namestring, directory-namestring, host-namestring, and enough-namestring. Can you guess what they do?

Extra Credit

Try out the extra namestring functions, namestring, directory-namestring, host-namestring, and enough-namestring. Can you get them to run? Can you predict the results you get from them?