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diff --git a/users/wpcarro/lisp/f/README.md b/users/wpcarro/lisp/f/README.md
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+# f.lisp
+
+In this project, I'm attempting to port the Elisp library [`f.el`][1] to Common Lisp.
+
+[1]: https://github.com/rejeep/f.el
diff --git a/users/wpcarro/lisp/f/default.nix b/users/wpcarro/lisp/f/default.nix
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+++ b/users/wpcarro/lisp/f/default.nix
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+{ depot, briefcase, ... }:
+
+depot.nix.buildLisp.library {
+  name = "f";
+  deps = with briefcase.lisp; [
+    prelude
+  ];
+  srcs = [
+    ./main.lisp
+  ];
+}
diff --git a/users/wpcarro/lisp/f/main.lisp b/users/wpcarro/lisp/f/main.lisp
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+++ b/users/wpcarro/lisp/f/main.lisp
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+(in-package #:cl-user)
+(defpackage #:main
+  (:documentation "Modern API for working with files and directories.")
+  (:use #:cl)
+  (:shadow #:type))
+(in-package #:main)
+
+;; Common Lisp distinguishes between `namestrings` and `pathnames` as two types
+;; of filename representations.
+;;
+;; A `pathname` is a structured representation of the name of a file, which
+;; consists of six parts:
+;; 1. host
+;; 2. device
+;; 3. directory
+;; 4. name
+;; 5. type
+;; 6. version
+
+;; TODO: Should I be using `string` as a type or `namestring`?
+
+(defmacro type (name in out)
+  `(declaim (ftype (function ,in ,out) ,name)))
+
+(type join (&rest namestring) pathname)
+(defun join (&rest args)
+  "Join ARGS to a single path."
+  (apply #'merge-pathnames args))
+
+(type ext (pathname) string)
+(defun ext (path)
+  "Return the file extension of PATH."
+  (pathname-type path))
+
+;; TODO: Define these tests elsewhere.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; join
+(string= (join "path") "path")
+(string= (join "path" "to") "path/to")
+(string= (join "/" "path" "to" "heaven") "/path/to/heaven")
+
+;; ext
+(string= (ext #p"path/to/file.ext") "ext")
+(string= (ext #p"path/to/directory") nil)