The why of version.lisp-expr

In all projects I manage, instead of including the version string verbatim in .asd files, I relegate it to an external file named version.lisp-expr which I then include in all .asd files, typically at least two:

 1;; foo.asd
 2
 3(asdf:defsystem :foo
 4  :version #.(with-open-file (f (merge-pathnames "version.lisp-expr"
 5                                                 (or *compile-file-pathname*
 6                                                     *load-truename*)))
 7               (read f))
 8...)
 9
10;; and version.lisp-expr
11
12;; -*- lisp -*-
13"0.0.1"

Q: Why this approach ?
A: Because it’s script-friendly: I have a generic release script that overwrites version.lisp-expr before committing and tagging, and it’s much safer to do it this way rather than using sed to replace the version string in .asd files. Also because this way the version info says in one place, instead of duplicating it in multiple files.

Q: Why use cl:read ?
A: Because it allows me to add comments to the file. Since I use #. to include the version, the format must be a CL string.

Q: Why .lisp-expr instead of .lisp ?
A: To make it obvious that the file isn’t meant to be compiled, but that the syntax is CL. It’s technically unnecessary.

Q: Where did I get this idea from ?
A: SBCL. Its build system includes version.lisp-expr only in release tarballs and computes the version using git-describe in source checkouts, but for most needs that’s unnecessarily complex.