I have been copy-pasting the same code over and over and decided to wrap it up in trivial-project-pathname, which is a very simple Common Lisp library for resolving pathnames relative to some directory, usually the sources.
The typical use case looks like this: I am working on, say, some data analysis and I have the following directory structure:
| data/ | where data files are kept |
| latex/ | LaTeX sources |
| lisp/ | Common Lisp code, including ASDF system definition |
| plots/ | plots (potentially generated from Lisp) |
I want to be able to write something like
(analyze-data-and-save-plot (project-path "mydata.csv" :data) (project-path "myplot.pdf" :plots))
and have the data read from data/mydata.csv and the plot saved to plots/myplot.pdf.
This is how I would do it using this library:
(project-pathname:define project-path (:asdf "myproject" '(:relative :up)) (:data "data/") (:plots "plots/"))
This finds the directory for the .asd file, takes its parent directory, and then defines a function that maps symbols to subdirectories.
Unless otherwise specified, NIL, the default for the optional parameter, would map to the base directory. You don't even need to use subdirectories, you can just
(project-pathname:define project-path2 (:asdf "myproject")) ;;; and then use (project-path2 "my-data-file.csv")
You can also use *load-truename* to define *base-directory* in the .asd file as recommended by Zach Beane and then define
(project-pathname:define project-path (:directory *base-directory*))
