Going back in (dist) time

So you updated Quicklisp with (ql:update-dist "quicklisp"), and now a library you depended on is broken. It returns 42 from a function that previously returned 21, or maybe the API changed in an important incompatible way. Now your project is stalled and you have to figure out how to make things work again.

Quicklisp is designed with this situation in mind. Old project sources are kept around in indefinitely, and at a permanent URL, so you can go back to old sets of projects.

The user interface needs to be cleaned up, but here's how it works today:

* (use-package :ql-dist)
* (available-versions (dist "quicklisp"))
(("2011-07-30" . "http://beta.quicklisp.org/dist/quicklisp/2011-07-30/distinfo.txt") 
 ("2011-06-19" . "http://beta.quicklisp.org/dist/quicklisp/2011-06-19/distinfo.txt") 
 ("2011-05-22" . "http://beta.quicklisp.org/dist/quicklisp/2011-05-22/distinfo.txt") 
 ("2011-04-18" . "http://beta.quicklisp.org/dist/quicklisp/2011-04-18/distinfo.txt") 
 ("2011-03-20" . "http://beta.quicklisp.org/dist/quicklisp/2011-03-20/distinfo.txt") 
 ("2011-02-19" . "http://beta.quicklisp.org/dist/quicklisp/2011-02-19/distinfo.txt") 
 ("2011-01-10" . "http://beta.quicklisp.org/dist/quicklisp/2011-01-10/distinfo.txt") 
 ("2010-12-07" . "http://beta.quicklisp.org/dist/quicklisp/2010-12-07/distinfo.txt") 
 ("2010-11-07" . "http://beta.quicklisp.org/dist/quicklisp/2010-11-07/distinfo.txt") 
 ("2010-10-07" . "http://beta.quicklisp.org/dist/quicklisp/2010-10-07/distinfo.txt"))
* (install-dist "http://beta.quicklisp.org/dist/quicklisp/2011-06-19/distinfo.txt" :replace t)
lots of output

That set of commands will revert to the June, 2011 Quicklisp dist. So if your project critically depends on the versions you got in June, you can always go back to it.