In Emacs, what does this error mean? “Warning: cl package required at runtime”

Question

I am byte-compiling a module. It gives me this warning:

 Warning: cl package required at runtime

Why is this a warning? I am well aware that I

Answer 1

Common Lisp has lots of namespace clashes with elisp, often the functions seem to do the same thing, but differ in some subtle detail. Mixing the two is a risk that is best not done behind the user's back. For this reason, most of the more useful functions in cl.el are defined as macros, so that cl.el can be required at compile time only, and the macros will then only affect the code that uses them in future sessions of Emacs.