Question

Is there a way in Common Lisp to create a generic function
where the argument to specialize on is a keyword argument?

For example rather than this:

CL-USER> (defgeneric tst (a))
#<STANDARD-GENERIC-FUNCTION TST (0)>
CL-USER> (defmethod tst ((a list)) 
           (print a))
#<STANDARD-METHOD TST (LIST) {1004ECC903}>

..which specifies on the argument ‘a’.
I’d like to have the following:

CL-USER> (defgeneric tst (&key a))
#<STANDARD-GENERIC-FUNCTION TST (0)>
CL-USER> (defmethod tst (&key (a list)) 
           (print a))
#<STANDARD-METHOD TST (LIST) {1004ECC903}>

Obviously this clashes with the syntax for setting the
default values of keyword arguments so I’m a tad stuck. In the actual code this problem relates to I chose a different argument layout so this wasn’t an issue (and was a bit tidier to boot!) but in case I run into this again I thought I best ask the experts!

Cheers all

Answer 1

No, that’s not provided by Common Lisp.