See for example: https://github.com/vindarel/cl-str#empty-emptyp-s
Or this: https://stackoverflow.com/a/33379360/12400477
I have seen this in several places, and I don't know what it means. Does the p suffix indicate the type the function returns? Does it always mean it returns a bool? Why do there seem to be two alternative symbols for the same function in the first example?
For Common Lisp there are a bunch of naming conventions. Adding a p or -p is for functions which return a truth value.
This does not mean that the function will return only t or nil. Other values of true are also possible:
Example of a predicate which will return a different value from t for true.
CL-USER 1 > (digit-char-p #\a)
NIL
CL-USER 2 > (digit-char-p #\1)
1 ; any object other than nil
; also means *true*
The convention to use ? instead is coming from the Scheme programming language.