Lisp format directive that interprets nil argument to empty string instead of “NIL”

问题

I know I can process the argument before passing it, I just wonder if there is a directive for this

回答1:

CL-USER> (format nil "~@[~A~]" "foo")
"foo"
CL-USER> (format nil "~@[~A~]" nil)
""

来源：https://stackoverflow.com/questions/5729032/lisp-format-directive-that-interprets-nil-argument-to-empty-string-instead-of-n