Use of # a.k.a. read-macro

Question

Reading book \"Let Over Lambda\" by Doug Hoyte, I found the following description of #. sign, a.k.a. read-macro:

A basic read macro that come

Answer 1

Another difference between using #'foo and 'foo as function designators is that #'foo evaluates to a function object, but 'foo evaluates to a symbol. So using 'foo transfers the work of finding the function object to a later time. That can be a noticeable performance hit if you do it in every cycle of a loop instead of just once.

CL-USER> (defun foo () 42)
FOO
CL-USER> (read-from-string "'foo")
=> (QUOTE FOO), 4

CL-USER> (eval *)
FOO
CL-USER> (read-from-string "#'foo")
=> (FUNCTION FOO), 5

CL-USER> (eval *)
=> #