common-lisp Functions as first class values Referring to Existing Functions
Example
Any symbol in Common Lisp has a slot for a variable to be bound and a separate slot for a function to be bound.
Note that the naming in this example is only for illustration. Global variables should not be named foo, but *foo*. The latter notation is a convention to make it clear that the variable is a special variable using dynamic binding.
CL-USER> (boundp 'foo) ;is FOO defined as a variable?
NIL
CL-USER> (defvar foo 7)
FOO
CL-USER> (boundp 'foo)
T
CL-USER> foo
7
CL-USER> (symbol-value 'foo)
7
CL-USER> (fboundp 'foo) ;is FOO defined as a function?
NIL
CL-USER> (defun foo (x y) (+ (* x x) (* y y)))
FOO
CL-USER> (fboundp 'foo)
T
CL-USER> foo
7
CL-USER> (symbol-function 'foo)
#<FUNCTION FOO>
CL-USER> (function foo)
#<FUNCTION FOO>
CL-USER> (equalp (quote #'foo) (quote (function foo)))
T
CL-USER> (eq (symbol-function 'foo) #'foo)
T
CL-USER> (foo 4 3)
25
CL-USER> (funcall foo 4 3)
;get an error: 7 is not a function
CL-USER> (funcall #'foo 4 3)
25
CL-USER> (defvar bar #'foo)
BAR
CL-USER> bar
#<FUNCTION FOO>
CL-USER> (funcall bar 4 3)
25
CL-USER> #'+
#<FUNCTION +>
CL-USER> (funcall #'+ 2 3)
5
