I’m reading An Introduction to Programming in Emacs Lisp. I like this passage:
A symbol can have both a function definition and a value attached to it at the same time. Or it can have just one or the other. The two are separate. This is somewhat similar to the way the name Cambridge can refer to the city in Massachusetts and have some information attached to the name as well, such as “great programming center”.
Another way to think about this is to imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice-versa.
English works this way. The symbol
drive has (several) function/verb and
value/noun definitions, as do many other English symbols. This overloading of
symbols is common in human languages but is uncommon in computer programming
languages. Emacs Lisp is a so-called “Lisp-2” because it can associate a symbol
with two or more different kinds of things at the same time, as opposed to a
“Lisp-1” like Scheme.