What is the difference between 1 and '1 in Lisp?

Question

I had never really thought about whether a symbol could be a number in Lisp, so I played around with it today:

> \'1
1
> (+ \'1 \'1)
2
> (+ \'1 1)
2
>         


        

Answer 1

Quoting prevents expressions from being evaluated until later. For example, the following is not a proper list:

(1 2 3)

This is because Lisp interprets 1 as a function, which it is not. So the list must be quoted:

'(1 2 3)

When you quote a very simple expression such as a number, Lisp effectively does not alter its behavior.

See Wikipedia: Lisp.