Question

Both

(not 'nil)

and

(not nil)

evaluate to T, so is there any difference between 'nil and nil? And what about ''nil? If ''nil evaluates to 'nil, shouldn’t ''nil evaluate to nil as well?

Answer 1

(quote <anything>) evaluates to <anything>, literally. The notation '<anything> means (quote <anything>), and regardless of what <anything> is, it is simply returned without being evaluated.

Furthermore, nil evaluates to itself.

Other objects, when quoted as literals, also evaluate to themselves: certain symbols and all non-symbolic atoms are this way.

What is the difference between '2 and 2? They both evaluate to 2!

Also, what is the difference between '"abc" and "abc", and between :foo and ':foo?

The difference is that '2 is the form (quote 2) whereas 2 is just 2. They evaluate to the same thing but aren’t the same thing.

To evaluate Lisp means that a datum is treated as the source code of an expression. Two expressions can have the same value, yet be made of different data. For instance 4, (+ 2 2) and (* 2 2).

Say, what’s the difference between 4 and (+ 2 2)?

If 4 and (+ 2 2) both produce 4, why does '4 produce 4, but '(+ 2 2) produces (+ 2 2)?

Quote means, “give me this piece of program code as a datum, rather than the value which it denotes.”