CLHS: Section 1.4.1.4.4

nil has a variety of meanings. It is a symbol in the COMMON-LISP package with the name "NIL", it is boolean (and generalized boolean) false, it is the empty list, and it is the name of the empty type (a subtype of all types).

Within Common Lisp, nil can be notated interchangeably as either NIL or (). By convention, the choice of notation offers a hint as to which of its many roles it is playing.

For Evaluation? Notation Typically Implied Role ---------- Yes nil use as a boolean. Yes 'nil use as a symbol. Yes '() use as an empty list No nil use as a symbol or boolean. No () use as an empty list.

Yes nil use as a boolean. Yes 'nil use as a symbol. Yes '() use as an empty list No nil use as a symbol or boolean. No () use as an empty list.

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