Hofstadter on Lisp (1983)

Interestingly, this is no longer the case. Modern Lisps now evaluate (car nil) and (cdr nil) to nil. In the original Lisp defined by John McCarthy, indeed CAR and CDR were undefined for NIL. Quoting from <https://dl.acm.org/doi/pdf/10.1145/367177.367199>:

> Here NIL is an atomic symbol used to terminate lists.

> car [x] is defined if and only if x is not atomic.

> cdr [x] is also defined when x is not atomic.

However, both Common Lisp and Emacs Lisp define (car nil) and (cdr nil) to be nil. Quoting from <https://www.lispworks.com/documentation/HyperSpec/Body/f_car...>:

> If x is a cons, car returns the car of that cons. If x is nil, car returns nil.

> If x is a cons, cdr returns the cdr of that cons. If x is nil, cdr returns nil.

Also, quoting from <https://www.gnu.org/software/emacs/manual/html_node/elisp/Li...>:

> Function: car cons-cell ... As a special case, if cons-cell is nil, this function returns nil. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil.

> Function: cdr cons-cell ... As a special case, if cons-cell is nil, this function returns nil; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil.

Scheme doesn't. Taking the CAR or CDR of nil is an error.

Ruby (not a lisp but bear with me) started to do this more correctly IMHO where a nil would start throwing errors if you tried to do things with it BUT it would still be equivalent to false in boolean checks.

if from a syntactic-flavor perspective, endless parentheses turn me off, but also cleanly map to significant indentation (where any new open paren is a new indentation level and a close paren maps to a backdent), has anyone tried a Lisp that uses indentation instead of parens?

I'm probably failing to consider edge cases but it seems like a potentially simple tweak that might make lisps more palatable to many

imagine that, a lisp without parens... (empty cons literals... crap, that's 1 edge case!)