[junke]

> in Lisp all expressions evaluate to something

Minor nitpick, but note that if you define:

    (defun foo () (values))

Then (foo) does not return a value and accordingly, the REPL prints nothing. But in a context where you need a value, that value would be NIL: if A evaluates to 3, then after (setf a (foo)) it will evaluate to NIL.

[kazinator]

According to a famous epigram by Alan Perlis, Lisp programmers know "the value of everything, but the cost of nothing". It reflects the expectation that a language which calls itself Lisp is expected to produce a value out of any expression which evaluates.

Though Common Lisp adds the nuance of multiple values, the behavior you describe is how it conforms to this general expectation. Code written in an everything-really-has-one-value dialect of Lisp can be easily transported to Common Lisp (or at least transported without without difficulties specifically caused by this issue).

Scheme, a Lisp-like language, allows some evaluable expressions to have an "undefined" or "unspecified" result value. Logic translated to Scheme from a Lisp dialect without attention to this issue can have a surprising or incorrect behavior. For instance, if the original code executes a do loop, with the expectation that it yields nil (or some similar false/empty value in the original dialect). In Scheme's do loop, if the result expression is present then it specifies the value; otherwise the value is not specified.

[flavio81]

> Scheme, a Lisp-like language, allows some evaluable expressions to have an "undefined" or "unspecified" result value.

That's why they are heretics !!

(Just joking of course... Scheme is the other great Lisp dialect.)