uiop:when-let*

(uiop:when-let* ((var form)...) body...)

The sequential uiop:when-let: each binding's form sees the variables bound before it, and the first variable that comes out nil short-circuits the whole form to nil without evaluating the remaining forms. That is the difference that matters — a later form may safely assume the earlier bindings are non-nil.

The second line never evaluates (/ 1 a), which would signal.

uiop is ASDF's portability layer, not part of Common Lisp: the name is only reachable with the uiop: qualifier.

Backend support

Works on all four backends: it is a built-in macro expansion shared by the interpreter and both compilers. Like the other built-in macros it has no function value (#'uiop:when-let* is an error).