Some control-flow macros in Common Lisp alexandria libs

Previous post recorded my curiosity about the if-let macro in alexandria.

Recently, I stumbled upon another file, control-flow.lisp, and became curious about what’s inside. Once again, I played around with it a bit to explore its potential for future use.

`nth-value-or`

(alexandria:nth-value-or 1
    (values 0 nil 2)
    (values 0 1 3)
    (values 0 1 4))

This code will return (values 0 1 3) because (nth 1 (multiple-value-list (values 0 1 3))) is not nil.

`xor`

This one is a bit weird:

(assert (equal '(nil nil) (multiple-value-list (alexandria:xor 1 2))))
(assert (equal '(2 t) (multiple-value-list (alexandria:xor nil 2))))
(assert (equal '(1 t) (multiple-value-list (alexandria:xor 1 nil nil))))
(assert (equal '(nil nil) (multiple-value-list (alexandria:xor 1 2 3))))
(assert (equal '(3 t) (multiple-value-list (alexandria:xor nil nil 3 nil))))
(assert (equal '(nil nil) (multiple-value-list (alexandria:xor nil nil 3 4))))
(assert (equal '(nil nil) (multiple-value-list (alexandria:xor nil 2 3 4))))

It looks like it keeps comparing pairs of values until it finds a pair that evaluates to (t nil) and then returns the corresponding value.

`switch`

This one has a couple of siblings: eswitch and cswitch.

(alexandria:switch (10 :test 'equal)
  (1 (format t "1") 2)
  (2 (format t "2") 3)
  (3 (format t "3") 4)
  ((1+ 9) (format t "match") 11))

At first, I thought it might behave like Go’s switch, but it seems more like a supercharged cond.

Written on January 15, 2025