Corece to boolean in Common Lisp

Question

Is there a build-in function in Common Lisp to coerce any thing into it's boolean value, like the bool function in Python does?

Venture

Answer 1

There's nothing specifically for this. You can't use boolean as the type argument to coerce. You can use:

(defun boolean-value (x)
  (not (not x)))

This is analogous to the !!x idiom used in many other languages.

Cheree

I've seen the equivalent (not (null x)) more often as idiom, but both work. – Christly

Me, too, I just didn't want to open up the "what's the difference between NOT and NULL" can of worms. – Cheree

... so what is the difference? – Luedtke

@Luedtke There's no operational difference. The difference to programmers is that NOT is for logical testing, while NULL is for structural testing. – Cheree

Answer 2

Sort of. (and form t) will return t if form is not one of the false values nil or ().

I.e. it is a macro, not a function, and it requires the additional argument t to do the trick.

Prehension