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I would like to evaluate a list of booleans like

(nil nil t)

into a single boolean, such that if any element is true, then the expression evaluates to true.

I first looked at the (or) function in elisp, but it does not take a list of booleans as an argument. Instead it requires its arguments to be "un-wrapped" outside of a list. In other words,

(or (nil nil t))

simply evaluates to the entire list (nil nil t) because it treats the it as a single argument. I would like something that instead evaluates exactly to t. - I understand that (nil nil t) is truthy, but that's beside the point.

How can I convert (or (nil nil t)) into just (or nil nil t) or just t?

After some thought, I can think of

(eval (cons 'or '(nil nil t)))

but is there some other more idiomatic way that I'm missing?

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If you wanted this for a function you would use apply, but as or is a special form you can't do that. In particular, or only evaluates as many arguments as it needs to.

You could write a macro:

(defmacro or-list (list)
  `(or ,@(eval list)))

(setq mylist '(a b c))

(or-list mylist) then expands to (or a b c)

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How about this?

(defun or-list (list)
  (cl-some #'identity list))

Here cl-some takes two arguments: a predicate and a list and returns non-nil if the predicate applied to some member of the list is non-nil. In our case, the identity function is a good predicate since we only want to test whether our list members are themselves non-nil.

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  • Good. But it might help to elaborate a bit, explaining how the code does what's requested. – Drew Jan 12 at 16:44
  • @Drew: done! (Though I think the code is pretty self-explanatory.) – Fran Burstall Jan 12 at 23:13

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