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Is it possible to have keywords used\called conditionally? Essentially instead of:

(defun test (var) (if var (some-function :para var) (some-function)))

I'd want something like this:

(defmacro test (var) `(some-function (if ,var [add-keyword])))

where the function is(for quick easy example purposes):

(defun some-function (&key para) (cond ((equal para 'yes) "yes") ((null para) "no") (t "other")))
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2 Answers 2

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3

It’s usually possible to write code in a way to not need to do this (just pass nil or the default). But if it really is unavoidable you can use apply:

(apply #'some-function args... (when var (list :para var)))

If you want to pass keyword arguments on, you should use a &rest argument with apply and &allow-other-keys. If you want to pass them on to the next method in CLOS, you should use call-next-method. In general it is best to have the set of keyword arguments known at compile time because then the compiler can make things much faster than the unknown case.

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Thanks! Yeah I ended up rewriting the function(for other reasons), but the curiosity remained, haha.
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One can also detect if a function was called with a keyword parameter:

CL-USER 44 > (flet ((example (&key (para nil para-supplied-p)) (list para para-supplied-p))) (list (example) (example :para nil) (example :para t))) ; para para-supplied-p ((NIL NIL) (NIL T) ( T T))
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This doesn't allow for not passing a key value into the function though? The issue is I am working with SDL, and I wanted to not pass a color parameter if you didn't supply one, but SDL doesn't allow for a nil value on color, thereby forcing you to select a color(it probably defaults to main\screen color, so I rewrote it to mimic a related object's color). So while this would work if I developed the SDL library, I didn't thereby I needed my function to be able to select the passing of a key(pre-rewrite). Eitherway, thanks for your answer! I learned something new!

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