Let's say, I want to call function foo, which requires 2 arguments a and b. (foo a b).
And in some use cases, the values of those parameters depend on each other, like this:
(let (a
b)
(if (something-p)
(progn
(setq a 'a1)
(setq b 'b1))
(progn
(setq a 'a2)
(setq b 'b2)))
(foo a b))
Is there a (lisp) pattern for this kind of problem? (Or do I need to write my own macro for that?)
Examples:
Applying a list:
(apply #'foo
(if (something-p)
(list 'a1 'b1)
(list 'a2 'b2)))
Similar, but with multiple values:
(multiple-value-call #'foo
(if (something)
(values 'a1 'b1)
(values 'a2 'b2)))
Binding multiple values:
(multiple-value-bind (a b)
(if (something)
(values 'a1 'b1)
(values 'a2 'b2))
(foo a b))
Nested let:
(let ((something (something-p)))
(let ((a (if something 'a1 'b2))
(b (if something 'b1 'b2)))
(foo a b)))
Your example does not really illustrate the parameters' mutual interdependence. As for the conditional aspect, you can embed arbitrary conditions into your function call. So you could have e.g.:
(foo (if (something-p) 'a1 'a2)
(if (something-p) 'b1 'b2))
EDIT:
If you're concerned about multiple evaluations of something-p, you can always store the value of a single evaluation in a let variable like this:
(let ((c (something-p)))
(foo (if c 'a1 'a2)
(if c 'b1 'b2)))
