CODEWITHSUNDEEP
common-lisp
derefed
derefed

Reputation: 689

How do I splice into a list outside of a macro in Common Lisp?

Say I have a function foo:

(defun foo (x y &rest args) ...)

And I later want to wrap it with a function bar:

(defun bar (x &rest args) (foo x 100 args))

Assume bar was then called like this: (bar 50 1 2 3)

With this setup, args is a list within the body of bar that holds the trailing parameters, so when I pass it to foo, instead of getting the equivalent of (foo 50 100 1 2 3) I of course get (foo 50 100 '(1 2 3)). If these were macros, I would use ``(foo ,x 100 ,@args)` within the body of bar to splice args into the function call. ,@ only works inside a backtick-quoted list, however.

How can I do this same sort of splicing within a regular function?

Upvotes: 22

Views: 5507

Answers (2)

Daniel Cussen
Daniel Cussen

Reputation: 149

This method is slow, but it may be what you're looking for.

Backquotes, commas, and comma-ats aren't only for macros. They can also be used in functions if you use eval too. Again, this is not fast or efficient. At any rate, here it is:

(defun bar (x &rest args)
  (eval `(foo ,x 100 ,@args)))

Upvotes: 1

Xach
Xach

Reputation: 11854

APPLY will call its first argument with its subsequent arguments, and the last argument must be a list. So:

(apply #'foo x 100 args)

Upvotes: 38

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