How do you insert declarations into loops?

[Robert L.]

Thomas A. Russ wrote:

> > How do you insert declarations into loops?
> >
> > Example: This function loops over a list of associations
> > collecting the cdrs.
> >
> > (defun list-cdrs (list)
> > (loop for (unwanted-var . wanted-var) in list
> > collect wanted-var))
> >
> > When I compile list-cdrs, I get an "unused lexical variable, UNWANTED-VAR"
> > message (this is fine.). Normally, if I wanted to inhibit this
> > warning I would stick a (declare (ignore unwanted-var)) in the
> > beginning of the function body immediately following the declaration.
> > It doesn't seem that you can do this using loop.
>
> The solution is not to introduce an ignore declartion, but instead to
> use a hack in the destructuring pattern matcher:
>
> (defun list-cdrs (list)
> (loop for (NIL . wanted-var) in list
> collect wanted-var))

(map cdr '((a . 2) (b . 3)))
===>
'(2 3)


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