Add do-nothing

Closes #159.

functions.lisp

@@ -562,3 +562,17 @@ If there are not VALUES, returns nothing."
               (lambda (&rest args)
                 (declare (ignore args))
                 (values ,@temps)))))))
+
+
+(define-compiler-macro do-nothing (&rest args)
+  `(progn ,@args (values)))
+
+(-> do-nothing (&rest t) (values &optional))
+(declaim (inline do-nothing))
+(defun do-nothing (&rest args)
+  "Do nothing and return nothing.
+This function is meant as a placeholder for a function argument.
+
+From LispWorks."
+  (declare (ignore args))
+  (values))

package.lisp

@@ -199,6 +199,7 @@
    #:fnil
    #:mvconstantly
    #:fuel
+   #:do-nothing
    ;; Trees.
    #:reuse-cons
    #:car+cdr

tests/functions.lisp

@@ -256,3 +256,11 @@
   (let ((fuel (fuel most-positive-double-float)))
     (signals error
       (funcall fuel double-float-epsilon))))
+
+(test do-nothing ()
+  (is (null (do-nothing)))
+  (is (equal '() (multiple-value-list (do-nothing))))
+  ;; Make sure we preserve side effects, in order.
+  (let ((list '()))
+    (do-nothing (push 1 list) (push 2 list))
+    (is (equal list '(2 1)))))