Add two of On Lisp's utility functions: group and once-only

src/utils.lisp

@@ -10,6 +10,7 @@
   (:export :it
            :aif
            :awhen
+           :group :once-only
            :restartable
            :add-hook :remove-hook :run-hook
            :update-current-time :get-delta-time
@@ -32,6 +33,25 @@
      (when it
        ,@body)))
 
+(defun group (source n)
+  "Regroup the list `source' elements by n."
+  (when (zerop n)
+    (error "zero length"))
+  (labels ((rec (source acc)
+             (let ((rest (nthcdr n source)))
+               (if (consp rest)
+                   (rec rest (cons (subseq source 0 n) acc))
+                   (nreverse (cons source acc))))))
+    (if source (rec source nil) nil)))
+
+(defmacro once-only ((&rest names) &body body)
+  "Evaluate the symbols in `names' only once, for use in a macro, as per Peter Siebel's Practical Common-Lisp."
+  (let ((gensyms (loop for n in names collect (gensym))))
+    `(let (,@(loop for g in gensyms collect `(,g (gensym))))
+       `(let (,,@(loop for g in gensyms for n in names collect ``(,,g ,,n)))
+          ,(let (,@(loop for n in names for g in gensyms collect `(,n ,g)))
+                ,@body)))))
+
 (defmacro restartable (unprotected &body body)
   "Provide a Continue restart unless `unprotected' is t."
   `(if ,unprotected