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copied!<p>Kaz explained it beautifully and extensively.</p> <p>However, if you wouldn't care much about the double-hygiene problem, you might find this one easier to understand:</p> <pre><code>(defmacro once-only ((&rest symbols) &body body) ;; copy-symbol may reuse the original symbol name (let ((uninterned-symbols (mapcar 'copy-symbol symbols))) ;; For the final macro expansion: ;; Evaluate the forms in the original bound symbols into fresh bindings ``(let (,,@(mapcar #'(lambda (uninterned-symbol symbol) ``(,',uninterned-symbol ,,symbol)) uninterned-symbols symbols)) ;; For the macro that is using us: ;; Bind the original symbols to the fresh symbols ,(let (,@(mapcar #'(lambda (symbol uninterned-symbol) `(,symbol ',uninterned-symbol)) symbols uninterned-symbols)) ,@body)))) </code></pre> <p>The first <code>let</code> is backquoted twice, because it'll be part of the final expansion. The purpose is to evaluate the forms in the original bound symbols into fresh bindings.</p> <p>The second <code>let</code> is backquoted once, because it'll be part of the user of <code>once-only</code>. The purpose is to rebind the original symbols to the fresh symbols, since their forms will have been evaluated and bound to them in the final expansion.</p> <p>If the rebinding of the original symbols was before the final macro expansion, the final macro expansion would refer to the uninterned symbols instead of the original forms.</p> <p>An implementation of <code>with-slots</code> that uses <code>once-only</code> is an example that requires double-hygiene:</p> <pre><code>(defmacro with-slots ((&rest slots) obj &body body) (once-only (obj) `(symbol-macrolet (,@(mapcar #'(lambda (slot) `(,slot (slot-value ,obj ',slot))) slots)) ,@body))) ;;; Interaction in a REPL > (let ((*gensym-counter* 1) (*print-circle* t) (*print-level* 10)) (pprint (macroexpand `(with-slots (a) (make-object-1) ,(macroexpand `(with-slots (b) (make-object-2) body)))))) ;;; With the double-hygienic once-only (let ((#1=#:g2 (make-object-1))) (symbol-macrolet ((a (slot-value #1# 'a))) (let ((#2=#:g1 (make-object-2))) (symbol-macrolet ((b (slot-value #2# 'b))) body)))) ;;; With this version of once-only (let ((#1=#:obj (make-object-1))) (symbol-macrolet ((a (slot-value #1# 'a))) (let ((#1# (make-object-2))) (symbol-macrolet ((b (slot-value #1# 'b))) body)))) </code></pre> <p>The second expansion shows that the inner <code>let</code> is shadowing the binding to the variable <code>#:obj</code> of the outter <code>let</code>. Thus, accessing <code>a</code> within the inner <code>with-slots</code> would actually access the second object.</p> <p>Note that in this example, the outter macro-expansion gets a gensym named <code>g2</code> and the inner <code>g1</code>. In normal evaluation or compilation, it would be the opposite, as forms are walked from the outter to the inner.</p>

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