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Mini-tutorial on explicit renaming macros in Chicken
Author: Juergen Lorenz ju (at) jugilo (dot) de Date: Jun 9th, 2009
The following is an attempt to explain, how to write hygienic macros in Chicken with explicit renaming. It supposes, that the reader knows, how to write other low-level-macros like define-macro in Chicken-3 and other Schemes. As you will see, explicit renaming macros are even easier to write than those with define-macro. This is because there is a brute force method to avoid variable capture.
Let's consider a trivial macro a la Chicken-3 without any considerations on hygiene and variable capture:
(define-macro swap! ; wrong (lambda (x y) `(let ((tmp ,x)) (set! ,x ,y) (set! ,y tmp))))
You might be surprised, but the replacement text, that within the backquotes, need not be changed in an explicit renaming macro, at least in a first step. What has to be changed, is the signature: explicit renaming transformers are always procedures of three variables, usually called form, rename, compare:
(define-syntax swap! (lambda (form rename compare) ... `(let ((tmp ,x)) (set! ,x ,y) (set! ,y tmp))))
and you have to destructure the form argument, which is the whole macro-from, (swap! x y) in the example, either by hand, or by means of a macro from the matchable package, to fill in the ellipsis above. That is, without any helper macros, you get
(define-syntax swap! (lambda (form rename compare) (let ( (x (cadr form)) (y (caddr form)) ) `(let ((tmp ,x)) (set! ,x ,y) (set! ,y tmp)))))
In this form, the new macro is the same as our first attempt with define-macro. We haven't bothered about variable capture or hygiene at all. But this macro already works. You can test it, say with
(let ((x 'x) (y 'y)) (swap! x y) (list x y))
and see if it does what it is supposed to do. Of course, it is not correct, since if a client uses tmp as one of his or her arguments, the macro will crash.
In the classical macro systems, defmacro in Common Lisp or define-macro in some Schemes, you now have to think carefully, which macro variables are in danger of variable capture and use an uninterned symbol for them, tmp in the current example. Worse than that, contrary to Common Lisp, Scheme allows to use any name for variables, even keyword names as let, set!, ... so that define-macro can never create a hygienic swap! macro. Here is, where renaming comes into the play. You don't need gensym, use rename instead. And you needn't bother, which symbol to rename, rename all.
What does that mean, "all"? It's easy in the present example, everything within the backquote, except those symbols, which are already unquoted. But in more complicated examples, you should use expand, to see the replacement text of your macro calls.
(pp (expand '(swap! x y)))
will result in the replacement text
(let ((tmp x)) (set! x y) (set! y tmp))
Only x and y are arguments of your swap! call, hence everything else in the replacement text should be renamed: namely let, tmp and set!. So a hygienic version of swap! would be
(define-syntax swap! (lambda (form rename compare) (let ( (x (cadr form)) (y (caddr form)) ) `(,(rename 'let) ((,(rename 'tmp) ,x)) (,(rename 'set!) ,x ,y) (,(rename 'set!) ,y ,(rename 'tmp))))))
If you repeat the expand call above, you will get the same replacement text, but with numbers suffixed to the renamed symbols, something like
(let11 ((tmp12 x)) (set!13 x y) (set!13 y tmp12))
These renamed symbols have the same meaning as the original names without suffixes, but serve as aliases which the client cannot use under any circumstances.
Note, that the two appearances of tmp and set! are renamed to the same alias, the rename operator is referentially transparent. Note also, that these renamed symbols are much easier to interpret than gensym'ed ones!
I personally do not like these rename calls within the backquoted expression, because you loose the visual similarity between the backquoted expression and the resulting replacement text. Therefore I prefer this version, which is equivalent
(define-syntax swap! (lambda (form rename compare) (let ( (x (cadr form)) (y (caddr form)) (%tmp (rename 'tmp)) (%let (rename 'let)) (%set! (rename 'set!)) ) `(,%let ((,%tmp ,x)) (,%set! ,x ,y) (,%set! ,y ,%tmp)))))
You can think of the two characters ,% as an identity operator.
You see, explicit renaming macros are a bit more verbose than classical ones with define-macro. But everybody who is able to write the latter, is able to write the former as well. It's even easier!