I am writing a rather long series of articles about Scheme on Artima,
"The Adventures of a Pythonista in Schemeland" (maybe somebody here
has
heard of it). Last week I arrived at point of discussing hygienic
macros
(http://www.artima.com/weblogs/viewpost.jsp?thread=260195)
and I wanted to spent a few words about the differences between
Scheme and other lisps. In particular I want to asset the status of
Clojure
macros with respect to hygiene.
Since they look a lot like Common Lisp macros, I initially assumed
that
Clojure macros were not hygienic, but after performing some experiment
I
realized that I was half wrong. For instance, I expected this code to
fail due to free-symbol capture:
user=> (defmacro m [x] `(list ~x))
#'user/m
user=> (let [list 1] (m 2))
(2)
Instead, it works correctly. Therefore, I assume that Clojure
is renaming the free symbols in the macro body (in this case 'list).
On the other hand, Clojure macros are not fully hygienic, otherwise
a would not be defined in this example:
user=> (defmacro def-a[x] `(def a ~x))
#'user/def-a
user=> (def-a 1)
#'user/a
user=> a
1
It seems that in order to avoid variable capture for bound symbols
I need to use good old (gensym):
user=> (defmacro def-a[x] (let [a (gensym)] `(def ~a ~x)))
#'user/def-a
user=> (def-a 1)
#'user/G__77
Am I right in my assumptions? Is there a document explaining how
Clojure macros work? The macro system seems quite unique, in the sense
that I do not know of any other macro system working in the same way.
Michele Simionato
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