On Jul 27, 11:56 am, Dmitry Gutov <[email protected]> wrote:
> > First: Why doesn't macroexpand expand the inner when-lets?
>
> It's not supposed to, see the doc. To do full expansion, you can use
> `clojure.walk/macroexpand-all`.
>
> > Is the gensym in the two expands the same thing, or do "they" get the
> > same name? That was surprising to me. I can't think of any real
> > example where that is a problem. But what if I had wanted to write a
> > macro like the second when-lets?
>
> Looks like a bug/limitation of the gensym reader macro, these symbols
> are treated as belonging to the same quoted form, so they get the same
> name.
Nothing to do with being the same quoted form: foo# symbols are
gensymmed at read time, so by the time the macro runs there's no
evidence that they were ever gensyms: just a symbol named
temp__1551__auto__, and it has no reason to suppose you intend them to
be different.
I don't think this is a problem for many (any?) macros, but if you
want more control you can gensym yourself:
(defmacro when-lets
[bindings & body]
(if-not (seq bindings)
`(do ~@body)
(let [temp (gensym) form (bindings 0) tst (bindings 1) rst (drop 2
bindings)]
`(let [~temp ~tst]
(when ~temp
(when-lets ~(vec rst)
(let [~form ~temp]
~@body)))))))
user> (use 'clojure.walk) (macroexpand-all '(when-lets [a 1 b 2] [a
b]))
(let* [G__1815 1] (if G__1815 (do (let* [G__1816 2] (if G__1816 (do
(do (let* [b G__1816] (let* [a G__1815] [a b])))))))))
user> (when-lets [a 1 b 2] [a b])
[1 2]
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