> 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.
But I don't think the second macro does what you're saying - the
evaluation order is the same, since `tst` is evaluated in the `let`
form above `when-lets` in both cases, only the order of binding the
values is reversed.
To reverse the evaluation order, you need to move that `let` inside:
(defmacro when-lets
[bindings & body]
(if-not (seq bindings)
`(do ~@body)
(let [form (bindings 0) tst (bindings 1) rst (drop 2 bindings)]
`(when-lets ~(vec rst)
(let [temp# ~tst]
(when temp#
(let [~form temp#]
~@body)))))))
user=> (when-lets [b (inc a) a 3] (+ b a))
7
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