Both of these work:

user> (let [[x & y] [1 2]] [x y])
[1 (2)]
user> (let [[x :as y] [1 2]] [x y])
[1 [1 2]]

And this works:

user> ((fn [x & y] [x y]) 1 2)
[1 (2)]

But this gives an exception (unsupported binding form):

user> ((fn [x :as y] [x y]) 1 2)
; Evaluation aborted.

I would have expected this to work and return [1 [1 2]], with y bound
to the whole vector implicitly passed to the function (just as it's
bound to the tail of the whole vector implicitly passed in the third
case).

This seems to happen because (apparently) the internals of fn* handle
rest parameters already, so that clojure.core/maybe-destructured
doesn't actually need to do anything to top-level elements of the
binding vector:

clojure.core> (maybe-destructured '[a b] ())
([a b])
clojure.core> (maybe-destructured '[a & b] ())
([a & b])
clojure.core> (maybe-destructured '[a [b & c]] ())
([a p__8059] (clojure.core/let [[b & c] p__8059]))

Since & is a symbol, it's just passed right through and left to fn* to
deal with. But :as is not a symbol, so we end up with this:

clojure.core> (maybe-destructured '[a :as b] ())
([a p__8064 b] (clojure.core/let [:as p__8064]))

Is there a reason for this apparent irregularity?

-- 
Ben Wolfson
"Human kind has used its intelligence to vary the flavour of drinks,
which may be sweet, aromatic, fermented or spirit-based. ... Family
and social life also offer numerous other occasions to consume drinks
for pleasure." [Larousse, "Drink" entry]

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