The as-> macro doesn't work with destructuring. This is invalid code:
(-> [1 2]
(as-> [a & b]
[a (inc b)]
[(inc a) b]))
Because it is expanded to:
(let [[a & b] [1 2]
[a & b] [a (inc b)]
[a & b] [(inc a) b]]
[a & b]) ;; this last expression will not compile
But with a little redefinition is possible to make as-> work with
destructuring:
(defmacro as->
"Binds name to expr, evaluates the first form in the lexical context
of that binding, then binds name to that result, repeating for each
successive form, returning the result of the last form."
{:added "1.5"}
[expr name & forms]
`(let [~name ~expr
~@(interleave (repeat name) (butlast forms))]
~(last forms)))
Now the previous example will expand to:
(let [[a & b] [1 2]
[a & b] [a (inc b)]]
[(inc a) b])
The following example shows why an as-> destructuring compatible
macro can be useful. This code parses a defmulti like parameter list
by reusing a destructuring form:
(defmacro defmulti2 [mm-name & opts]
(-> [{} opts]
(as-> [m [e & r :as o]]
(if (string? e)
[(assoc m :docstring e) r]
[m o])
(if (map? e)
[(assoc m :attr-map e :dispatch-fn (first r)) (next r)]
[(assoc m :dispatch-fn e) r])
...
Compare with the original defmulti:
(defmacro defmulti [mm-name & options]
(let [docstring (if (string? (first options))
(first options)
nil)
options (if (string? (first options))
(next options)
options)
m (if (map? (first options))
(first options)
{})
options (if (map? (first options))
(next options)
options)
dispatch-fn (first options)
options (next options)
m (if docstring
(assoc m :doc docstring)
m)
...
I added a JIRA issue to report & track this (I think) enhacement:
http://dev.clojure.org/jira/browse/CLJ-1418
Saludos,
Nahuel Greco.
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