On 10 Feb, 12:23, Mark Carter <alt.mcar...@googlemail.com> wrote:
> Any ideas how I could implement
> this?
I've made a stab at it, but I'm not there yet.
(import (java.util ArrayList List))
(defn arraylist->list [aList]
(let [size (. aList size)]
(loop [accum nil
index 0]
(if (< index size)
(recur (concat accum (list (. aList get index))) (inc index))
accum))))
(defn collect-lambda [f]
(let [alist (new ArrayList)
emit (fn [new-member] (. alist add new-member))]
(f emit)
(arraylist->list alist)))
So far, so good. Now I can type
(collect-lambda (fn [emit]
(emit 1)
(emit 2)))
and get the result (1 2) - which is what we expect. There's a lot of
goo there, though, which would be nice to eliminate via a macro. I
really want to be able to say that emit is part of my syntax.
To try to solve this problem, I started out by defining unpartial -
which is kind of the opposite of partial:
(defmacro unpartial [x & body]
`(fn [~x] ~...@body))
Seems an odd thing to do just now, but there's a logic to it. I can do
things like
((unpartial y (+ y 2)) 3)
and get the value 5 - again, this is what I expect.
I now try to define the macro that I'm really interested in:
(defmacro collect [& body]
`(let [f# (unpartial emit ~...@body)]
(collect-lambda f#)))
Admittedly, my understanding of composing macros is very shaky.
However, when I do
(collect
(emit 1)
(emit 2))
expecting the result (1 2), I instead get the error
java.lang.Exception: Can't use qualified name as parameter: user/emit
Grrr. How can I fix this?
I can get closer if I define collect as
(defmacro collect [emitter & body]
`(let [f# (unpartial ~emitter ~...@body)]
(collect-lambda f#)))
and use it by
(collect emit
(emit 15)
(emit 6))
to return (15 6). But I'd rather like to be able to omit the emit
parameter.
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