On Wed, Jul 27, 2011 at 2:13 PM, Alan Malloy <a...@malloys.org> wrote:
> On Jul 27, 11:11 am, Alan Malloy <a...@malloys.org> wrote:
>> On Jul 27, 5:50 am, Feng Shen <shen...@gmail.com> wrote:
>>
>> > Clojure core.clj has a macro when-let,
>> > I am wondering how to write a macro `when-lets`
>>
>> > (when-lets [symbol-1 test-1
>> > symbol-2 test-2
>> > ...
>> > ]
>> > body
>> > )
>>
>> > body only get evaluated when (and test-1 test-2 ....)
>> > I am thinking about it, anybody has any clue?
>>
>> (defmacro when-lets [tests & body]
>> (reduce (fn [body test]
>> `(when-let ~(vec test)
>> ~body))
>> `(do ~@body)
>> (partition 2 (reverse tests))))
>>
>> This would be cleaner if we had foldr in the language already - I find
>> myself wanting it quite often for macros like this.
>
> Sorry, should of course be
>
> (defmacro when-lets [tests & body]
> (reduce (fn [body test]
> `(when-let ~(vec test)
> ~body))
> `(do ~@body)
> (reverse (partition 2 tests))))
>
> Apparently the reason I want foldr is because I get it wrong when I do
> it myself.
(defn foldr
([f coll]
(reduce f (reverse coll)))
([f init coll]
(reduce f init (reverse coll))))
There. :)
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