Hi all,
On Sat, Oct 17, 2009 at 2:12 AM, John Harrop <jharrop...@gmail.com> wrote:
> On Fri, Oct 16, 2009 at 7:58 PM, mbrodersen <morten.broder...@gmail.com>wrote:
>
>>
>> Hi,
>>
>> I am new to Clojure and I am wondering if there is anything similar to
>> the following macro already built in:
>>
>> (defmacro let-while
>> "Makes it easy to continue processing an expression as long as it
>> is
>> true"
>> [[name expr] & forms]
>> `(loop []
>> (let [~name ~expr]
>> (when ~name
>> ~...@forms
>> (recur)))))
>>
>
> Nice.
>
> But name is multiply evaluated.
>
It's a valid point: since name is used in test-position in the chen clause
we must ensure that's it's a symbol and not a destructuring form (map or
vector).
eg
(let-while [[a b] nil] (println "you should never see this message"))
But the proposed version doesn't solve anything.
Here is a better one:
(defmacro while-let
"Makes it easy to continue processing an expression as long as it is true"
[binding & forms]
`(loop []
(when-let ~binding
~...@forms
(recur))))
(I cheated: rather than manually working around this bug, I used when-let
which already does the right thing.)
hth,
Christophe
--
Professional: http://cgrand.net/ (fr)
On Clojure: http://clj-me.blogspot.com/ (en)
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