https://github.com/joegallo/robert-bruce
for retrying things
On Fri, Mar 18, 2011 at 6:36 PM, Fiel
<e4696wyoa63emq6w3250kiw60i4...@gmail.com> wrote:
> Thank you to all who posted all the helpful examples. I'm sure these
> will help other clojure learners too.
>
> On Mar 18, 5:27 pm, Ken Wesson <kwess...@gmail.com> wrote:
>> On Fri, Mar 18, 2011 at 1:13 PM, Shantanu Kumar
>>
>> <kumar.shant...@gmail.com> wrote:
>> > The `try-times` macro above is buggy (doesn't work when body of code
>> > returns logical false). Fixed version is below:
>>
>> > (defmacro try-times
>> > [n & body] {:pre [(posnum? n)]}
>> > `(let [c# (repeat-exec (dec ~n) #(maybe ~@body))
>> > r# (some #(if (last %) nil %) c#)]
>> > (first (or r# [(do ~@body)]))))
>>
>> You might want to unwrap everything after:
>>
>> (defmacro try-times
>> [n & body] {:pre [(posnum? n)]}
>> `(let [c# (repeat-exec (dec ~n) #(maybe ~@body))
>> r# (some #(if (last %) nil %) c#)
>> [r# e#] (first (or r# [(do ~@body)]))]
>> (if e#
>> (throw e#)
>> r#)))
>>
>> This will re-throw the last exception on failure. On success it will
>> evaluate to the return value of the successful execution of the body.
>> So this try-times trying to acquire a network socket may throw a
>> socket unavailable exception or return the socket, rather than
>> returning a vector of a maybe-socket and a maybe-exception.
>>
>> The other thing you will probably want is a delay:
>>
>> (defmacro try-times
>> [delay n & body] {:pre [(posnum? n)]}
>> `(let [c# (repeat-exec (dec ~n) #(do (maybe ~@body) Thread/sleep ~delay))
>> r# (some #(if (last %) nil %) c#)
>> [r# e#] (first (or r# [(do ~@body)]))]
>> (if e#
>> (throw e#)
>> r#)))
>>
>> Now you can (try-times 30000 10 (acquire-socket 80)) to spend five
>> minutes trying to grab a socket at half-minute intervals, throw on
>> failure, and return the socket on success. That would be quite handy
>> for low level networking, where you usually want to wait a bit between
>> retries for congestion to ease or other conditions to change that were
>> impeding success.
>
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