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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