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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