On Mar 19, 2:27 am, Ken Wesson <[email protected]> wrote:
> On Fri, Mar 18, 2011 at 1:13 PM, Shantanu Kumar
>
> <[email protected]> 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.
My version already does that if I understand correctly. Ken Wesson's
variation on `delay` is neat.
I came up with another way to retry on exception:
(defmacro try-while
"Return the result after executing code body; on exception keep re-
trying as
long as pred returns true. The predicate function accepts thrown
exception
as argument. Unless pred throws an exception, no exception will
escape the
code body itself."
[pred & body] {:pre [`(fn? ~pred)]}
`(first (some #(let [e# (last %)]
(if e# (if (~pred e#) false
(throw e#))
%))
(repeat-exec #(maybe ~@body)))))
Regards,
Shantanu
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