On Mon, 17 May 2010 14:36:23 -0700 (PDT)
Base <basselh...@gmail.com> wrote:
> Hi All -
>
> I am trying convert a function to use loop/recur and am getting the
> dreded
>
> "java.lang.UnsupportedOperationException: Can only recur from tail
> position (repl-1:414)" error
>
> (at least dreded for newbies...)
The "tail" position means it's the last call at the top level of the
loop. I.e. - you can't call anything after the recur. In your case
> The function takes 2 joda dates and returns a sequence of dates
> starting at the first of the month until it is past the second date.
> This works using standard recursion but I wanted to use TCO.
>
> Any help is most appreciated (as usual...!)
>
> Thanks
>
> Base
>
>
> (def date-seq
> (fn [d1 d2]
> (loop [b (-> d1
> (.dayOfMonth)
> (.withMinimumValue))
> e d2]
> (cons b (if (time/before? b e)
> (recur (.plusMonths b 1) e))))))
In this case, you're calling cons with the result of the recur, which
means it happens "after" the recur, which is the error.
I suspect your original wasn't tail recursive, since the two loop
arguments are two dates (assuming my guesses about time/before? are
correct), and the values in the list you returns would be stored on
the call stack, to be pasted together by the string of conses you've
left to run after each recursive call.
To make it tail recursive, you need to make the list you're going to
return one of the arguments of the loop, adjust the code for that,
then push the cons into the if, sort of like so:
(defn date-seq
[d1 d2]
(loop [b (-> d1
(.dayOfMonth)
(.withMinimumValue))
e [d2]]
(if (time/before? b (first e))
(recur (.plusMonths b 1) (conj e b))
(reverse b))))
This is just a swag; I'm not familiar enough with Java to reconstruct
your arguments...
<mike
--
Mike Meyer <m...@mired.org> http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.
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