I do not see any straightforward change to Clojure that would enable this
to work (someone else might, though). Given that adding a loop form inside
the function is a fairly straightforward workaround to the limitation, and
the difficulty of enhancing it to avoid the limitation, I would personally
be a bit surprised to see such a ticket accepted.
It doesn't cost anything but some time to create one, though.
Andy
On Thu, Jul 24, 2014 at 9:13 PM, Michael O'Keefe <michael.p.oke...@gmail.com
> wrote:
> Thank you everyone, for the replies:
>
>
> - The macro expansion was helpful
> - Andy, thanks for pointing out where the lines are -- very helpful
> - Ambrose, both (number? %) and number? are valid post-condition
> forms; the issue is that the post-condition check bumps the recur out of
> tail position
>
> I guess the real question: is this worth filing a ticket for?
>
> Thanks all,
>
> Michael
>
>
>
> On Thursday, July 24, 2014 9:17:23 PM UTC-6, Ambrose Bonnaire-Sergeant
> wrote:
>
>> Hi Michael,
>>
>> I believe your post condition should read {:post [(number? %)]}.
>>
>> Thanks,
>> Ambrose
>>
>>
>> On Fri, Jul 25, 2014 at 6:56 AM, Michael O'Keefe <michael....@gmail.com>
>> wrote:
>>
>>> Hello All,
>>>
>>> I encountered the following behavior in Clojure 1.6 and wanted to check
>>> if it should be considered a bug or not. I would say yes but wanted to
>>> double check on the list first.
>>>
>>> Here's a minimal test case that elicited the error:
>>>
>>> (defn f
>>>
>>> [xs acc]
>>> (if (nil? xs)
>>> acc
>>> (recur (next xs) (+ (first xs) acc))))
>>>
>>> (f [1 2 3 4] 0) => 10
>>>
>>>
>>> Now, if I want to add pre/post conditions, the following happens:
>>>
>>> (defn g
>>>
>>> [xs acc]
>>> {:pre [(or (nil? xs) (sequential? xs)) (number? acc)]
>>> :post [number?]}
>>> (if (nil? xs)
>>> acc
>>> (recur (next xs) (+ (first xs) acc))))
>>>
>>>
>>> => this fails to compile with "CompilerException
>>> java.lang.UnsupportedOperationException:
>>> Can only recur from tail position"
>>>
>>>
>>> In fact, it is only the post-condition that triggers the issue.
>>>
>>> My guess would be that the recur statement is being knocked out of tail
>>> position by the mechanism for handling the post-condition assertion. It can
>>> be fixed in the code by adding an explicit loop:
>>>
>>> (defn g2 [xs acc]
>>> {:pre [(or (nil? xs) (sequential? xs)) (number? acc)]
>>> :post [number?]}
>>> (loop [xs xs
>>> acc acc]
>>> (if (nil? xs)
>>> acc
>>> (recur (next xs) (+ (first xs) acc)))))
>>>
>>>
>>> Thanks,
>>>
>>> Michael O'Keefe
>>>
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