is std.algorithm.joiner lazy?

[Puming via Digitalmars-d-learn]

On Thursday, 7 April 2016 at 08:27:23 UTC, Edwin van Leeuwen 
wrote:
> On Thursday, 7 April 2016 at 08:17:38 UTC, Puming wrote:
>> On Thursday, 7 April 2016 at 08:07:12 UTC, Edwin van Leeuwen 
>> wrote:
>>
>> OK. Even if it consumes the first two elements, then why does 
>> it have to consume them AGAIN when actually used? If the 
>> function mkarray has side effects, it could lead to problems.
>
> After some testing it seems to get each element twice, calls 
> front on the MapResult twice, on each element. The first two 
> mkarray are both for first element, the second two for the 
> second. You can solve this by caching the front call with:
>
> xs.map!(x=>mkarray(x)).cache.joiner;

Thanks! I added more elements to xs and checked that you are 
right.

So EVERY element is accessed twice with joiner. Better add that 
to the docs, and note the use of cache.