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[bearophile]

%u:

> i have qustion why filter can't return int[]

Because a lazy filter is handy, you often don't need a real array result, a lazy sequence is enough. A lazy sequence avoids the memory allocation of the output array. In D programs often the slowest parts are the memory allocations.
On the other hand I have asked to add amap/afilter functions that return arrays. Andrei has not answered about this yet.

Bye,
bearophile