Very best way to concatenate array of arrays, while applying a function

[mcabbott]

@Per

reduce can check the sizes beforehand to know how much memory to allocate, but mapreduce cannot know the sizes of arrays output by the user function before it is called.

This is true, although it could still do better than pairwise vcat, for instance by guessing that all are the same size as the first. Or by repeated doubling like append!.

@Mason

this will only work if fun always returns a vector of the same length

Indeed, this is one of the things which makes stack easier.

@BambOoxX

Do you know why mapreduce cannot hit the same methods than reduce ?

Mostly nobody has got around to it. PR 31636 had a go. Another option would simply be to send this to reduce(vcat, map(f, xs)), as this will usually be better. (This is also what happens for mapreduce(f, op, xs, ys) at present.)

The other option would be to write a flatten companion for stack, since these clever reduce overloads are hard to discover & quite fragile – it’s easy to step off the fast path, as with init keyword for example.