How can I do list comprehension for vcat?

General Usage
11

For some reason, the init slows it down quite a bit:

julia> @btime reduce(vcat, $v, init=Int[]);
  23.151 μs (101 allocations: 210.23 KiB)

julia> @btime reduce(vcat, $v);
  962.444 ns (2 allocations: 5.12 KiB)

There is a related open issue for mapreduce (#31137) — it seems like we didn’t optimize a sufficiently low-level method of reduce.

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