perf(array): pre-size Ap and Flatten results#190
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array.Ap and array.Flatten both routed through MonadChain, which grows the result from nil and (for Ap) allocates an intermediate slice per function. Their output lengths are known up front -- len(fab)*len(fa) for the applicative cartesian product and the sum of inner lengths for flatten -- so build the result in a single pre-sized allocation. Benchmarks (added): BenchmarkAp 6 -> 1 allocs/op (-83%), 155 -> 66 ns/op BenchmarkFlatten 3 -> 1 allocs/op (-67%), 51 -> 24 ns/op Signed-off-by: Nishant Mehta <nishantmehta.n@gmail.com>
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Problem
array.Apandarray.Flattenboth route throughMonadChain, which grows theresult from
nil;Apadditionally allocates an intermediate slice per function(via
MonadMap).Change
Both output lengths are known up front —
len(fab)*len(fa)for the applicativecartesian product, and the sum of the inner slice lengths for flatten — so the
result is built in a single pre-sized allocation. Element order and values are
unchanged (the change is in
array/generic, which the publicarray.Ap/array.Flattendelegate to).Result
Benchmarks (added),
-benchmem -count=6:ApbeforeApafterFlattenbeforeFlattenafter−83% / −67% allocations respectively.
Testing
go test -race ./array/...passes (existing tests cover element order/values).BenchmarkApandBenchmarkFlattenwere added to demonstrate and guard the win.