`std::accumulate` is defined to have sequential semantics, so the analysis required to make it parallel is probably not that different than starting from the loop version. I guess you could have an alternate `accumulate_associative` that uses the same interface but assumes the reduction is associative and has unspecified evaluation order?

std::accumulate is sequential and guarantes in order traversal. std::reduce is parallel version of it

1) afaik accumulate cannot be parallelized

2) the map part is included in the accumulate lambda, so the map part cannot be parallelized either -> you'd have to split it out into a transform step (iirc)