Dust off MapReduce

[eholk]

There's a wiki page about the design of a map reduce library for Rust:

https://github.com/mozilla/rust/wiki/Bikeshed-mapreduce

A version of this lives in the wordcount benchmarks, but it would be good to polish this and move it into libstd.

[eholk]

Some polish has happened as a result of moving parts of MapReduce over to pipes. We still could stand to move it into libstd.

[bstrie]

Nowadays the only mentions of anything named map_reduce are in a single test case in the test suite. I'd suggest vetting a good design on that wiki page before trying to implement anything serious.

[catamorphism]

This doesn't seem to have been nominated properly. Re-nominating to de-milestone it.

[catamorphism]

Removing nomination and triage label.

[pongad]

std::iter already has a map function. My opinion is that we can just add a reduce instead of map_reduce.

[sfackler]

@pongad MapReduce is not just map/fold on iterators.

[pongad]

Oh whoops, I guess I did not have enough sleep. Sorry!

[cosmosgenius]

isn't the functionality of reduce same as what is for fold in iterator?

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#637