Github user srowen commented on the pull request:
https://github.com/apache/spark/pull/11217#issuecomment-184692578
They don't need to be since `fold` will operate on the input in some order.
Spark's `fold` is built on `Iterator.fold` which would naturally apply the
operator in a fixed order from first to last, so commutativity does not matter.
See its doc:
http://www.scala-lang.org/api/2.11.7/index.html#scala.collection.Iterator ;
most Spark docs for `fold` methods do not mention commutativity either.
The scaladoc does mention the non-trivial things that happen if it's not
commutative, but it's not required.
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