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https://issues.apache.org/jira/browse/TINKERPOP-1849?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16282342#comment-16282342
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Marko A. Rodriguez commented on TINKERPOP-1849:
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After bagging to [~dkuppitz] about my solution, he said: "Does it work on
GraphComputer?" I immediately thought: "no" and tested it and it doesn't. Why?
{{FoldStep}} is a {{ReducingBarrierStep}} and thus, can only work with
associative and commutative operations. {{addWithIndex}} is not. It requires a
serial ordering during "folding" in order to get an index. Thus, this raises
two solutions:
Do we provide a "flag" on {{Operators}} to say whether they are associative and
commutative?
1. If so, then if it is not, then all reduction must happen on the master
traversal.
2. Or, simply through a {{ComputerVerificationException}} is such operators
are used.
The more general problem would be provided lambdas that are NOT commutative nor
associative. How is Gremlin to know?
> Provide a way to fold() with an index
> -------------------------------------
>
> Key: TINKERPOP-1849
> URL: https://issues.apache.org/jira/browse/TINKERPOP-1849
> Project: TinkerPop
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.3.0
> Reporter: stephen mallette
>
> In Groovy you can call {{withIndex()}} to generate output like this:
> {code}
> gremlin> g.V().fold().next().withIndex()
> ==>[v[1],0]
> ==>[v[2],1]
> ==>[v[3],2]
> ==>[v[4],3]
> ==>[v[5],4]
> ==>[v[6],5]
> {code}
> We can currently simulate this with Gremlin via:
> {code}
> gremlin>
> g.V().project("a","b").by().by(select("x").count(local)).store("x").map(union(select('a'),
> select('b')).fold()).fold().next()
> ==>[v[1],0]
> ==>[v[2],1]
> ==>[v[3],2]
> ==>[v[4],3]
> ==>[v[5],4]
> ==>[v[6],5]
> {code}
> but it's not easy to follow, efficient, or potentially foolproof. Perhaps we
> can add an option to {{fold()}} that would take an enum of "fold operators".
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