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https://issues.apache.org/jira/browse/TINKERPOP-1506?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15595686#comment-15595686
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Marko A. Rodriguez commented on TINKERPOP-1506:
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So, we could make it so that {{next()}} returns the result from {{hasNext()}}.
However, the way {{BranchStep}} is implemented (for which {{ChooseStep}} is a
subclass (for which {{optional()}} is implemented with)), the selection
traversal is not the same as the branch traversal. Where the former is a "local
child" and the latter, a "global child." Thus, {{optional()}} uses {{clone()}}
to get a selection and branch traversal. We could always write an
{{OptionalStep}} which would do as you recommend and in fact, this might be the
best way forward.
> Optional/Coalesce should not allow sideEffect traversals.
> ---------------------------------------------------------
>
> Key: TINKERPOP-1506
> URL: https://issues.apache.org/jira/browse/TINKERPOP-1506
> Project: TinkerPop
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.1.4, 3.2.2
> Reporter: Marko A. Rodriguez
>
> It took me a long time to realize what was wrong with a traversal I wrote
> that used {{optional(blah.sideEffect.blah)}}. {{optional()}} maps to
> {{ChooseStep}} under the hood and the provide traversal is first tested for a
> {{hasNext()}}. If so, the it plays itself out. The problem is that if there
> is a side-effect in the traversal child, then it gets executed twice.
> {code}
> gremlin> g = TinkerGraph.open().traversal()
> ==>graphtraversalsource[tinkergraph[vertices:0 edges:0], standard]
> gremlin> g.inject(1).optional(addV('twin'))
> ==>v[1]
> gremlin> g.V().valueMap(true)
> ==>[id:0,label:twin]
> ==>[id:1,label:twin]
> {code}
> We should NOT allow {{optional()}} to have {{SideEffectStep}} steps in it so
> as not to cause unexpected behavior. {{StandardVerificationStrategy}} can
> analyze and throw an exception if necessary.
> Also, {{coalesce()}} has a similar problem, though perhaps it can be a useful
> 'technique.'
> {code}
> gremlin> g = TinkerGraph.open().traversal()
> ==>graphtraversalsource[tinkergraph[vertices:0 edges:0], standard]
> gremlin> g.inject(1).coalesce(addV('twin1').limit(0), addV('twin2'))
> ==>v[1]
> gremlin> g.V().valueMap(true)
> ==>[id:0,label:twin1]
> ==>[id:1,label:twin2]
> gremlin>
> {code}
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