Major Description
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.
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]
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.'
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>
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