Robert Dale commented on TINKERPOP-1506:

I don't use optional() but I wonder if that's more of a bug. Why execute twice? 
Shouldn't next() just return the result of hasNext()?  It almost sounds like 
this bug https://github.com/apache/tinkerpop/pull/346
Also, I disagree on coalesce(). It should continue to work as-is.  It's very 
useful for upsert (update or insert) where you definitely want the second 
traversal to create an element. I'm open to a better way.  But in general, just 
because you can shoot yourself in the foot, it doesn't make the tool defective. 
Maybe it would be better to put a NOTE: in the ref guides about doing such 
things to make to explicitly clear.

> 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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