Marko A. Rodriguez created TINKERPOP-1506:

             Summary: Optional/Coalesce should not allow sideEffect traversals.
                 Key: TINKERPOP-1506
             Project: TinkerPop
          Issue Type: Improvement
          Components: process
    Affects Versions: 3.2.2, 3.1.4
            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. 

gremlin> g =
==>graphtraversalsource[tinkergraph[vertices:0 edges:0], standard]
gremlin> g.inject(1).optional(addV('twin'))
gremlin> g.V().valueMap(true)

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 

gremlin> g =
==>graphtraversalsource[tinkergraph[vertices:0 edges:0], standard]
gremlin> g.inject(1).coalesce(addV('twin1').limit(0), addV('twin2'))
gremlin> g.V().valueMap(true)

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