Add edge if not exist using gremlin

It is possible to do with edges. The pattern is conceptually the same as vertices and centers around coalesce(). Using the "modern" TinkerPop toy graph to demonstrate:

gremlin> g.V().has('person','name','vadas').as('v').
           V().has('software','name','ripple').
           coalesce(__.inE('created').where(outV().as('v')),
                    addE('created').from('v').property('weight',0.5))
==>e[13][2-created->5]

Here we add an edge between "vadas" and "ripple" but only if it doesn't exist already. the key here is the check in the first argument to coalesce().