Link 2 vertices in Gremlin

link.groovy

// link two vertices, if the link does not already exists
// return the new or the existing link / edge
Gremlin.defineStep('link', [Vertex,Pipe], { direction, label, other ->
  exists = false
  dirs   = direction.toString()
  dir    = dirs.toLowerCase()
  edge   = dir + "E"
  oppV   = direction.opposite().toString().toLowerCase() + "V"
  link   = "link" + dirs.substring(0,1) + dir.substring(1)
  _()."$dir"(label).filter{it==other}.sideEffect{exists=true}.optional(3).ifThenElse{exists} \
    {it."$edge"(label)."$oppV"().retain([other]).back(2)}                                    \
    {it."$link"(label, other)."$edge"(label)."$oppV".retain([other]).back(2)}
})

// example
v1 = g.addVertex()
v2 = g.addVertex()
v3 = g.addVertex()

v1.link(OUT,  'knows', v3)   // v1 knows v3
v2.link(IN,   'knows', v3)   // v3 knows v2
v1.link(BOTH, 'knows', v2)   // v1 knows v2, v2 knows v1