Step - fail

fail() and fail(message) stop the whole traversal with an error as soon as a traverser reaches the step (TinkerPop FailStep). Use it to assert that a branch is never taken.

g.V().fail("msg").to_list()
// Error: ... Caused by: msg (fail() reached by v[1])

g.V().coalesce(__.values("name"), __.fail("no name")).count().next()   // 6: fail() is never reached
g.V().union(__.out(), __.fail()).to_list()
// Error: ... Caused by: fail() step triggered (fail() reached by v[1])

• The error is TraverserError::Fail, with the message (fail() step triggered without one) and a short rendering of the value that reached the step (v[1], e[7], or the JSON text of a value). Its help() shows how to guard the step with coalesce() or choose().
• Like every failing traversal, a traversal that changed the graph before fail() is rolled back as a whole (see Transactions).
• fail() is a side-effect step: the optimizer never moves a filter across it, so only the traversers that reach it as written fail the query.

Rust: GraphTraversalSource::fail(None | Some(message)), __::fail(..).