cayleygraph/cayley · error

not implemented: need a function from predicates to their as

Error message

not implemented: need a function from predicates to their associated edges

What it means

predicatesMorphism() has no Reversal: reversing it would require a function mapping predicates to their associated edges, which the path layer does not provide. Evaluating a path in reverse that contains the predicates step panics with this message.

Source

Thrown at query/path/morphism_apply_functions.go:224

// labelsMorphism iterates to the uniqified set of labels from
// the given set of nodes in the path.
func labelsMorphism() morphism {
	return morphism{
		Reversal: func(ctx *pathContext) (morphism, *pathContext) {
			panic("not implemented")
		},
		Apply: func(in shape.Shape, ctx *pathContext) (shape.Shape, *pathContext) {
			return shape.Labels(in), ctx
		},
	}
}

// predicatesMorphism iterates to the uniqified set of predicates from
// the given set of nodes in the path.
func predicatesMorphism(isIn bool) morphism {
	return morphism{
		Reversal: func(ctx *pathContext) (morphism, *pathContext) {
			panic("not implemented: need a function from predicates to their associated edges")
		},
		Apply: func(in shape.Shape, ctx *pathContext) (shape.Shape, *pathContext) {
			return shape.Predicates(in, isIn), ctx
		},
	}
}

// savePredicatesMorphism tags either forward or reverse predicates from current node
// without affecting path.
func savePredicatesMorphism(isIn bool, tag string) morphism {
	return morphism{
		Reversal: func(ctx *pathContext) (morphism, *pathContext) {
			return savePredicatesMorphism(isIn, tag), ctx
		},
		Apply: func(in shape.Shape, ctx *pathContext) (shape.Shape, *pathContext) {
			return shape.SavePredicates(in, isIn, tag), ctx
		},
	}

View on GitHub (pinned to 81dcd7d73e)

Solutions

1. Restructure the query so the predicates step is only applied forward.
2. Replace predicates() with an explicit traversal over predicate edges that supports reversal.
3. If you own the code, implement Reversal by deriving the edge set from predicates.

Example fix

// before
p := cayley.StartPath(qs).Predicates(true) // later reversed via Back() -> panic
// after: keep predicates forward-only or use explicit edge traversal
p := cayley.StartPath(qs).Out(predQuad.Predicate)...

if pathWillBeReversed && pathContainsPredicatesStep {
    return errors.New("predicates() step cannot be reversed; restructure the query")
}

When it happens

Trigger: A reversed/backtracking traversal that includes predicates()/Predicates(in, isIn), forcing evaluation of the morphism's Reversal function.

Common situations: Gremlin-style queries like Predicates(true) used within Back() or reverse evaluation; converting old Cayley gremlin code where the predicates step appears on the reverse side.

Related errors

• not implemented
• did not pass a string predicate or a Path to FollowRecursive
• Building an iterator from a morphism. Bind a QuadStore with
• ErrEmptyPath
• error updating node: %v

AI-assisted analysis of cayleygraph/cayley@81dcd7d73e (2026-09-06). Data as JSON: /api/errors/97af6491bc2e38b0. Report an issue: GitHub.