cayleygraph/cayley · error

wrong quad format: '%s': no predicate

Error message

wrong quad format: '%s': no predicate

What it means

The schema parser failed to extract a predicate from a quad-shaped rule string. Rules of the form 'predicate[ value]' must contain a non-empty predicate before the space; an empty predicate part makes the rule meaningless, so generation aborts. This comes from strings.SplitN of the rule string where the first segment trimmed to empty.

Source

Thrown at schema/schema.go:194

			return nil, fmt.Errorf("wrong quad tag format: '%s'", rule)
		}
	} else { // s-p>o // default
		tri = strings.SplitN(rule, spo, 3)
		if len(tri) > 2 { //len(tri) != 2 {
			return nil, fmt.Errorf("wrong quad tag format: '%s'", rule)
		}
	}
	var ps, vs string
	if rev {
		ps, vs = strings.Trim(tri[0], trim), strings.Trim(tri[1], trim)
	} else {
		ps, vs = strings.Trim(tri[0], trim), any
		if len(tri) > 1 {
			vs = strings.Trim(tri[1], trim)
		}
	}
	if ps == "" {
		return nil, fmt.Errorf("wrong quad format: '%s': no predicate", rule)
	}
	p := c.toIRI(ps)
	if vs == "" || vs == any && fld.Type != reflEmptyStruct {
		return saveRule{Pred: p, Rev: rev, Opt: opt}, nil
	}
	return constraintRule{Pred: p, Val: c.toIRI(vs), Rev: rev}, nil
}

func checkFieldType(ftp reflect.Type) error {
	for ftp.Kind() == reflect.Ptr || ftp.Kind() == reflect.Slice {
		ftp = ftp.Elem()
	}
	switch ftp.Kind() {
	case reflect.Array: // TODO: support arrays
		return fmt.Errorf("array fields are not supported yet")
	case reflect.Func, reflect.Invalid:
		return fmt.Errorf("%v fields are not supported", ftp.Kind())
	default:

View on GitHub (pinned to 81dcd7d73e)

Solutions

1. Fix the struct tag so it begins with a valid predicate IRI or string, e.g. `tag:"name"`
2. Remove stray leading spaces/colons from the rule string before the predicate
3. Print the offending rule (the %s in the message) and compare against documented quad format 'predicate [value]'

Example fix

// before
type Person struct {
    Name string `tag:" nickname"`
}
// after
type Person struct {
    Name string `tag:"nickname"`
}

for _, s := range rules {
    s = strings.TrimSpace(strings.SplitN(s, " ", 2)[0])
    if s == "" {
        return fmt.Errorf("rule %q has empty predicate", s)
    }
}

When it happens

Trigger: Calling Generate on a struct whose tagged rule string starts with a space, contains only whitespace before the value separator (e.g. `tag:",someIRI"`), or is effectively empty after trimming.

Common situations: Typos in struct tags like `tag:":value"` or `tag:" value"`; programmatically built rule strings with accidental leading separators; copy-pasted RDF lines missing the predicate column.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• can't load rules: %v
• HTTP Request needed
• ErrEmptyPath
• replication: name '" + name + "' is not registered
• not found

AI-assisted analysis of cayleygraph/cayley@81dcd7d73e (2026-09-06). Data as JSON: /api/errors/1a8cb3c20cebb34f. Report an issue: GitHub.