usememos/memos · error

equality predicate expects exactly two arguments

Error message

equality predicate expects exactly two arguments

What it means

buildEqualsPredicate translates t == "value" inside a comprehension. It requires the == call to have exactly two arguments so it can determine which side is the iteration variable and which side is the constant. Any other arity is rejected.

Solutions

1. Use the textual == operator inside the macro: tag.exists(t, t == "value")
2. When building ASTs, always give _==_ exactly two args

if err := r.Render(cond); err != nil {
    if strings.Contains(err.Error(), "exactly two arguments") {
        return errors.New("== inside a macro must compare two operands")
    }
    return err
}

When it happens

Trigger: A programmatically built _==_ call with 0, 1, or 3+ args; textual CEL cannot produce this because == is a binary operator. So in practice this guards against malformed ASTs only.

Common situations: Custom macro/AST pipelines; tests constructing Expr_Call nodes directly with wrong arg counts.

Related errors

• comprehension accumulator must be initialized with a…
• comprehension loop step must be a call expression
• comprehension loop step must have two arguments
• comprehension predicate must be a function call
• contains argument must be a string

AI-assisted analysis of usememos/memos@14d757ce1f (2026-08-15). Data as JSON: /api/errors/2b7be42e998981cc. Report an issue: GitHub.

Appendix: source

Thrown at internal/filter/parser.go:921

	// Handle different predicate functions
	switch predicateCall.Function {
	case "_==_":
		return buildEqualsPredicate(predicateCall, comp.IterVar)
	case "startsWith":
		return buildStartsWithPredicate(predicateCall, comp.IterVar)
	case "endsWith":
		return buildEndsWithPredicate(predicateCall, comp.IterVar)
	case "contains":
		return buildContainsPredicate(predicateCall, comp.IterVar)
	default:
		return nil, errors.Errorf(`unsupported predicate function %q in comprehension (supported: ==, startsWith, endsWith, contains)`, predicateCall.Function)
	}
}

// buildEqualsPredicate extracts the value from t == "value".
func buildEqualsPredicate(call *exprv1.Expr_Call, iterVar string) (PredicateExpr, error) {
	if len(call.Args) != 2 {
		return nil, errors.New("equality predicate expects exactly two arguments")
	}

	var constExpr *exprv1.Expr
	switch {
	case isIterVarExpr(call.Args[0], iterVar):
		constExpr = call.Args[1]
	case isIterVarExpr(call.Args[1], iterVar):
		constExpr = call.Args[0]
	default:
		return nil, errors.Errorf("equality predicate must compare against the iteration variable %q", iterVar)
	}

	value, err := getConstValue(constExpr)
	if err != nil {
		return nil, errors.Wrap(err, "equality argument must be a constant string")
	}

	valueStr, ok := value.(string)

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