@Override
    public DataType getFieldType(String fieldName, Expression exp) {
        try {
            return input.getDataType().buildFieldPath(fieldName).getResultingDataType();
        } catch (IllegalArgumentException e) {
            throw new VerificationException(exp, "Input field '" + fieldName + "' not found");
        }
    }
Creates an instance with empty output
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/SimpleDocumentFieldValues.java