[Feature Request] Error handling for expand and flatten functions

[JunyiYi]

Today MM will not check errors in expand or flatten, which is not always correct. For example, to expand/flatten a date.Time type:

func expandStringToDate(input interface{}) (*date.Time, error) {
	v := input.(string)

	dateTime, err := date.ParseTime(time.RFC3339, v)
	if err != nil {
		return nil, err
	}

	result := date.Time{
		Time: dateTime,
	}
	return &result, nil
}

Actual Result

When we call it in the function, MM will generate something like

parameters = xxx.yyy {
    MyDate: expandStringToDate(dt)
};

Expected Result

But ideally, it should generate:

parameters = xxx.yyy {};

if date, err := expandStringToDate(dt); err != nil {
    return err
} else {
    parameters.MyDate = date;
}

Notice that we should propagate all errors up until the top level instead of handling it silently in sub-functions.

Proposed Solution

We need to introduce some special flags to indicate whether an expand/flatten function returns error or not, and handle that in the caller.