DateTimeFormatter with nested optionals [duplicate]

I've got three types of ISO dates: yyyy, yyyy-MM, yyyy-MM-dd. In other words, increasingly getting more precise from year to month to day. Should a field be absent, it should be mapped to the lowest possible value (missing day -> first day of the month // missing month -> first month of the year).

A day cannot ever be there without a month, so I thought it would work fine using nested brackets:

 DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy[-MM[-dd]]"); LocalDate ld = LocalDate.parse("2019-08-10", dtf); // ok ld = LocalDate.parse("2019-08", dtf); // error ld = LocalDate.parse("2019", dtf); // error 

But alas, the two bottom ones spit out a DateTimeParseException. Is this even possible?

EDIT: Got it. Turns out, as mentioned in the comments, defaults aren't set implicitly. This fixes it:

 DateTimeFormatter dtf = new DateTimeFormatterBuilder() .append(DateTimeFormatter.ofPattern("yyyy[-MM[-dd]]")) .parseDefaulting(ChronoField.DAY_OF_MONTH, 1) .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1) .toFormatter();