Inconsistent:

kotlinx-datetime

Instant

,

LocalDate

, etc. • uses ISO-8601 for `toString()`/`parse()` • provides serializer

kotlin.time

Duration

OTOH • uses human-readable output for

toString()

, returns ISO-8601 for

toIsoString()

, and has no

parse()

• has no serializer Still two different date/time worlds in Kotlin and usage is confusing and error-prone.

We want to change the format of

Duration.toString

but it will still remain human readable, not based on ISO (https://youtrack.jetbrains.com/issue/KT-42851) We also have

Duration.parse

in near plans, that would support both that human-readable and the ISO formats: https://youtrack.jetbrains.com/issue/KT-45325

Thanks @ilya.gorbunov. So,

.toString()

is supposed to lose precision to make it human-readable.

.parse()

is supposed to parse the human-readable format. That would make the result

d2 = Duration.parse(d1.toString())

very confusing because often

d1 != d2

. The

toString()

(&

parse()

) issue is a general issue in Java and now Kotlin. Is

toString()

supposed to return a human-readable or a machine-readable result? Human-readable involves many aspects like locale, precision, time zone, etc. While machines can always serialize and parse consistently (e.g. ISO 8601). Same discussion with

Double.toString()

- locale-dependent or not? I’d argue to implement

toString()

for fundamental types consistently. Either always for humans or always for machines. Never mixed. Then developers always know what to expect. And

parse()

should never try to parse two different formats. If I build an API that expects an ISO 8601 duration then I don’t want it to accept some human-readable format too just because Kotlin happens to do so.

There's human readable, machine readable, and in the middle, sits developer readable. The problem with toString in Java is that it's sometimes one, sometimes another, and that leaks into Kotlin.

Yes. That’s why it would be nice if Kotlin would focus on increasing consistency and predictability here.

The consistency is currently to have none 😅