Handling TOML timestamps

[ear7h]

TOML has native support for dates and times and I'm wondering how this should be handled in dhall. I can add to the discussion standardizing date/times in dhall, but perhaps a more specialized solution would be better.

I don't think toml-to-dhall poses too much of a problem, we can just turn the timestamps into strings. But dhall-to-toml requires some mechanism of distinguishing strings from timestamps since the syntax for times are different:

a-string = "1970-01-01"
a-time = 1970-01-01

In the above thread one approach mentioned is to use a newtype { date : Text } to represent times, but overloading a record with a date field seems like it would run into problems. I think a better approach, with a similar effect is to use an enum Date : Type = <Date : Text>. This could live in the prelude under Prelude.Toml.Date. However, it requires special-casing the type in dhall-to-toml and I'm not sure if that's acceptable or if there's a precedent for doing that in other dhall (en|de)coders.

[sjakobi]

I think it does make sense to rekindle the discussion in dhall-lang/dhall-lang#80. The Dhall community has changed quite a bit in the two years since the last comment on that thread. In particular, there are quite a few more implementations that may or may not want to provide built-in support for dates.

Until we have a decision on the language support we might as well do the easiest thing which I think is to use a simple newtype like { date : Text } or maybe { tomlDate : Text }. The < Date : Text > idea is nice too, but I think it's a bit less convenient to extract the field from it.

The main precedent for types that carry special meaning during marshalling is the JSON.Nesting type used in dhall-json.

How do the Haskell TOML libraries handle TOML's dates and times BTW? That might give us some inspiration.

[ear7h]

Both tomland and toml haskell packages use the time package, where the types are records. For example UTCTime is:

data UTCTime = UTCTime {
    -- | the day
    utctDay :: Day,
    -- | the time from midnight, 0 <= t < 86401s (because of leap-seconds)
    utctDayTime :: DiffTime
}

Something I remembered is that there are different kinds of times in TOML:

• offset datetime (has a time zone)
• local datetime (no time zone)
• local date (no time zone)
• local time (no time zone)

So there would actually need to be multiple newtypes { offsetDatetime : Text}, {localDatetime : Text}, etc. I was thinking that a single { tomlDate : Text} would work, but the the date kind is necessary in for making the correct Toml.Codec value

[Gabriella439]

I wrote up my thoughts on how this could be done as a change to the language standard here: dhall-lang/dhall-lang#80 (comment)

[sjakobi]

For reference:

• Support for dates, times and timezones was standardized in Standardize support for dates / times / time zones dhall-lang#1191.
• The Haskell implementation was updated in Add support for Date / Time / TimeZone #2247.
• Dhall.Pretty.temporalToText might be handy for implementing date support in dhall-to-toml.