[swift-evolution] [Pitch] DateComponents{Encoding/Decoding}Strategy in JSON{Encoder/Decoder}
Itai Ferber
iferber at apple.com
Wed Sep 6 13:03:34 CDT 2017
Hi Pitiphong,
Don’t worry — your original email was clear, and we are on the same
page about `Date{En,De}codingStrategy` and
`DateComponents{En,De}codingStrategy` being separate things.
To clarify my points, though, there are two main things I want to say:
1. I think there is a mismatch here between your goal of representing
the components of a date (and what `DateComponents` can specifically
hold) and the goal of ISO 8601
2. I think that there is an inherent problem in parsing `DateComponents`
due to ambiguity
I think both of these issues can be solved by reading and writing a
`Date` (formatted however you need it to be) instead of
`DateComponents`.
To elaborate:
* `DateComponents` is meant to be a container for an _arbitrary_ subset
of information about a `Date`. A `Date` represents a specific instant in
time, but `DateComponents` are effectively meaningless without
additional context. In the examples that you give, it’s possible to
represent the concepts at hand with `DateComponents`, but in order to
make those components actionable and meaningful, you still need to
convert them to `Date`s. Note also that:
* It’s entirely possible to create a `DateComponents` which
represents a date which does not exist, or a time which does not exist
* Any of these concepts can also be represented by a `Date` instead
of just components; e.g., an all-day event can be represented by a
`Date` that represents the beginning of the day (`00:00:00`) and a flag
that indicates that the time of the event can be ignored, or by a start
`Date` that represents the start of the day and and end `Date` that
represents the end of the day
* Unlike `DateComponents`, ISO 8601 strings have some structure to them.
They cannot represent just a time zone, for instance, or some singular
components of a date/time (e.g. a month without a year, a day without a
month and year, a minute without an hour, a second without a minute and
hour, etc.). I think this is a relatively large conceptual mismatch that
is worth considering deeply. There are a lot of `DateComponents`
instances which simply cannot be represented by an ISO 8601 string
* There is also the issue of decoding arbitrary ISO 8601 strings into
`DateComponents`. `DateComponents`, having no structure at all, have no
specified format they can expect to decode from, and ISO 8601 does not
always provide that structure. Consider the following example:
* ISO 8601 allows for date representations by year, month, and day
(`YYYY-MM-DD`), among other forms. But it also allows days to be left
unspecified (`YYYY-MM`), and even months (`YYYY`)
* Similarly, it allows for a time representations by hour, minute,
and second (`hh:mm:ss`), but also just hour and minute (`hh:mm`), and
just hour (`hh`). Importantly, it allows time separators to be omitted
(`hhmmss`, `hhmm`, `hh`)
* Consider then, attempting to parse the string `"2017"` without
any context — what `DateComponents` should be read out? Intuitively,
`2017` looks like a year (`YYYY`), but it is equally valid to parse as
the time `20:17` (`hhmm`). Without knowing the expected format, parsing
is ambiguous
We cannot promise to parse `DateComponents` in all cases because
there are many combinations of strings that are just completely
ambiguous.
* So, to get at the core of this — if there is a specific format that
you would like to encode to and from, why not do so with a `Date` and a
`DateFormatter` (or if you need ISO 8601 specifically,
`ISO8601DateFormatter`)? With a formatter, the format is unambiguous
because you explicitly provide it, and there is nothing the date can’t
represent that `DateComponents` can. You can always parse the date and
pull out only those components that you care about. You also mention
interoperability with an external JSON source — how is that source
producing a string/parsing one back? [What I’m getting at here is:
what is the value of adding a new, potentially risky strategy over
existing methods that might work just as well, or better?]
* And lastly, if `.iso8601` is not necessarily a good fit for this
strategy, what separates `.custom` from just overriding `encode(to:)`
and `init(from:)` and writing the components out in the format that you
need?
I think answers to these questions can help us push this forward. :)
— Itai
On 5 Sep 2017, at 10:41, Pitiphong Phongpattranont wrote:
> Hi Itai,
>
> I think my first pitch email was not clear enough and want to sorry
> for that. I have been working on a calendar app for awhile and
> understand the concept of calendar or date and time programming in
> some level. I didn’t pitch the idea of encoding and decoding `Date`
> value with this `DateComponents{Encoding/Decoding}Strategy`. I still
> agree that `Date` value should be encoded/decoded with the
> `Date{Encoding/Decoding}Strategy`. The
> DateComponents{Encoding/Decoding}Strategy I pitched only apply for
> `DateComponents` value only.
>
> About the use case, I think there are some application which store an
> information of a `Date` value that is not include a time value (A date
> of September 6th, 2017) for example a calendar app which want to store
> the Start and End date of an `All Day Event` with a value of
> DateComponents type or an alarm app which want to store just a time of
> the day that user want to set an recurring alarm (10:30am.)
>
> The problem I found with the current implementation is that I have no
> control on how the DateComponents implement the conformance methods of
> the Encodable and Decodable protocol. This means that if I have a
> service that serialize those properties with a difference notation
> (ISO 8601 in my case) then I cannot rely on the auto synthesized
> implementation from the compiler and need to do a manual
> encoding/decoding by manually implement the Encodable and Decodable
>
> Lastly, on the issue that `ISO8601` standard does not support every
> components in DateComponents, I still haven’t thought this though
> and still thinking about it. I want to pitch the idea first and would
> like to have a discussion/brainstorm on should we do this and how we
> can do it. My backup plan is doesn’t include the `iso8601` strategy
> but still have the `custom` strategy for those who need to apply a
> custom encoding/decoding strategy which will be apply to all values in
> a payload. Since we encode/decode a JSON from one source at a time and
> the encoding/decoding strategy of DateComponents of that source should
> be consistency throughout its types (which may be the types that I own
> or the types from a 3rd party service), I think this still is a valid
> use case for providing a custom strategy.
>
> Thank you
> — Pitiphong P.
>
>
>> On 6 Sep BE 2560, at 00:15, Itai Ferber <iferber at apple.com> wrote:
>>
>> Hi Pitiphong,
>>
>> Thanks for pitching this! My main question here is about the use
>> case. Since encoding/decoding strategies apply to all values in a
>> payload (whether or not those belong to types that you own), they
>> inherently come with some risk.
>> What is the use case in mind for needing to encode and decode
>> DateComponents directly, as opposed to encoding and decoding a Date
>> instance and pulling the components you need from that?
>>
>> From a correctness standpoint, I also want to point out that
>> DateComponents is really just a "bag of stuff" that doesn’t
>> necessarily mean much until converted into a Date through a Calendar
>> and a TimeZone. There is somewhat of a mismatch between this "bag of
>> stuff" and what ISO 8601 intends to represent — an actual date and
>> time. It’s possible to represent things in a DateComponents that
>> don’t really make sense for (or are not supported by)
>> ISO-8601-formatted dates. For instance, you can have a DateComponents
>> which just has a TimeZone, but ISO 8601 does not allow representing a
>> time zone without a corresponding time. DateComponents also, for
>> instance, has a quarter component (among others) which I’m almost
>> certain ISO 8601 has no equivalent for.
>>
>> Given that conceptual mismatch, I think we’d need a very compelling
>> use case to support this over simply using Date.
>>
>> — Itai
>>
>> On 3 Sep 2017, at 0:55, Pitiphong Phongpattranont via swift-evolution
>> wrote:
>>
>> Hi folks, I have an idea on improving the JSON{Encoder/Decoder} to
>> pitch.
>>
>> Since JSON doesn’t have a native representation for
>> `DateComponents` like it doesn’t have for `Date` too so that
>> there’re many ways to represent it in JSON, for example ISO 8601,
>> UNIX timestamp, etc. for Date. There are also a few ways to represent
>> `DateComponents` too, for example ISO 8601
>> (https://en.wikipedia.org/wiki/ISO_8601
>> <https://en.wikipedia.org/wiki/ISO_8601>) also describes how to
>> represent some of the valid date components (e.g. "2017-09-03”).
>> Unlike what JSON{Encoder/Decoder} does to represent `Date` value with
>> several strategy but there is no support like that for
>> `DateComponents`.
>>
>> The current implementation DateComponents is to encode/decode with
>> KeyedContainer and cannot provide a custom or ISO 8601 compatible
>> implementation. So I think JSON{Encoder/Decoder} should have a
>> strategy for encoding/decoding `DateComponents` just like for Date
>>
>> Here’s an initial `DateComponentsStrategy` strategy that I want
>> JSON{Encoder/Decoder} I can think of now, any suggestion is welcomed.
>>
>> ```swift
>> /// The strategy to use for encoding `DateComponents` values.
>> public enum DateComponentsStrategy {
>> /// Defer to `Date` for choosing an encoding. This is the default
>> strategy.
>> case deferredToDateComponents
>>
>> /// Encode the `Date` as an ISO-8601-formatted string (in RFC 3339
>> format).
>> case iso8601
>>
>> /// Encode the `Date` as a custom value encoded by the given closure.
>> ///
>> /// If the closure fails to encode a value into the given encoder,
>> the encoder will encode an empty automatic container in its place.
>> case custom((DateComponents, Encoder) throws -> Void)
>> }
>> ```
>>
>> What do you guys think about this pitch?
>>
>>
>> Pitiphong Phongpattranont
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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