RFC: Temporal API bindings

This is an RFC, not a task: the author already has ~270 bindings in 11 files and is asking for a direction decision on three points (stdlib placement, the == change, CI on Node.js 22). Start by reading how Intl is laid out in the stdlib (Stdlib_Intl.res and Stdlib_Intl_Common.res) since the proposal mirrors it with Stdlib_Temporal.res and Stdlib_Temporal_Common.res, and look at the Date equality fix in #3455 before judging the == change. Done means a maintainer answers the three questions and the Node.js 26 test-step question is settled, not a patch.

Summary

Add bindings to JavaScript's Temporal API to the standard library as Temporal. There is one module per Temporal type, plus Temporal.Now and Date.toTemporalInstant. The bindings are externals that compile to direct calls on the global Temporal. They need a runtime that provides it, such as Node.js 26, or a polyfill that defines globalThis.Temporal.

let meeting = Temporal.ZonedDateTime.fromStringOrThrow(
  "2024-03-09T09:30:00-05:00[America/New_York]",
)
let nextMeeting = meeting->Temporal.ZonedDateTime.addFields({days: 1.}) // JS: meeting.add({ days: 1 })

nextMeeting->Temporal.ZonedDateTime.toString
// "2024-03-10T09:30:00-04:00[America/New_York]"

// The day daylight saving time starts is 23 hours long
meeting->Temporal.ZonedDateTime.until(nextMeeting)->Temporal.Duration.toString
// "PT23H"

I have a working implementation: around 270 bindings in 11 files, with 126 doc examples. I can push it to a branch of my fork if seeing the code helps. Before finishing it and opening a PR, I'd like feedback on the direction, especially on these three points:

• adding Temporal to the standard library;
• the change to ==;
• testing while CI runs on Node.js 22.

Motivation

• Temporal replaces Date. It covers exact times, wall-clock dates and times, time zones, calendars and durations. Node.js 26 ships it without a flag.
• The standard library binds the ECMAScript built-ins, like Date, Intl and BigInt. Temporal is one of them.
• Its signatures are heavily overloaded. Hand-written externals are hard to get right, and each project would type them differently.
• No ReScript bindings for Temporal exist on npm.

Design

Modules

Module	Represents
Temporal.Instant	An exact time.
Temporal.ZonedDateTime	An exact time in a time zone and calendar.
Temporal.PlainDateTime, PlainDate, PlainTime, PlainYearMonth, PlainMonthDay	Wall-clock values without a time zone.
Temporal.Duration	A duration.
Temporal.Now	Reads the current time.
Temporal.Common	Types shared by the modules.

Each module is a file, like Stdlib_Temporal_PlainDate.res, aliased from Stdlib_Temporal.res, as with Intl. Each module has an abstract type t and data-first functions.

Shared types

Temporal types convert into each other in both directions, like PlainDate.toPlainDateTime and PlainDateTime.toPlainDate. If each file defined its own type, the files would form a dependency cycle. So the value types are defined once, in Temporal.Common, and each module re-exports its own, as Intl.Common does for Intl:

// Stdlib_Temporal_Common.res
@notUndefined @editor.completeFrom(Stdlib.Temporal.PlainDate)
type plainDate

// Stdlib_Temporal_PlainDate.res
type t = Stdlib_Temporal_Common.plainDate

@editor.completeFrom keeps pipe completion working on a Common.plainDate.

Splitting JavaScript overloads

A ReScript function has one type, so each JavaScript overload is bound as its own function. The functions for one overloaded method compile to the same JavaScript call:

JavaScript	ReScript
new Temporal.PlainDate(2024, 2, 29)	PlainDate.make(~year=2024, ~month=2, ~day=29)
Temporal.PlainDate.from("2024-02-29")	PlainDate.fromStringOrThrow("2024-02-29"), or fromString, which returns an option
Temporal.PlainDate.from({year: 2024, month: 2, day: 29})	PlainDate.fromFields({year: 2024, month: 2, day: 29})
date.add(duration)	date->PlainDate.add(duration)
date.add({days: 1})	date->PlainDate.addFields({days: 1.})
Temporal.Duration.compare(a, b, {relativeTo})	Duration.compareWithOptions(a, b, {relativeTo: …})

All modules follow this pattern. A gap means JavaScript has no such function:

Module	make	fromStringOrThrow, fromString	fromFields	add, subtract	addFields, subtractFields	compare
Duration	✓	✓	✓	✓	✓	✓, and compareWithOptions
Instant	fromEpochNanoseconds, fromEpochMilliseconds	✓	—	✓	✓	✓
PlainDate	✓	✓	✓	✓	✓	✓
PlainTime	✓	✓	✓	✓	✓	✓
PlainDateTime	✓	✓	✓	✓	✓	✓
PlainYearMonth	✓	✓	✓	✓	✓	✓
PlainMonthDay	✓	✓	✓	—	—	—
ZonedDateTime	fromEpochNanoseconds	✓	✓	✓	✓	✓

• Options are an optional labeled ~options argument.
• compare returns Ordering.t, so Array.toSorted(Temporal.PlainDate.compare) works.

Types

• Calendars are a closed polymorphic variant of the 16 calendars Temporal accepts. They are a subset of Intl.Common.calendar, so (calendar :> Intl.Common.calendar) works.
• Time zones and eras are strings.
• Units are singular polymorphic variants. Each function takes only the units its type supports.
• Numbers. Duration fields are float, date and time fields are int, and epochNanoseconds is bigint.
• relativeTo is an abstract type, created from a PlainDate or a ZonedDateTime.
• Nullable results are options.

Errors

The bindings let Temporal's errors through. fromString is the exception: it returns None for a RangeError and rethrows any other error. That way, a missing Temporal global isn't reported as an invalid string.

Equality

ReScript's == compared objects by their own keys. Temporal values have none, so any two of them were equal. Date had the same bug (#3455). Following that fix, == now uses the type's equals, and compares durations field by field.

compare, < and > are unchanged, because Temporal has no single order for them to use. For ordering, each module has its own compare.

Compiler and tooling

• JS globals. Temporal is added to the JS globals, so a nested module named Temporal is emitted as $$Temporal, as with Intl.
• GenType maps the types to TypeScript's Temporal.* types from the esnext.temporal lib.

Breaking changes

• Shadowing. The stdlib Temporal module shadows a project's own top-level Temporal module, such as bindings to the Temporal.io SDK.
• Export name. A nested module named Temporal is exported to JavaScript as $$Temporal.

Testing

• CI on Node.js 22. Node.js 22 doesn't have Temporal, so the Temporal tests in the stdlib suite don't run there. The doc examples are still type checked: doc examples that need a newer Node.js are now emitted as test.skip instead of being dropped.
• Node.js 26. Locally, all 871 doc examples and the stdlib suite pass.

Not included

• Duration.toLocaleString options, which are Intl.DurationFormat options.
• largestUnit: "auto", and relativeTo as a PlainDateTime or a string.
• String arguments to add, subtract and compare, from with another Temporal value, and plural unit names.
• Intl.DateTimeFormat.format with Temporal values.
• valueOf, which always throws.
• A CI step that runs the tests on Node.js 26. This could be a separate PR.