DTC (Datetime Type Classes)

Type classes for datetime values. Works both on JVM & ScalaJS.

DTC provides type classes for local and zoned datetime values, and type class instances for both JVM and ScalaJS.

It serves 2 main purposes:

1. Allows to write generic polymorphic code, that operates on datetime values.
2. Gives possibility to write universal datetime logic, that compiles both for JVM and ScalaJS. Currently, there's no truly cross-platform datetime instance, as scala-js-java-time does not yet provide java.time.LocalDateTime and java.time.ZonedDateTime.

As a bonus, you get immutable datetime values for ScalaJS that behave like java.time._ counterparts.

Table of contents

1. Dependencies
2. Usage
   1. Setup
   2. Simple example
   3. Type classes
      1. TimePoint
      2. Local
      3. Zoned
      4. Capture
      5. Provider
   4. Instances
      1. JVM instances
      2. JS instances
   5. Syntax and Cats integration
3. Motivation
4. Modules
5. Known issues
6. Changelog

Dependencies

DTC core depends on:

• scala-js-java-time to take advantage of java.time._ API parts, that are already available for ScalaJS
• small cats-kernel module for Order type class.

Usage

Setup

Core

Add this line to your build.sbt.

libraryDependencies += "ru.pavkin" %%% "dtc-core" % "2.6.0"

MomentJS instances

If you want to use momentjs instances for ScalaJS runtime (see JS instances), also add dtc-moment module dependency to your scalajs subproject as well:

libraryDependencies += "ru.pavkin" %%% "dtc-moment" % "2.6.0"

This will add momentjs to your JS and scala-js-momentjs to your scalaJS dependencies.

Cats instances

Some additional cats type class instances for DTC type classes (like Invariant) are available via dtc-cats module:

libraryDependencies += "ru.pavkin" %%% "dtc-cats" % "2.6.0"

This will bring in cats-core dependency.

Simple example

Let's create a simple polymorphic class that works with local datetime values.

Say we want a period entity that operates on local datetime values and knows both it's start and end:

import java.time.Duration // this is provided crossplatformly by scala-js-java-time

import dtc.Local
import dtc.syntax.local._ // syntax extensions for Local instances

case class Period[T: Local](start: T, end: T) {

  def prolong(by: Duration): Period[T] = copy(end = end.plus(by)) // syntax in action

  def durationInSeconds: Long = start.secondsUntil(end) // syntax in action
  def durationInMinutes: Long = start.minutesUntil(end) // syntax in action
}

It is 100% cross-platform, so we can put it into a "shared" cross-project, to use later on both platforms.

Let's create two simple apps to demonstrate the concept.

First, let's start with JVM:

import java.time.LocalDateTime

import dtc.instances.localDateTime._ // provide implicit typeclass instance for java.time.LocalDateTime
import dtc.examples.Period

object Main extends App {

  // with implicit typeclass instance in scope we can put LocalDateTime instances here.
  val period = Period(LocalDateTime.now(), LocalDateTime.now().plusDays(1L))
  println(period.durationInMinutes)
  println(period.durationInSeconds)
  println(period.hours.mkString("\n"))
}

Next, nothing stops us from creating a JS app as well:

import java.time.Duration

import dtc.js.JSDate // this is special wrapper around plain JS date, that provides basic FP guarantees, e.g. immutability
import dtc.instances.jsDate._ // implicit Local instance for JSDate
import dtc.examples.Period

import scala.scalajs.js.annotation.JSExportTopLevel

object Main  {
  
  @JSExportTopLevel("dtc.examples.Main.main")
  def main() = {

    val period = Period(JSDate.now, JSDate.now.plus(Duration.ofDays(1L)))
    println(period.durationInMinutes)
    println(period.durationInSeconds)
    println(period.hours.mkString("\n"))
  }
}

These examples demonstrate the core idea of the project. Read further to check out the list of available type classes and instances.

---

Type classes

Disclaimer: although following entities are called type classes, there are not "pure". For example, they can throw exceptions for invalid method parameters. This is intentional:

Primary goal is to provide API that looks like java.time._ as much as it's possible.

DTC provides 4 type classes.

TimePoint

TimePoint extends cats.kernel.Order (api)

Base type class, that can be used for both local and zoned datetime instances.

All instances in DTC are extending TimePoint Most of the APIs are same for any datetime value, so with this typeclass you get:

• all common methods and syntax except ones that are specific to local or zoned datetime values (e.g. constructors)
• you can use both zoned and local datetime instances to fill in the type parameter (not simultaneously, of course)
• almost no laws within the polymorphic code context :)

Local

Local extends TimePoint (api)

Type class for values, that behave similarly to java.time.LocalDateTime. Instances hold local datetime laws.

Zoned

Zoned extends TimePoint (api)

Type class for values, that behave similarly to java.time.ZonedDateTime. Instances hold zoned datetime laws.

Capture

Api

Instance of Capture[T] means, that a specific instant can be represented by a value of type T. Here an instant is represented as a product of (LocalDate, LocalTime, TimeZoneId).

While for a Zoned type it's clear, how to represent such instant (Zoned, actually, extends Capture to show that), for Local it becomes tricky.

DTC provides only one instance of local Capture, which is a UTC instant representation with java.time.LocalDateTime. Such behaviour allows to retain consistent value construction in polymorphic context: LocalDateTime values, created with Capture will represent same instants as ZonedDateTime instances, created from the same input.

Provider

Api

Type class, that abstracts over the notion of "current" time. Provides API to get current date/time in a particular time zone.

In polymorphic context a Provider is the only way to get current time in DTC. This facilitates explicit DI of current time, which leads to better design. For example, it allows to work with artificial time, controlled from the outside (useful for tests).

---

Instances

To make your polymorphic code work on a specific platform, you'll need to supply typeclass instances for concrete datetime types you use.

DTC provides instances for both JVM and ScalaJS.

JVM instances

For JVM everything is straightforward:

1. java.time.LocalDateTime has an instance of Local and a special UTC instance of Capture.
2. java.time.ZonedDateTime has an instance of Zoned (which includes Capture).

To get the instances, just import respectively

import dtc.instances.localDateTime._
// or
import dtc.instances.zonedDateTime._

Also both LocalDateTime and ZonedDateTime have an implicit instance of real system time Provider, available in:

import dtc.instances.providers._

JS instances

First of all, DTC does not provide instances for raw js values (neither Date nor moment). They are mess to work with directly for two reasons:

1. they are mutable
2. they have totally different semantics, comparing to java.time._.

Instead, DTC provides simple wrappers that delegate to underlying values (or even enrich the available API). These wrappers provide immutability guarantees and adapt the behaviour to follow java.time._ semantics.

For ease of direct use, they reflect typeclass API as much as possible. Though, amount of actual direct use of them should be naturally limited, because, well... you can write polymorphic code instead!

JSDate

JSDate wraps native ECMA-Script Date and provides instance for Local.

Instance can be imported like this:

import dtc.instances.jsDate._

Javascript date has a very limited API which doesn't allow to handle time zones in a proper way. So there's no Zoned and no Capture instance for it.

If you need a Zoned instance for your ScalaJS code, take a look at moment submodule, which is following next.

As on JVM, to get a real time Provider[JSDate], add this to your imports:

import dtc.instances.providers._

MomentLocalDateTime and MomentZonedDateTime

These are based on popular MomentJS javascript library as well as ScalaJS facade for it.

To add them to your project, you'll need an explicit dependency on dtc-moment module:

libraryDependencies += "ru.pavkin" %%% "dtc-moment" % "2.0.0"

Both classes wrap moment.Date and, as you can guess:

• MomentLocalDateTime has a Local instance with and a special UTC Capture instance
• MomentZonedDateTime has a Zoned instance (includes Capture)

You can get all instances in scope by adding:

import dtc.instances.moment._

Provider instances for real time can be obtained here:

import dtc.instances.moment.providers._

---

Syntax and Cats integration

DTC syntax

When writing polymorphic code with DTC, it's very convenient to use syntax extensions. They are similar to what Cats or Scalaz provide for their type classes.

Just add following to your imports:

import dtc.syntax.local._  // for Local syntax
// or
import dtc.syntax.zoned._  // for Zoned syntax

If you need syntax for TimePoint or for both local and zoned type classes in the same file, just import all syntax at once:

import dtc.syntax.all._

cats.kernel.Order syntax.

Though, DTC provides basic API for datetime values comparison, it's more convenient and readable to use operators like <, >= and so on. To pull this off, you will need syntax extensions for cats.kernel.Order, that is extended by all DTC type classes.

Unfortunately, kernel doesn't have syntax extensions. So, to get this syntax, you'll need to add dtc-cats module or define an explicit cats-core dependency in your project:

libraryDependencies += "org.typelevel" %%% "cats-core" % "1.0.1"

After that just add regular cats import to get Order syntax for datetime values:

import cats.syntax.order._

Motivation

See my article for original motivation and implementation overview.

Modules

DTC modules with published artifacts:

1. dtc-core - all type classes and instances for java.time._ and JSDate
2. dtc-moment - momentjs instances (ScalaJS only)
3. dtc-cats - additional cats instances for dtc type classes, like Invariant (adds cats-core dependency).
4. dtc-laws - discipline laws to test your own instances

Known issues

MomentJS #3029

There's an open longstanding bug in MomentJS. In some rare cases it gives incorrect diffs for monthsUntil method.

As of current version of DTC, this bug leaks into momentjs instances as well.

Changelog

2.6.0

No major changes. Dependencies updated.

2.5.0

No major changes. Dependencies updated.

2.4.0

Support Scala.js 1.0.0

2.3.1

Scala 2.13 support has been provided

2.0.0

It appeared that variability of equality semantics was totally missed in previous versions.

Zoned values can be compared in two ways:

1. Strict: identical instants are considered equal only if their time zones are equal as well.
2. Relaxed: identical instants are considered equal regardless of time zones.

Since 2.0.0 dtc provides both kind of instances for java.time.ZonedDateTime and MomentZonedDateTime:

• zonedDateTimeWithCrossZoneEquality and zonedDateTimeWithStrictEquality for JVM
• momentZonedWithCrossZoneEquality and momentZonedWithStrictEquality for moment

Migration from 2.0.0-M1 to 2.0.0:

• dtc.instances.zonedDateTime.zonedDateTimeDTC was renamed to zonedDateTimeWithStrictEquality
• dtc.instances.moment.momentZonedDTC was renamed to momentZonedWithCrossZoneEquality

2.0.0-M1

This major release aims to fix a specific design flaw, which is impossibility to construct Local values from zone-aware instants. Prior to 2.0, such option was an exclusive privilege of Zoned.

In practice this introduced limitations in correctly abstracting over UTC/Zoned time representations. Inability to grasp this aspect from the beginning leaded to creation of arcane localDateTimeAsZoned instance. This trick allowed to use LocalDateTime in Zoned context, which provided an ability to construct values polymorphically. Such abuse created it's own issues, in particular - ability to create LocalDateTime values in some Zoned contexts, where it didn't make any sense.

Version 2.0 resolves this issue by extracting time creation functionality in a separate Capture typeclass.

Now it's possible to specify an exact polymorphic context you need:

• TimePoint for a generic instant
• TimePoint + Capture for a generic instant that can be constructed in a instant-preserving way.
• Zoned for a zone-aware value for full control over zoning (no locals here from now on)
• Local specifically for local (or UTC) instant, without zone information and control over it.

Migration from 1.* to 2.0.0-M1:

• Now there's no Zoned instance for LocalDateTime.

  For each place you use it there're two options:

  • It's zoned-only code, that doesn't make any sense in local context. In such case you have a better protection now from accidentally using it in an incorrect context.
  • It's an abstraction over UTC/Zoned contexts. In such case you should be able to replace the context bound from Zoned to TimePoint + Capture.

• Zoned.of[T](...) was removed. Use Capture[T](...)

• Lawless was renamed to TimePoint.

Other potentially breaking changes:

• Explicit implementation of hashCode was added to moment wrapper classes, which can lead to different behaviour in Maps.
• [Laws] Zoned.constructorConsistency law is gone. Proper laws for Capture are work in progress.