Making the Transition�to Scala 3

Seth Tisue / Lightbend, Inc.

Twitter: @SethTisue�GitHub: SethTisue

Scala in a nutshell

Scala = object-oriented programming + functional programming

targets: JVM (Java Virtual Machine),� but also JavaScript (Scala.js) and LLVM (Scala Native)

built around a Java-like core;� Java programmers can begin by writing Java-like code

Scala prioritizes type safety

normal Scala code uses immutable values by default

Scala 2 timeline

Scala 3 timeline

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2012 — first commit

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�2020 — Dotty (experimental compiler)� becomes Scala 3

2021 — 3.0 (May), 3.1 (just released!)

2003 — first commit

2007 — Scala 2.6

2008 — Scala 2.7

2010 — Scala 2.8

2012 — Scala 2.9, 2.10

2014 — Scala 2.11

2016 — Scala 2.12

2019 — Scala 2.13

2021 — Scala 2.13.6

Scala @ Twitter

Spark

Akka

Kafka

Reactive Manifesto

The Scala organization

nonprofit / academic

companies

Prof. Martin Odersky

Who am I?

Full-time Scala coder (2008–)

Scala community organizer (2009–2016)

Scalawags podcast (2014–2016)

Scala Improvement Process committee (2014–)

Compiler hacking, open source, community relations

@ Lightbend (2015–)

1. Why upgrade?�
2. When to upgrade?�
3. How to upgrade?

• Why upgrade?�
• When to upgrade?�
• How to upgrade?

Why upgrade?

because Scala 3 is…

• the same�huge emphasis on compatibility�& smooth upgrade paths
• yet better!�tons of features & benefits

Scala 3 is still Scala

The Scala 3 uses the Scala 2.13 standard library AS IS.

Scala 3 is still Scala

Nearly all Scala 2 code is also�valid Scala 3 code.

Scala 3 is still Scala

Library & tooling ecosystem is already large.

Scala 3: benefits

Quality

stronger theoretical foundations� Scala 3 = Dotty = DOT = Dependent Object Types

stronger practical foundations� new compiler = less tech debt

“Fixed in Scala 3”

(the true numbers are�surely much higher)

Benefits: UX

Design goal:

• Deprecate or remove unneeded features

Benefits: UX

Design goal:

• Bake common usage patterns into the language

Features are “new”, yet already familiar

Benefits: Implicits

“Implicits” redesigned

• extension methods are now supported directly
• given (at definition site) and using (at use site) replace implicit
• givens rarely need names anymore

Benefits: Implicits

“Implicits” redesigned

• implicit conversions now safer,�yet discouraged
• “context functions” enable new coding patterns

Benefits: Context functions

Brachthäuser,�Boruch-Gruszecki,�& Odersky��ICFP 2021��https://www.youtube.com/watch?v=UmO-f0qTRSU

Benefits: Syntax

cleaner, indentation-based syntax

Scala 2

Scala 3

Benefits: Enumerations

Enumerations

• Simple enumerations, like Java
• More complex use cases

Benefits: Enumerations

Enumerations — simple

Scala 2

Scala 3

Benefits: Enumerations

Enumerations — advanced

Scala 2

Scala 3

Benefits: Opaque types

“Opaque types” provide zero-overhead abstraction

Benefits: Metaprogramming

new metaprogramming features

Scala 2: experimental macro system� workable, but cumbersome and fragile to create

Scala 3:

• macros needed less often
• new, superior macro system fully supported

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Benefits: Metaprogramming

Benefits: Metaprogramming

Macros are needed less often.

• inline enables zero-cost abstraction
• “Match types” replace macro-based or implicit-based type-level computations
• “Typeclass derivation” is supported directly

Benefits

https://docs.scala-lang.org/scala3/new-in-scala3.html

also, talks yesterday by:�• Dean Wampler�• Bill Venners

• Why upgrade?�
• When to upgrade?�
• How to upgrade?

Advice

Be bold!

Type safety will protect you.

Strong compatibility story will protect you.

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Yet be cautious, too...

Advice

• Start small!
• Start with small projects
• Start with a greenfield project
• Start with internal projects

Advice

Migrate advanced, Scala-friendly teams first

Scala 2 remains the safest choice�• for less experienced teams�• for less enthusiastic teams

Advice on dependencies

• Scala 3 can use 2’s libraries

Caveat #1: Macros

Caveat #2: Diamond dependencies

Advice on dependencies

• Be cautious when using Scala 2 libraries from Scala 3��use CrossVersion.for3Use2_13�in applications, not libraries

App 2

Library C for 3

Library D for 3

Library E for 3

Library E for 2.13

for3Use2_13

App 1

Library A for 3

for3Use2_13

Library B for 2.13

Library D�shouldn’t�do this!

Advice

• Consider all of your tooling

IDE support is already pretty good�(IntelliJ, Metals + VS Code)

Advice

• Consider your tooling

3.0 shipped when:

design decisions were made�quality & tooling good enough to create Scala 3 libraries with

3.0 did not wait for:� tooling to be fully on par with Scala 2

• Why upgrade?�
• When to upgrade?�
• How to upgrade?

Advice

• Get on Scala 2.13 first
• Drop Scala 2.12 if you can
• Clean up warnings
• Enable -Xsource:3

Advice on cross-building

• Enable -Xsource:3 today
• Start cross-building very soon
• Leave 2 behind when ready

Advice

• Read the Scala 3 Migration Guide�https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html

Advice

• Use the Scala 3 Migration Tool�https://docs.scala-lang.org/scala3/guides/migration/tooling-tour.html

Advice

• Use -source:3.0-migration
• Use -rewrite

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Advice

• Choose a strategy for adopting indentation-based syntax

Not at all?�Gradually?� by file, by class, by method�All at once?

most common?

Learning materials: Videos

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https://www.youtube.com/playlist?list=PLTx-VKTe8yLxYQfX_eGHCxaTuWvvG28Ml

“Let’s Talk About Scala 3”�12 videos, 10–25 mins each

Learning materials: Free online courses

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new course:

the classics, updated:

Learning materials: The Scala 3 Book

free on Scala website

author: Alvin Alexander

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https://docs.scala-lang.org/scala3/book/introduction.html

Learning materials: Books

Learning materials: Books

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MEAP = in progress

“Functional Programming in Scala”

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by Runar Bjarnason & Paul Chiusano��new co-author:�Michael Pilquist (of Typelevel, FS2 fame)

Questions?

FAQ

1. How is Scala 3 adoption going?
2. Do you like the new indentation-based syntax?
3. What IDE do you recommend?
4. When will {Akka, Play, Spark, sbt, …} move to Scala 3?