In this course we will:
Start from an existing sbt build (Scala 2.13)
Use sbt-scala3-migrate to migrate the build to Scala 3
The objective is to learn the four steps of migrating a project:
Updating the dependencies
Updating the compiler options
Patching the syntax of the code
Fixing the type inference errors
Dotty: New research project started in 2012 by Martin Odersky's lab at EPFL
Dotty announced to become Scala 3 (April 2018)
3.0.0 released on May 13, 2021
3.3.0 relased on May 30th, 2023. First LTS (Long Term Support) release.IDE support
There's the Scala 3 Migration Guide !
Before Scala 2.10: no compatibility between any two versions
Different standard library
Different binary interface
Since Scala 2.10: strict binary compatibility in minor versions
2.13.11 is compatible with 2.13.8
2.13.11 is not compatible with 2.12.19
=> Ecosystem jump between 2.12 and 2.13 Compatibility between Scala 2.13 and Scala 3
Scala 3 uses the Scala 2.13 standard library
Scala 3 produces Scala 2.13 compatible binaries (class files, sjsir, ...)
Scala 3 can read Scala 2.13 signatures (Pickle format)
Scala 2.13 can read Scala 3 signatures (TASTy) using -Ytasty-reader
=> Backward and forward compatibility between Scala 2.13 and Scala 3
lazy val foo = project.in(file("foo"))
.settings(
scalaVersion := "3.0.0",
libraryDependencies +=
("org.bar" %% "bar" % "1.0.0").cross(CrossVersion.for3Use2_13)
)
For 2.13 use 3 (TASTy reader)
lazy val foo = project.in(ile("foo"))
.settings(
scalaVersion := "2.13.6",
scalacOptions += "-Ytasty-reader",
libraryDependencies +=
("org.bar" %% "bar" % "1.0.0").cross(CrossVersion.for2_13Use3)
)
[error] Modules were resolved with conflicting cross-version suffixes in main:
[error] org.typelevel:cats _3, _2.13
Scala 2.13 macros are experimental
Scala 3 macros are based on new foundations.
They are simpler and more stable.
Scala 2.13 and Scala 3 macros are incompatible
The community started migrating the macro libraries to Scala 3.
See Table of macro libraries.
The Scala 3 compiler is completely new
Scala 2.13 plugins cannot be used in Scala 3
Scala 3 plugins are more constrained
Some important plugins were integrated in the compiler, and can be activated with an option:
-Ykind-projector-scalajs-semanticdb
Compiler Options (scalacOptions)
Most options are available
Some are renamed
Some are dropped
Some of the most used Scala 2 options are ported to Scala 3. For example, we ported -Wunused to Scala 3.3.0.
See table of compiler options .
Contextual abstractions (Implicits)
Type checker and type inference
It can assist you during the migration to Scala 3 of your build
$ cmtc install -f -s scalacenter/scala3-migration-course
Downloading studentified course from 'https://github.com/scalacenter/scala3-migration-course/releases/download/0.1.0/scala3-migrate-course-student.zip' to courses directory
Project scalacenter/scala3-migration-course (0.1.0) successfully installed to:
/home/user/Library/Caches/com.lunatech.cmt/Courses/scala3-migration-course
Current course set to '/home/user/Library/Caches/com.lunatech.cmt/Courses/scala3-migration-course'
Exercises in repository:
1. * exercise_000_initial_state
2. exercise_001_install_sbt_scala3_migrate
3. exercise_002_migrate_dependencies
4. exercise_003_migrate_scalac_options
5. exercise_004_migrate_syntax
6. exercise_005_migrate_types
7. exercise_006_update_scala_version
