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Migration guide Scala 2 -> Scala 3

NOTE: This guide is deprecated and will not be updated, as all Scala 2 code is migrated to Scala 3 in this repo.

Currently already completed migrations

  • All code in [compendium/examples] has been migrated using -rewrite
  • All code in [compendium/workspace] has been migrated using -rewrite

Migrations TODO

See issues and progress here: https://github.com/lunduniversity/introprog/issues

  • Exercises - see instructions below
  • Slides of lectures and text in compendium
  • Discussion and decision on what new stuff to teach in Scala 3 in the second study period of the course, see general discussion here #509

Installing Scala 3

Unfortunately there are neither .msi files for Windows nor .deb package for Linux, for Scala 3 on the download page. Instead you can do one or more of the following:

  1. Use sbt with at least sbt.version=1.5.5 in project/build.properties and scalaVersion := "3.0.1" in build.sbt and you can launch the REPL with sbt console

  2. Use scala-runners.

  3. Use Coursier according to here and then: cs install scala3-compiler and cs install scala3-repl to get scala3-compiler and scala3-repl on your path

  4. Download the latest Scala 3 zip on github and make the binaries available on your path so you can write scala and scalac in terminal.

Migrating exercises

Exercises are migrated manually (i.e. not using the Scala 3 compilers -rewrite option) as they are snippets that should work in REPL that often do not compile on their own and they are embedded in the latex code.

Exercises are located here [compendium/modules] prefixed with week and suffixed with -exercise.tex

  1. Study the syntactic differences between Scala 2 and Scala 3 here: https://docs.scala-lang.org/scala3/guides/migration/incompat-syntactic.html and the new coding style here: https://docs.scala-lang.org/scala3/guides/migration/tooling-syntax-rewriting.html

  2. Fork this repo and clone it locally, etc.

  3. Make sure you can sbt build with a working local Latex installation (e.g. TexLive, on linux/WSL sudo apt install texlive-full) so that you get pdflatex on your path

  4. Study how to make a contribution to this repo in Chapter 0 here on page 16 [http://cs.lth.se/pgk/compendium]

  5. Locate the issue corresponding to the exercise you are migrating here called something like Migrate exercise w01 expressions

  6. Focus on fixing these most common cases first:

    1. Fix things that does not work anymore, e.g. calling def f() = 42 with just f does not work anymore - you need to match the parenthesis at definition site with the call site.
    2. Use new control syntax: if then, for do, for yield, while do
    3. Make braces optional where sensible. One-liners can for space reasons or convenience still use braces.
    4. Avoid unnecessary braces in lambdas; this is legal in Scala 3: table.map((k,v) => multiple line-breaks here)

  7. Migrate both the task and the facit.

  8. Remember to check everything in the Scala 3 REPL before you commit.

  9. When you make a Pull Request on a fix to an exercises, then mention fix #999 in the title of the PR message where #999 is replaced with the number of the corresponding migration issue. Then the issue will be automatically closed when the PR is merged.

How to see current versions in all subprojects?

In terminal on project top level run:

sh show-scala-versions.sh

How to migrate using the scala3 compiler in terminal?

  • make scala3-compiler available on your path or as an alias
  • open a terminal in this repo's top folder
  • run in terminal: ./rewrite-to-scala3.sh path/to/where/code/is/*.scala
  • check that the rewriting looks ok

How to migrate sub-projects of this repo inside sbt?

Start sbt in terminal on project top level and then:

sbt:introprog root> project w03_irritext
sbt:introprog root> set scalacOptions := Seq("-rewrite","-new-syntax")
sbt:introprog root> compile
sbt:introprog root> set scalacOptions := Seq("-rewrite","-indent")
sbt:introprog root> compile

It is important to rewrite to -new-syntax before rewriting -indent.

The rewritings can be reverted:

sbt:introprog root> set scalacOptions := Seq("-rewrite","-no-indent")
sbt:introprog root> compile
sbt:introprog root> set scalacOptions := Seq("-rewrite","-old-syntax")
sbt:introprog root> compile

It is important to revert to -no-indent before reverting to -old-syntax.