Scala 3 Migration Guide

Scala 3 Migration Mode


The Scala 3 compiler provides some helpful utilities to ease the migration.

Try running scalac to have a glimpse of those utilities:

scalac is the executable of the Scala compiler, it can be downloaded from Github.

It can also be installed using Coursier with cs install scala3-compiler, in which case scalac is aliased scala3-compiler.

$ scalac
Usage: scalac <options> <source files>
where possible standard options include:

-explain           Explain errors in more detail.
-rewrite           When used in conjunction with a `...-migration` source version, rewrites sources to migrate to new version.
-source            source version
                   Default: 3.0.
                   Choices: 3.0, future, 3.0-migration, future-migration.

Migration mode

The -source:3.0-migration option makes the compiler forgiving on most of the dropped features, printing warnings in place of errors. Each warning is a strong indication that the compiler is even capable of safely rewriting the deprecated pieces of code into their cross-compiling counterparts.

We call this the Scala 3 migration compilation.

Automatic rewrites

Once your code compiles in the migration mode, almost all warnings can be resolved automatically by the compiler itself. To do so you just need to compile again, this time with the -source:3.0-migration and the -rewrite options.

Beware that the compiler will modify the code! It is intended to be safe. However you may want to commit the initial state so that you can print the diff applied by the compiler and revert it if necessary.

Good to know

  • The rewrites are not applied if the code compiles in error.
  • You cannot choose which rules are applied, the compiler runs all of them.

You can refer to the Incompatibility Table to see the list of Scala 3 migration rewrites.

Error explanations

The -source:3.0-migration mode handles many of the changed features but not all of them. The compiler can give you more details about the remaining errors when invoked with the -explain and/or the -explain-types options.

The -explain and -explain-types options are not limited to the migration. They can, in general, assist you to learn and code in Scala 3.

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