izumi-reflect

@quote: Looks a bit similar to TypeTag

izumi-reflect is a fast, lightweight, portable and efficient alternative for TypeTag from scala-reflect.

izumi-reflect is a lightweight model of Scala type system and provides a simulator of the important parts of the Scala typechecker.

Why izumi-reflect

1. izumi-reflect compiles faster, runs a lot faster than scala-reflect and is fully immutable and thread-safe,
2. izumi-reflect supports Scala 2.11, 2.12, 2.13 and Scala 3,
3. izumi-reflect supports Scala.js and Scala Native,
4. izumi-reflect works well with GraalVM Native Image,
5. izumi-reflect allows you to obtain tags for unapplied type constructors (F[_]) and combine them at runtime.

Credits

izumi-reflect has been created by Septimal Mind to power Izumi Project, as a replacement for TypeTag in reaction to a lack of confirmed information about the future of scala-reflect/TypeTag in Scala 3 (Motivation), and donated to ZIO.

Limitations

izumi-reflect model of the Scala type system is not 100% precise, but "good enough" for the vast majority of the usecases.

Known limitations are:

1. Recursive type bounds (F-bounded types) are not preserved and may produce false positives,
2. Existential types, both written with wildcards and forSome may produce unexpected results, the support is limited,
3. Path-Dependent Types are based on variable names and may cause unexpected results when variables with different names have the same type or vice-versa (vs. Scala compiler)
4. This-Types such as X.this.type are ignored and identical to X
5. izumi-reflect is less powerful than scala-reflect: it does not preserve fields and methods when it's not necessary for equality and subtype checks, it does not preserve code trees, internal compiler data structures, etc.
6. There are some optimizations in place which reduce correctness, namely: subtype check for scala.Matchable will always return true, no distinction is made between scala.Any and scala.AnyRef.
7. Lower bounds are not preserved in abstract higher-kinded type members which may produce false comparisons.

Debugging

Set -Dizumi.reflect.debug.macro.rtti=true to enable debug output during compilation when tags are constructed and at runtime when they are compared.

sbt -Dizumi.reflect.debug.macro.rtti=true

To see debug output when compiling in Intellij, add the above flag to VM options in Preferences -> Build, Execution, Deployment -> Compiler -> Scala Compiler -> Scala Compile Server

You may also set it in .jvmopts file during development. (.jvmopts properties will not apply to Intellij compile server, only to sbt)

Set -Dizumi.reflect.debug.macro.rtti.assertions=true to enable additional assertions.

Other useful system properties are:

• izumi.reflect.rtti.optimized.equals
• izumi.reflect.rtti.cache.compile

Build

build.sbt is generated by sbtgen. During development you may not want to mess with ScalaJS and ScalaNative, you may generate a pure-JVM Scala project:

./sbtgen.sc

Once you finished tinkering with the code you may want to generate full project and test it for all the platforms:

./sbtgen.sc --js --native
sbt +test

To develop using Scala 2 invoke sbtgen with a scala version argument:

./sbtgen.sc 2 // 2.13
./sbtgen.sc 2.12 // 2.12

Likewise with Scala 3:

./sbtgen.sc 3

In Intellij, you may also set Scala version by changing the option sbt -> sbt settings -> Open cross-compiled projects Scala 3 / Scala 2 projects as:

Talks

• Kit Langton — Scala 3 Macro Fun (Open Source Hackery)
• Pavel Shirshov — Izumi Reflect: Scala Type System Model

See also

gzoller/scala-reflection

• Scala 3 only
• No support for subtype checks
• Requires compiler plugin
• Type lambdas are not supported
• Preserves field information

airframe-surface

• Scala 2 and Scala 3
• No support for subtype checks
• Preserves field information

And even more

1. https://github.com/gaeljw/typetrees - a very basic type tag substitute for Scala 3
2. https://stackoverflow.com/questions/75752812/is-there-a-simple-scala-3-example-of-how-to-use-quoted-type-as-replacement-for - discussion on StackOverflow
3. https://contributors.scala-lang.org/t/scala-3-and-reflection/3627 - original discussion on Scala Contributors forum