TypeTag without scala-reflect. Supports Scala 2 and Scala 3.

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Izumi


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.

Izumi

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. At the moment Scala 3 port does not support Path-Dependent Types, and Structural Refinements. This will be fixed in the future.
  6. 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.

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:

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:

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