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Version: ZIO 2.x

IntelliJ Support

It is highly recommended that you use Metals instead of IntelliJ for ZIO-Direct development since Metals correctly infers types for defer blocks (and other whitebox macros). This is especially true in Scala 3 where IntelliJ does not properly support union-types upon which the defer mechanism relies for ZIO error type composition.

It is well known that IntelliJ does not support type inference with whitebox macros because it does not read types from the Scala compiler. Since defer is a whitebox macro, IntelliJ will not be able to infer the type of the defer block.

IntelliJ Not Works

To circumvent this limitation, IntelliJ provides a mechanism to load custom library-specific plugins called Library-Extensions that can provide type information to the IDE (info here). Library-Extension plugins are automatically loaded by IntelliJ when a library like zio-direct "asks" for one by providing a configuration file in the library's jar.

ZIO-Direct provides a Library-Extension plugin that is loaded by IntelliJ to provide type information.


  1. When adding a library dependency on zio-direct in the build.sbt file:

    // Build.sbt
    libraryDependencies += "dev.zio" %% "zio-direct" % "..."
  2. Click the reload button in the SBT panel of the IntelliJ project.

    IntelliJ SBT Reload

  3. Once the SBT configuration is reloaded, the following message will appear in the bottom right corner of the IntelliJ project window. Click "Yes".

    IntelliJ Import Plugin

  4. IntelliJ will automatically download the zio-direct library and the Library-Extension plugin. The typing of defer blocks should then work as expected.

    IntelliJ Works


If the "Extensions Available" dialog does not appear or a manual reload of the zio-direct-intellij plugin is required. This typically involves manually clearing the imported libraries and then forcing a re-import from SBT. This is necessary so that IntelliJ runs the correct triggers to load the zio-direct-intellij plugin.

  1. Go to the Project Structure -> Libraries dialog. Select all the libraries (Cmd+A) and click the - button.

    IntelliJ Clear Libraries

  2. Once this is complete and all the libraries are removed. Go back to the SBT panel and click the reload button.

    IntelliJ SBT Reload

  3. Once the SBT project structure is reloaded, the "Extensions Available" dialog should appear.

    IntelliJ Import Plugin


Type Incongruence

Since the zio-direct-intellij plugin is still experimental, the types that it infers may be incorrect. Use the function to check the Scala-compiler inferred type of the defer block.

IntelliJ Info

Scala 3 Union-Type Limitations

Additionally, since IntelliJ does not properly support union-types, it is impossible to correctly infer the error type of a defer that composes multiple ZIO effects with errors in Scala 3. Instead, the zio-direct-intellij plugin will use a least-upper bound instead of a union-type.

IntelliJ Lub VS Console

While in practice, this is not a problem since the least-upper bound is always a supertype of the union-type, it is still a limitation of the zio-direct-intellij plugin. If any unexpected issues occur, you can force zio-direct to infer least-upper-bound types by using defer(Use.withAbstractError) { ... } however as this setting is not fully supported (e.g. it does not correctly infer error-types that contain type-parameters) it must be used with caution.