Scala跨版本行为澄清：两种kind-projector编译器插件依赖配置方式的差异疑问

Great question—this is one of those sbt syntax quirks that can feel confusing at first, especially with compiler plugins that have strict cross-version requirements like kind-projector. Let’s break this down clearly:

First, let’s recap the core sbt dependency syntax basics:

• %% is sbt’s syntax sugar that automatically appends your project’s binary Scala version (e.g., 2.13) to the artifact ID. So "org.typelevel" %% "kind-projector" % "0.11.3" expands to "org.typelevel" % "kind-projector_2.13" % "0.11.3" by default.
• % is the raw dependency declaration—no automatic Scala version is added; you’d normally specify the full artifact ID (like "kind-projector_2.13") manually.

Now, cross CrossVersion.full changes everything here. For compiler plugins like kind-projector, which are tightly coupled to specific Scala compiler versions (not just binary-compatible families), this flag tells sbt to ignore the default binary version matching and instead fetch the artifact built for your project’s exact full Scala version (e.g., 2.13.4).

Why both %% and % work with cross CrossVersion.full #

Kind-projector publishes its compiler plugin artifacts using full Scala versions (e.g., kind-projector_2.13.4:0.11.3), not binary versions. So regardless of whether you use %% or %, cross CrossVersion.full overrides the default behavior to target these full-version artifacts.

The actual difference between the two lines #

Let’s walk through what each does step-by-step:

1. Using %%:

   addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.11.3" cross CrossVersion.full)
   

   • First, %% adds the binary Scala version: artifact ID becomes kind-projector_2.13.
   • Then, cross CrossVersion.full replaces the binary version suffix with the full Scala version: kind-projector_2.13 → kind-projector_2.13.4.
   • Result: sbt fetches org.typelevel:kind-projector_2.13.4:0.11.3.

2. Using %:

   addCompilerPlugin("org.typelevel" % "kind-projector" % "0.11.3" cross CrossVersion.full)
   

   • % leaves the artifact ID as kind-projector (no automatic version suffix).
   • cross CrossVersion.full directly appends the full Scala version to the artifact ID: kind-projector → kind-projector_2.13.4.
   • Result: sbt fetches the exact same artifact as the %% version: org.typelevel:kind-projector_2.13.4:0.11.3.

Which should you use? #

Both work identically in practice for kind-projector, but the %% syntax is generally preferred:

• It follows sbt’s standard dependency conventions, making your build file more readable to other Scala developers.
• It’s more flexible: if kind-projector ever starts publishing binary-version artifacts (unlikely for compiler plugins, but possible), you could just remove cross CrossVersion.full instead of rewriting the dependency string.

内容的提问来源于stack exchange，提问作者MaatDeamon