Scala Compilation Speed

Scala has been somewhat notorious for long compile times. This has improved a lot over the years. Compared to other high-level language compilers, it’s probably not too bad (though the usual comparison with Java hurts).

Scala has a working incremental compiler, so when used from sbt (or other build systems), recompilation is not a big issue in most cases.

Potential culprits for slow compile times

In user code (in no particular order):

• Heavy use of implicits
• Type inference (adding explicit types to public members can help a lot)
• Macros (can disturb incremental compilation when macros are defined in an internal dependency and that changes)
• Big Modules (if you avoid circular dependencies between files you can often restructure code into more modules to help with parallelism)
• Mixed Mode projects containing Java and Scala files (at least that’s what I observed in akka-http that sbt cannot infer well enough what to recompile and incremental compilation is slower than a full recompile)

In scalac’s compiler architecture:

• Mostly single-threaded execution (outline compilation might be a remedy in the future)

Other Scala compilers with a focus on speed

• triplequote hydra - a commercial parallel Scala compiler implementation
• twitter rsc - “Reasonable Scala compiler (also known as Rsc) is an experimental Scala compiler focused on compilation speed.”
• gkossakowski’s kentucky mule - “an exploration of an alternative architecture for Scala compiler design (specifically typechecker) with a focus on speed”
• dotty - The next-generation compiler for Scala 3 - a simpler type-system and a new compiler architecture may or may not bring performance improvements (haven’t seen benchmarks)

Tools

• scalac-profiling - “aims at providing the tools to understand and profile your Scala projects”
• scalafix ExplicitResultTypes refactoring - explicit result type will help compilation speed already now but it’s also mandatory for outline compilation (when that is available)
• bloop - “One toolchain, one build server, one developer experience”