Sunday, July 04, 2021

gradle to sbt for scala3

Problem and Audience

Scala 3 is an overhaul of the guts of the scala language type system and compiler that was recently released (in mid 2021) for general use. Unfortunately, the scala plugin for gradle does not yet support the new scala-3 build chain, so we ported littleware to scala's sbt build tool.

Porting a gradle build to an sbt build is straight forward. Both gradle and sbt define a graph of tasks for building projects, and they define the task library (types of tasks) via third-party plugins. Both system's define the task instances in a project's build graph with a user-supplied build file written in a DSL that makes API calls against the runtime. For example, the littleware sub-project in both gradle (build.gradle) and sbt (build.sbt) are similar to each other:


lazy val littleware = project
    name := "littleware",
    crossPaths := false,
    autoScalaLibrary := false,
    libraryDependencies ++= Seq(
      "javax.mail" % "javax.mail-api" % "1.5.5",
      "javax" % "javaee-web-api" % "8.0",
      "org.apache.derby" % "derby" % "",
      "org.apache.derby" % "derbyclient" % "",
      "org.postgresql" % "postgresql" % "42.2.18",
      "mysql" % "mysql-connector-java" % "8.0.23",
      "org.javasimon" % "javasimon-core" % "4.2.0",
      "javax.activation" % "activation" % "1.1.1"
    ) ++ junitRunnerSet ++ log4jJsonSet,


project( ':littleware' ) {
    dependencies {
        implementation ''
        implementation 'junit:junit:4.13.2'
        implementation ''
        compileOnly 'javax.mail:javax.mail-api:1.5.5'
        implementation 'javax:javaee-web-api:8.0'
        compileOnly 'org.apache.derby:derby:'
        compileOnly 'org.apache.derby:derbyclient:'
        compileOnly 'org.postgresql:postgresql:42.2.18'
        compileOnly 'mysql:mysql-connector-java:8.0.23'
        implementation 'org.javasimon:javasimon-core:4.2.0'
        runtimeOnly 'javax.activation:activation:1.1.1'

I am glad that the port to SBT was straight forward, but I'm also annoyed that scala has its own build tool, sbt, rather than simply invest in gradle - which is widely used for building java, Android, and kotlin projects. It would be nice if I could just learn gradle, and use it to build scala (version 3) too. The same could be said for golang and dotnet - which also implement their own build tools rather than use gradle, but gradle depends on the jvm, so it makes more sense for languages in that ecosystem. There is a trade-off between building a custom tool chain that is finely tuned for a particular domain, or using a more generic system that has a large community of users. I expect gradle to support scala 3 in a few months anyway, so we will soon have the best of both worlds.


It was easy to port littleware's gradle build to sbt, since the two systems implement a task-graph design.

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