Gradle Scala REPL

I recently switched a Scala project’s build from SBT to Gradle and was disappointed to find that Gradle has no support for launching the Scala REPL.

My workaround:

Let Gradle compile and output the classpath
Launch the REPL separately

At first this felt like a kludge around a deficiency in Gradle, but now I think this approach may actually make more sense than SBT’s conflation of build tool and REPL. The build tool doesn’t need to be responsible for everything.

Launching the REPL from the scala-compiler jar

Normally you’d launch the REPL using the scala script provided by the standard Scala installation. But I’d prefer to let Gradle download the appropriate version of Scala for the project rather than requiring developers to install it themselves. Gradle can help us with this because the artifact org.scala-lang:scala-compiler in the Maven Central repo contains the Scala REPL.

The main method that launches the REPL belongs to a class with the (rather non-obvious) name scala.tools.nsc.MainGenericRunner. Thus we need to run

java -Dscala.usejavacp=true \
     -classpath "$CLASSPATH" \
     scala.tools.nsc.MainGenericRunner

where $CLASSPATH includes the scala-compiler jar.

Fetching the jar with Gradle

To let Gradle provide the scala-compiler artifact for us, add it as a classpath dependency to buildscript.

buildscript {
    dependencies {
        classpath "org.scala-lang:scala-compiler:${scalaVersion}"
    }
    repositories {
        mavenCentral()
    }
}

Then add a function to look up the filesystem path of this artifact, which we’ll use later when assembling the full classpath for the REPL session.

def scalaPath = buildscript.configurations.classpath.find {
    it.name == "scala-compiler-${scalaVersion}.jar"
}

Generating the classpath

Getting the classpath for your project and its dependencies in Gradle is pretty simple.

def classpath = sourceSets.main.runtimeClasspath.asPath

Combined with the path of the Scala compiler, the result is the full classpath that we’ll use for launching the REPL.

task printClasspath << {
    println "${scalaPath}:${classpath}"
}

With this task defined, call gradle printClasspath --quiet to set the classpath in the startup script.

java -Dscala.usejavacp=true \
     -classpath "$(gradle printClasspath --quiet)" \
     scala.tools.nsc.MainGenericRunner

Initial REPL commands

SBT has a useful setting that lets you specify Scala expressions that run automatically when the REPL starts. This tends to save you the trouble of repeating a lot of imports every time you start a session.

initialCommands in console := "..."

You can accomplish this using the -i option on the Scala REPL, which loads commands from a file. In this example, the file containing initial commands is named repl.scala.

java -Dscala.usejavacp=true \
     -classpath "$(gradle printClasspath --quiet)" \
     scala.tools.nsc.MainGenericRunner \
     -i repl.scala

Full example

build.gradle

project(':repl') {

    def scalaVersion = '2.11.7'

    // Dependencies on any other projects that should be
    // accessible from the REPL context.
    dependencies {
        compile project(':example_project_1')
        compile project(':example_project_2')
    }

    // Require the scala-compiler jar
    buildscript {
        dependencies {
            classpath "org.scala-lang:scala-compiler:${scalaVersion}"
        }
        repositories {
            mavenCentral()
        }
    }

    // The path of the scala-compiler jar
    def scalaPath = buildscript.configurations.classpath.find {
        it.name == "scala-compiler-${scalaVersion}.jar"
    }

    // The classpath of this project and its dependencies
    def classpath = sourceSets.main.runtimeClasspath.asPath

    // Prints the classpath needed to launch the REPL
    task printClasspath << {
        println "${scalaPath}:${classpath}"
    }

}

repl.sh

#!/bin/bash
gradle :repl:compileScala && \
java -Dscala.usejavacp=true \
     -classpath "$(gradle :repl:printClasspath --quiet)" \
     scala.tools.nsc.MainGenericRunner \
     -i repl.scala

repl.scala

myproject.repl.init()
import myproject.repl._