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REPL

The repl command starts a Scala REPL, which lets you interactively run your code and inspect its results:

scala-cli repl
scala> println("Hello Scala")
Hello Scala

scala> :exit

Scala CLI uses the Scala REPL by default, except for pure-Java projects where it defaults to JShell.

JShell backend (experimental)​

You can force JShell as the REPL backend with --jshell (--jsh), including in mixed Scala/Java or pure Scala projects.

scala-cli repl --jshell
|  Welcome to JShell ...
jshell>

The repl command accepts the same arguments as the compile command. It first compiles any provided sources, and then exposes those results and any provided dependencies to the REPL session:

package mylibrary

object Messages {
  def message = "Hello"
  def print(): Unit = println(message)
}
scala-cli repl mylibrary/Messages.scala
Compiling project (Scala 3.0.2, JVM)
Compiled project (Scala 3.0.2, JVM)
scala> import mylibrary._

scala> Messages.print()
Hello

scala> :quit

Watch mode​

Use --watch to recompile your inputs and restart the REPL session when sources change:

scala-cli repl --watch Main.scala

--watching lets you include additional files or directories:

scala-cli repl --watch --watching ./data Main.scala

You can also configure extra watched paths in sources:

//> using watching ./data

Passing REPL options​

It is also possible to manually pass REPL-specific options. It can be done in a couple ways:

  • after the -- separator, as the REPL itself is the launched app, so its options are app arguments
scala repl -S 3.6.4-RC1 -- --repl-init-script 'println("Hello")'
Hello
Welcome to Scala 3.6.4-RC1 (23.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                 
scala>
  • with the -O, effectively passing them as compiler options:
scala repl -S 3.6.4-RC1 -O --repl-init-script -O 'println("Hello")'
Hello
Welcome to Scala 3.6.4-RC1 (23.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                 
scala>
  • via a using directive, treating them as Scala compiler options:
//> using toolkit default
//> using options --repl-init-script "import os.*; println(pwd)"
scala repl repl-options.scala
/current/directory/path
Welcome to Scala 3.7.4 (17, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                         
scala> pwd
val res0: os.Path = /current/directory/path
scala> :quit
  • directly, as a Scala CLI option (do note that newly added options from an RC version or a snapshot may not be supported this way just yet):
scala repl --repl-init-script 'println("Hello")'
Hello
Welcome to Scala 3.6.4-RC1 (23.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                         
scala> pwd
val res0: os.Path = /current/directory/path
scala> :quit

Using Toolkit in REPL​

It is also possible to start the scala-cli REPL with toolkit enabled

scala-cli repl --toolkit default
Welcome to Scala 3.7.4 (17, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                
scala> import os.*
                                                                                
scala> os.pwd
val res0: os.Path = /current/directory/path
scala> :quit

Since we started the repl with toolkit enabled, we can use the libraries included in the toolkit directly. In the above example, the os-lib library from the toolkit is used to print the current path.

Running snippets directly via the REPL​

It is possible to run code snippets via the REPL, with all the internal quirks of the REPL, rather than using standard runner execution. While the difference is subtle and should generally be invisible to users, it is useful for investigating REPL behavior. The key to doing that is to use the both --repl-init-script and --repl-quit-after-init options together.

scala repl --repl-init-script 'println(42)' --repl-quit-after-init # runs in the REPL and quits immediately
42
scala repl -e '//> using options --repl-init-script "println(42)" --repl-quit-after-init' # same, but via using directive
42
scala -e 'println(42)' # standard runner equivalent
42

Inject code as JAR file in class path​

If your application inspects its class path, and requires only JAR files in it, use --as-jar to put the Scala CLI project in the class path as a JAR file rather than as a directory:

scala-cli repl Foo.scala --as-jar