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REPL
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The Scala REPL is a tool (scala) for evaluating expressions in Scala.

The scala command will execute a source script by wrapping it in a template and then compiling and executing the resulting program.

In interactive mode, the REPL reads expressions at the prompt, wraps them in an executable template, and then compiles and executes the result.

Previous results are automatically imported into the scope of the current expression as required.

The REPL also provides some command facilities, described below.

An alternative REPL is available in the Ammonite project, which also provides a richer shell environment.

Features

Useful REPL features include:

  • the REPL's IMain is bound to $intp.
  • the REPL's last exception is bound to lastException.
  • use tab for completion.
  • use //print<tab> to show typed desugarings.
  • use :help for a list of commands.
  • use :load to load a file of REPL input.
  • use :paste to enter a class and object as companions.
  • use :paste -raw to disable code wrapping, to define a package.
  • use :javap to inspect class artifacts.
  • use -Yrepl-outdir to inspect class artifacts with external tools.
  • use :power to enter power mode and import compiler components.
  • use :settings to modify compiler settings; some settings require :replay.
  • use :replay to replay the session with modified settings.

Implementation notes:

  • user code can be wrapped in either an object (so that the code runs during class initialization) or a class (so that the code runs during instance construction). The switch is -Yrepl-class-based.
  • every line of input is compiled separately.
  • dependencies on previous lines are included by automatically generated imports.
  • implicit import of scala.Predef can be controlled by inputting an explicit import.

Example:

scala> import Predef.{any2stringadd => _, _}
import Predef.{any2stringadd=>_, _}

scala> new Object + "a string"
<console>:13: error: value + is not a member of Object
       new Object + "a string"
                  ^

scala> import Predef._
import Predef._

scala> new Object + "a string"
res1: String = java.lang.Object@787a0fd6a string

Power Mode

:power mode imports identifiers from the interpreter's compiler.

That is analogous to importing from the runtime reflective context using import reflect.runtime._, universe._.

Power mode also offers some utility methods as documented in the welcome banner.

Its facilities can be witnessed using :imports or -Xprint:parser.

Contributing to Scala REPL

The REPL source is part of the Scala project. Issues are tracked by the standard mechanism for the project and pull requests are accepted at the github repository.