How to Use the Capture Checker

Enabling Capture Checking

Use Scala 3 nightly for the latest features and fixes.

Add this import in any file that uses capture checking:

import language.experimental.captureChecking

Separation Checking

Requires the import above:

import language.experimental.captureChecking
import language.experimental.separationChecking

SBT Project Template

Alternatively, you can clone a pre-defined SBT project to get started: https://github.com/lampepfl/scala3-cc-template

The REPL/Scala-CLI

Using the command line through explicit parameters:

scala -S 3.nightly -language:experimental.captureChecking

or when reading from a file:

// foo.scala:
//> using scala 3.nightly
import language.experimental.captureChecking
...

Then, it suffices to run

scala foo.scala

API Documentation

Scaladoc supports capture checking. The nightly standard library API docs have it enabled by default: https://nightly.scala-lang.org/api/

Compilation Options

The following options are relevant for capture checking.

• -Vprint:cc Prints the program with capturing types as inferred by capture checking.
• -Ycc-verbose Prints capabilities and capturing types in more detail.
• -Ycc-debug Gives more detailed, implementation-oriented information about capture checking, as described in the next section.