Is it possible to implement the Abort capability using Scala 3.8 features?

philipschwarz · January 25, 2026, 8:14am

Hi all,

A couple of years ago, when I decided to learn the absolute basics of Unison’s support for abilities (that’s how capabilities are called in the language) I wrote this deck (for the purposes of this email, if you are considering taking a look at it, please jump to slide 20, at least on a first reading):

In the deck, having realised the following

I took this Scala code

and translated it to this Unison code

I then modified the Unison code so that it implemented the optionality effect using an ability (algebraic effect) called Abort:

I am writing this email because on the one hand, I can’t wait to have a go at implementing the optionality effect using capabilities in Scala 3, but on the other hand, before I even consider doing that, I thought I’d ask you if Scala 3.8 already provides sufficient capability-related features to allow that, and if not, what are the future milestones that will see Scala 3 reach the point where it does.

Thank you in advance for any help you might be able to provide.

Philip

bishabosha · January 26, 2026, 12:29pm

this sounds like a variant of CanThrow Capabilities with a fixed exception type

bjornregnell · January 26, 2026, 12:55pm

You may also want to check out the implementation of boundary.break:

github.com/scala/scala3

library/src/scala/util/boundary.scala

main

package scala.util

import language.experimental.captureChecking
import scala.annotation.implicitNotFound

/** A boundary that can be exited by `break` calls.
 *  `boundary` and `break` represent a unified and superior alternative for the
 *  `scala.util.control.NonLocalReturns` and `scala.util.control.Breaks` APIs.
 *  The main differences are:
 *
 *    - Unified names: `boundary` to establish a scope, `break` to leave it.
 *      `break` can optionally return a value.
 *    - Integration with exceptions. `break`s are logically non-fatal exceptions.
 *      The `Break` exception class extends `RuntimeException` and is optimized so
 *      that stack trace generation is suppressed.
 *    - Better performance: breaks to enclosing scopes in the same method can
 *      be rewritten to jumps.
 *
 * Example usage:
 *

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philipschwarz · January 26, 2026, 6:28pm

Thank you @bishabosha and @bjornregnell for your suggestions.