propensive / quarantine   0.4.0

Apache License 2.0 GitHub

Safe and total handling of domain-specific exceptions

Scala versions: 2.12

GitHub Workflow


Quarantine is a small library to make it easy to handle exceptions safely and exhaustively, in particular when combining modules which may throw different exception types.



Getting Started

The problem

Methods in Java may return a value, or throw an exception. It is possible to write very concise and readable code using Java exceptions, but only if you neglect to handle those exceptions; the try/catch handling code is verbose and ugly.

A type such as scala.util.Try[T] may capture a successful result, or an Exception. But it does not distinguish between different subtypes of Exception, so it can be difficult to know what exception types may be contained within a Try, and thus need handling.

Either[L, R] can be used to capture successful R values or exceptions of type L. If L is a sealed type, the compiler can use its exhaustivity-checking algorithm to warn if we have forgotten to match any subtypes of L. But it is verbose to have to write the full type-signature (including the L parameter) every time.

Quarantine offers, domain.Result[T], a parameterized, path-dependent type which captures succesful instances of type T, and exceptions of a type, which would ideally be sealed, determined by a domain. It would be typical to import the domain,

import domain._

so that the Result[T] type can be used without a prefix. So although the type prefix is an essential part of the type, from which its exception type will be inferred, imports can elide it from the syntax within a particular scope, while it would be common for a consistent prefix of Result[T] to be used within one such scope.

The types domain.Result[T] and domain2.Result[T] are distinct type, so using an instance of one where the other is expected results in a type error. This makes it impossible for a result containing an exception type from one domain to be accidentally encapsulated in another result type whose consumers are not expecting to handle it. Instead, it must be converted, manually or semi-automatically.


Exceptions can be thrown at any time on the JVM, and unless they are caught, they will bubble up through the call stack. Apart from exceptions representing error states which are best resolved by terminating the JVM, we usually want to capture any exceptions we were not expecting and store them in the result type.

Transforming Results

Result[T] provides a number of methods for transforming a result type.


Quarantine is classified as fledgling. Propensive defines the following five stability levels for open-source projects:

  • embryonic: for experimental or demonstrative purposes only, without guarantee of longevity
  • fledgling: of proven utility, seeking contributions, but liable to significant redesigns
  • maturescent: major design decisions broady settled, seeking probatory adoption and refinement of designs
  • dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0 or later
  • adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated


Quarantine’s source is available on GitHub, and may be built with Fury by cloning the layer propensive/quarantine.

fury layer clone -i propensive/quarantine

or imported into an existing layer with,

fury layer import -i propensive/quarantine

A binary is available on Maven Central as com.propensive:quarantine-core_<scala-version>:0.6.0. This may be added to an sbt build with:

libraryDependencies += "com.propensive" %% "quarantine-core" % "0.6.0"


Contributors to Quarantine are welcome and encouraged. New contributors may like to look for issues marked label: good first issue.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Quarantine easier.

Please do not contact project maintainers privately with questions, as other users cannot then benefit from the answers.


Quarantine was designed and developed by Jon Pretty, and commercial support and training is available from Propensive OÜ.


Quarantine is copyright © 2019-20 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.