Stability index

Each API in Folktale is marked with a particular “stability index”. They describe what you can expect from that API in the future releases of the library, so you can decide whether it’s safe for you to use it or not in a particular project.

The stability index is based on Node.js’s stability index.



The feature is problematic in some way, and will either be entirely removed from the system, or completely redesigned.

Features that are deprecated will not be shipped in the next major version of Folktale, however it’ll still be available in any minor or feature release. For example, Maybe#get was deprecated in Folktale 2.0, which means that it’ll be available until the Folktale 3.0 release.

When a feature is deprecated, warnings will be displayed in the standard output whenever something uses it. These warnings will also contain the location where these features are used. Note that, since the warnings use the current stack trace, this might not be entirely accurate if running in an engine with PTC enabled.

Deprecated features will list the reason for deprecating them in this documentation, along with any new APIs that have been designed to replace them. Code using deprecated APIs should migrate to new APIs as soon as possible.


The feature has not been tested enough to be considered stable yet, and is included in a particular release so people can test and see where its weak points are.

Note that experimental features are subject to change at any point in future releases. This means that if a feature is released as experimental in 2.0, it could be changed, or even removed, in a 2.1 release.

Experimental features should not be relied upon.


The feature is stable, and its API is unlikely to change, unless deemed necessary for security or other important reasons. Backwards compatibility will always be maintained in the next major release.

A feature becomes stable once it has seen enough tests or proof that the API is a good solution for the particular problem it tries to solve.


The feature will not change in any future release, although security patches and bug fixes may still be applied.

Even if a locked feature is deprecated, it will not be removed from Folktale, in any future release.