The goal of this document is to establish a shared vision for Fluent’s compatibility strategy after the 1.0 release. The vision will serve two primary purposes:
(External) Manage early-adopters’ expectations of Fluent Syntax and tooling.
(Internal) Contextualize feedback and inform design decisions during the 1.x lifecycle and beyond.
Backwards incompatible changes make old files not parse in the current
parsers. Removing a feature which was previously available and reporting
SyntaxError instead is a backwards incompatible change.
Extensions are changes which introduce syntax which was previously forbidden. Extensions make current files with the extended syntax not parse in old parsers. Old files continue to parse correctly in the current parsers.
Deprecations are non-breaking changes meant to encourage the use of more modern syntax and guarantee a transition period for migrating translations to the modern syntax. Deprecations herald future removals of syntax features.
In the 1.x lifetime, there won’t be any breaking changes to Fluent Syntax. The grammar and the AST must maintain backwards compatibility with 1.0.
The dot releases may introduce new syntax in a backwards compatible way. New features are only allowed as extensions to the 1.0 syntax and the AST.
Extensions may be accompanied by deprecations. Deprecated syntax will continue to be supported throughout the 1.x lifetime. Parsers will report deprecations as annotations in the AST.
Authoring tools should provide per-project settings for specifying the lower and upper bounds of accepted syntax versions, depending on the currently-used Fluent implementation in the target project. Syntax newer than the upper bound must not be allowed in the project’s translations; syntax deprecated in versions older than the lower bound should be reported as warnings, and later on, as errors—depending on the grace period chosen for the project.
In order to clean up the accrued deprecations, a 2.0 version may be published in the future, breaking the backward compatibility with 1.x. The 2.0 version will define the compatibility strategy for Fluent 2.x and beyond.
Removing deprecations will achieve the following goals.
Establish a fresh baseline for subsequent Fluent 2.x releases and the future standard.
Improve the learnability of Fluent by reducing the cognitive complexity of the syntax.
Allow implementations to streamline their codebases, which can increase their quality and give opportunities for new performance optimizations.
If Fluent succeeds as a future standard, the compatibility strategy will be redefined by the standards organization publishing it.