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@ -62,14 +62,18 @@ type-driven tooling is important for productivity. These needs result in a desig
\item \emph{strict mode}, aimed at professionals, focused on \item \emph{strict mode}, aimed at professionals, focused on
minimizing false negatives (i.e.~type soundness). minimizing false negatives (i.e.~type soundness).
\end{itemize} \end{itemize}
For both communities, type-driven tooling is important, so we %% For both communities, type-driven tooling is important, so we
provide \emph{infallible type inference}, which infers types %% provide \emph{infallible type inference}, which infers types
even for ill-typed or syntactically invalid programs. %% even for ill-typed or syntactically invalid programs.
\section{Progress} \section{Progress}
There are two unexpected changes to the type system: \emph{semantic subtyping}, In the two years since the position paper, we have been making changes
and treating gradual typing as \emph{type error suppression}. to the Luau type system to achieve the goals we set out. Most of the
changes were straightforward, but two were large changes in how we
thought about the design of the type system: replacing the existing
syntactic subtyping algorithm by \emph{semantic subtyping}, and
treating gradual typing as \emph{type error suppression}.
Semantic subtyping Semantic subtyping
interprets types as sets of values, and subtyping as set interprets types as sets of values, and subtyping as set
@ -103,11 +107,8 @@ inference~\cite{PT00:LocalTypeInference}.
Currently, non-strict mode operates in the style of gradual type Currently, non-strict mode operates in the style of gradual type
systems by inferring $\ANY$ as the type for local variables. This does systems by inferring $\ANY$ as the type for local variables. This does
not play well with type-directed tooling, for example $\ANY$ cannot not play well with type-directed tooling, for example $\ANY$ cannot
provide autocomplete suggestions. provide autocomplete suggestions. Local type inference will infer more
precise union types, and hence better type-driven tooling.
We hope that, together with error suppression, these changes will
result in a significant improvement to type inference and type error
reporting in non-strict mode.
\bibliographystyle{ACM-Reference-Format} \bibliography{bibliography} \bibliographystyle{ACM-Reference-Format} \bibliography{bibliography}