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@ -37,12 +37,12 @@
} }
\begin{abstract} \begin{abstract}
Luau is the scripting language used in creating Roblox experiences. Luau is the scripting language that powers user-generated experiences on the
It is a statically-typed language based on the dynamically-typed Lua language, Roblox platform. It is a statically-typed language with type inference based
with type inference. These types are used in the on the dynamically-typed Lua language. These types are used for providing
IDE, for example when providing autocomplete suggestions. In this autocomplete suggestions in Roblox Studio, the IDE for authoring Roblox experiences.
paper, we describe some of the goals of the Luau type system, In this paper, we describe some of the goals of the Luau type system,
focusing on where the goals are different from those of other type systems. focusing on where the goals differ from those of other type systems.
\end{abstract} \end{abstract}
\maketitle \maketitle
@ -50,49 +50,49 @@
\section{Introduction} \section{Introduction}
The Roblox~\cite{Roblox} platform allows anyone to create shared, The Roblox~\cite{Roblox} platform allows anyone to create shared,
immersive, 3D experiences. At the time of writing, there are immersive, 3D experiences. As of July 2021, there are
approximately 20~million experiences available on Roblox, created approximately 20~million experiences available on Roblox, created
by 8~million developers. Roblox creators are often young, for by 8~million developers. Roblox creators are often young. For
example there are over 200~Roblox kids' coding camps in 65~countries example, there are over 200~Roblox kids' coding camps in 65~countries
listed at~\cite{AllEducators}. listed by the company as education resources~\cite{AllEducators}.
The Luau programming language~\cite{Luau} is the scripting language The Luau programming language~\cite{Luau} is the scripting language
used by developers of Roblox experiences. Luau is derived from the Lua used by creators of Roblox experiences. Luau is derived from the Lua
programming language~\cite{Lua}, with additional capabilities, programming language~\cite{Lua}, with additional capabilities,
including a type inference engine. including a type inference engine.
This paper will discuss some of the goals of the Luau type system, This paper will discuss some of the goals of the Luau type system,
focusing on where the goals are different from those of other type systems. focusing on where the goals differ from those of other type systems.
\section{Human Aspects} \section{Human Aspects}
\subsection{Heterogeneous developer community} \subsection{Heterogeneous developer community}
Quoting a Roblox 2020 report \cite{RobloxDevelopers}: Quoting a Roblox 2020 report \cite{RobloxDevelopers}:
\begin{itemize} \begin{itemize}
\item Adopt Me! now has over 10 billion plays and surpassed 1.6 million concurrent users in game earlier this year. \item Adopt Me! now has over 10 billion plays and surpassed 1.6 million concurrent users earlier this year.
\item Piggy, launched in January 2020, has close to 5 billion visits in just over six months. \item Piggy, launched in January 2020, has close to 5 billion visits in just over six months.
\item There are now 345,000 developers on the platform who are monetizing their games. \item There are now 345,000 developers on the platform who are monetizing their games.
\end{itemize} \end{itemize}
This demonstrates how heterogeneous the Roblox developer community is: This demonstrates the heterogeneity of the Roblox developer community:
developers of experiences with billions of plays are on the same developers of experiences with billions of plays are on the same
platform as children first learning to code. Moreover, \emph{both of platform as children first learning to code. Moreover, \emph{both of
these groups are important}, as the professional development studios these groups are important}. The professional development studios
bring high-quality experiences to the platform, and the beginning creators bring high-quality experiences to the platform, and the beginning creators
contribute to the energetic creative community, and will form the next generation of developers. contribute to the energetic creative community, forming the next generation of developers.
\subsection{Goal-driven learning} \subsection{Goal-driven learning}
All developers are goal-driven, but this is especially true for All developers are goal-driven, but this is especially true for
learners. A learner will download Roblox Studio (the IDE) with an learners. A learner will download Roblox Studio with an
experience in mind, often designing an obstacle course (an ``obby'') experience in mind, such as designing an obstacle course (an ``obby'')
to play in with their friends. to play in with their friends.
The user experience of developing a Roblox experience is primarily a The user experience of developing a Roblox experience is primarily a
3D interactive one, seen in Fig.~\ref{fig:studio}(a). The user designs 3D interactive one, seen in Fig.~\ref{fig:studio}(a). The user designs
and deploys 3D assets such as terrain, parts and joints, and provides and deploys 3D assets such as terrain, parts and joints, providing
them with physics attributes such as mass and orientation. The user them with physics attributes such as mass and orientation. The user
can interact with the experience in Studio, and deploy it to a Roblox can interact with the experience in Studio, and deploy it to a Roblox
server so anyone with the Roblox app can play it. Physics, rendering server so anyone with the Roblox app can play it. Physics, rendering
and multiplayer are all immediately accessible to all creators. and multiplayer are all immediately accessible to creators.
\begin{figure} \begin{figure}
\includegraphics[width=0.48\textwidth]{studio-mow.png} \includegraphics[width=0.48\textwidth]{studio-mow.png}
@ -101,8 +101,8 @@ and multiplayer are all immediately accessible to all creators.
\label{fig:studio} \label{fig:studio}
\end{figure} \end{figure}
At some point during experience design, the user of Studio has a need At some point during experience design, the experience creator has a need
which can't be met by the physics engine alone. ``The stairs should which can't be met by the physics engine alone, such as ``The stairs should
light up when a player walks on them'' or ``a firework is set off light up when a player walks on them'' or ``a firework is set off
every few seconds.'' At this point they will discover the script every few seconds.'' At this point they will discover the script
editor, seen in Fig.~\ref{fig:studio}(b). editor, seen in Fig.~\ref{fig:studio}(b).
@ -112,7 +112,7 @@ programming, in that by the time the user first opens the script
editor, they have already built much of their creation, and have a editor, they have already built much of their creation, and have a
very specific concrete aim. It suggests a Luau goal for helping the very specific concrete aim. It suggests a Luau goal for helping the
majority of creators: \emph{support learning how to perform specific majority of creators: \emph{support learning how to perform specific
tasks} (for example through autocomplete suggestions). tasks}.
\subsection{Type-driven development} \subsection{Type-driven development}
@ -122,11 +122,11 @@ goals may be more abstract, such as ``decrease user churn'' or
\begin{itemize} \begin{itemize}
\item \emph{Code planning}: \item \emph{Code planning}:
code spends much of its development time in an incomplete state, code spends much of its time in an incomplete state, with holes
with holes that will be filled in later. that will be filled in later.
\item \emph{Code refactoring}: \item \emph{Code refactoring}:
experiences evolve over time, and it easy for changes to code evolves over time, and it is easy for changes to
break previously-held invariants. break previously-held invariants.
\item \emph{Defect detection}: \item \emph{Defect detection}:
@ -139,12 +139,12 @@ resulting in an array of techniques for establishing safety results,
surveyed for example in~\cite{TAPL}. Supporting code planning and surveyed for example in~\cite{TAPL}. Supporting code planning and
refactoring are some of the goals of \emph{type-driven refactoring are some of the goals of \emph{type-driven
development}~\cite{TDDIdris} under the slogan ``type, define, development}~\cite{TDDIdris} under the slogan ``type, define,
refine''. For example. a common use of type-driven development is to refine''. For example, a common use of type-driven development is renaming a
rename a property, which is achieved by changing the name in one place, property, which is achieved by changing the name in one place,
and then fixing the resulting type errors---once the type system stops and then fixing the resulting type errors---once the type system stops
reporting errors, the refactoring is complete. reporting errors, the refactoring is complete.
To help support the transition from novice to experienced developer, To \emph{help support the transition from novice to experienced developer},
types are introduced gradually, through API documentation and type discovery. types are introduced gradually, through API documentation and type discovery.
Type inference provides many of the benefits of type-driven development Type inference provides many of the benefits of type-driven development
even to creators who are not explicitly providing types. even to creators who are not explicitly providing types.
@ -152,14 +152,13 @@ even to creators who are not explicitly providing types.
\section{Types} \section{Types}
\subsection{Infallible types} \subsection{Infallible types}
Goal: \emph{support type-driven tools for all programs}. Programs spend much of their time under development in an ill-typed or incomplete state, even if the
final artifact is well-typed. Tools should support this by \emph{providing type information even for ill-typed programs}.
Programs spend much of their time under development in an incomplete state, even if the final artifact
is well-typed. Tools should support this by providing type information even for ill-typed programs.
An analogy is infallible parsers, which perform error recovery and provide an AST for all input texts. An analogy is infallible parsers, which perform error recovery and provide an AST for all input texts.
Program analysis can still flag type errors, for example with red Program analysis can still flag type errors, which may be presented
squiggly underlining. Formalizing this, rather than a judgment to the user with red squiggly underlining. Formalizing this, rather
than a judgment
$\Gamma\vdash M:T$, for an input term $M$, there is a judgment $\Gamma\vdash M:T$, for an input term $M$, there is a judgment
$\Gamma \vdash M \Rightarrow N : T$ where $N$ is an output term $\Gamma \vdash M \Rightarrow N : T$ where $N$ is an output term
where some subterms are \emph{flagged} as having type errors, written $\squnder{N}$. Write $\erase(N)$ where some subterms are \emph{flagged} as having type errors, written $\squnder{N}$. Write $\erase(N)$
@ -202,7 +201,7 @@ Some issues raised by infallible types:
\item Which heuristics should be used to provide types for flagged programs? For example, could one \item Which heuristics should be used to provide types for flagged programs? For example, could one
use minimal edit distance to correct for spelling mistakes in field names? use minimal edit distance to correct for spelling mistakes in field names?
\item How can we avoid cascading type errors, where a developer is \item How can we avoid cascading type errors, where a developer is
faced with type errors that are artifacts of the heuristics rather faced with type errors that are artifacts of the heuristics, rather
than genuine errors? than genuine errors?
\item How can the goals of an infallible type system be formalized? \item How can the goals of an infallible type system be formalized?
\end{itemize} \end{itemize}
@ -221,8 +220,8 @@ for programs with type errors.
Goal: \emph{no false negatives.} Goal: \emph{no false negatives.}
For developers who are interested in defect detection, Luau provides a \emph{strict mode}, For developers who are interested in defect detection, Luau provides a \emph{strict mode},
which acts much like a traditional, sound, type system. This has the goal of ``no false negatives'' that is any which acts much like a traditional, sound, type system. This has the goal of ``no false negatives''
run-time error is flagged. This is formalized using: where any possible run-time error is flagged. This is formalized using:
\begin{itemize} \begin{itemize}
\item \emph{Operational semantics}: a reduction judgment $M \rightarrow N$ on terms. \item \emph{Operational semantics}: a reduction judgment $M \rightarrow N$ on terms.
\item \emph{Values}: a subset of terms representing a successfully completed evaluation. \item \emph{Values}: a subset of terms representing a successfully completed evaluation.
@ -234,7 +233,7 @@ straightforwardly be adapted. We extend the operational semantics to flagged ter
where $M \rightarrow M'$ implies $\squnder{M} \rightarrow \squnder{M'}$, and where $M \rightarrow M'$ implies $\squnder{M} \rightarrow \squnder{M'}$, and
for any value $V$ we have $\squnder{V} \rightarrow V$, then show: for any value $V$ we have $\squnder{V} \rightarrow V$, then show:
\begin{itemize} \begin{itemize}
\item \emph{Progress}: if ${} \vdash M \Rightarrow N : T$, then either $N \rightarrow N'$ or $N$ is a value or $N$ has a flagged subterm. \item \emph{Progress}: if ${} \vdash M \Rightarrow N : T$, then $N \rightarrow N'$ or $N$ is a value or $N$ has a flagged subterm.
\item \emph{Preservation}: if ${} \vdash M \Rightarrow N : T$ and $N \rightarrow N'$ then $M \rightarrow^*M'$ and ${} \vdash M' \Rightarrow N' : T$. \item \emph{Preservation}: if ${} \vdash M \Rightarrow N : T$ and $N \rightarrow N'$ then $M \rightarrow^*M'$ and ${} \vdash M' \Rightarrow N' : T$.
\end{itemize} \end{itemize}
Some issues raised by infallible types: Some issues raised by infallible types:
@ -306,8 +305,7 @@ Goal: \emph{support mixed strict/nonstrict development}.
Like every active software community, Roblox developers share code Like every active software community, Roblox developers share code
with one another constantly. First- and third-party developers alike with one another constantly. First- and third-party developers alike
frequently share entire software packages written in Luau. To add to frequently share entire software packages written in Luau. To add to
this, many Roblox games are authored not by just one developer, but a this, many Roblox experiences are authored by a team.
team.
It is therefore crucial that we offer first-class support for mixing It is therefore crucial that we offer first-class support for mixing
code written in strict and nonstrict modes. code written in strict and nonstrict modes.
@ -332,7 +330,7 @@ Some issues raised by mixed-mode types:
In this paper, we have presented some of the goals of the Luau type In this paper, we have presented some of the goals of the Luau type
system, and how they map to the needs of the Roblox creator system, and how they map to the needs of the Roblox creator
community. We have sketched what a solution might look like, all that community. We have sketched what a solution might look like; all that
remains is to draw the owl~\cite{HowToDrawAnOwl}. remains is to draw the owl~\cite{HowToDrawAnOwl}.
\bibliographystyle{ACM-Reference-Format} \bibliography{bibliography} \bibliographystyle{ACM-Reference-Format} \bibliography{bibliography}