  Faceted Execution is a linguistic paradigm for dynamic information-flow
control. Under faceted execution, secure program data is represented by faceted
values: decision trees that encode how the data should appear to its owner
(represented by a label) versus everyone else. When labels are allowed to be
first-class (i.e., predicates that decide at runtime which data to reveal),
faceted execution enables policy-agnostic programming: a programming style that
allows privacy policies for data to be enforced independently of code that
computes on that data.
  To date, implementations of faceted execution are relatively heavyweight:
requiring either changing the language runtime or the application code (e.g.,
by using monads). Following Racket's languages-as-libraries approach, we
present Racets: an implementation of faceted execution as a library of macros.
Given Racket's highly-expressive macro system, our implementation follows
relatively directly from the semantics of faceted execution. To demonstrate how
Racets can be used for policy-agnostic programming, we use it to build a
web-based game of Battleship. Our implementation sheds light on several
interesting issues in interacting with code written without faceted execution.
Our Racets implementation is open source, under development, and available
online.
