  Traditional session types prescribe bidirectional communication protocols for
concurrent computations, where well-typed programs are guaranteed to adhere to
the protocols. However, simple session types cannot capture properties beyond
the basic type of the exchanged messages. In response, recent work has extended
session types with refinements from linear arithmetic, capturing intrinsic
attributes of processes and data. These refinements then play a central role in
describing sequential and parallel complexity bounds on session-typed programs.
The Rast language provides an open-source implementation of session-typed
concurrent programs extended with arithmetic refinements as well as ergometric
and temporal types to capture work and span of program execution. To further
support generic programming, Rast also enhances arithmetically refined session
types with recently developed nested parametric polymorphism. Type checking
relies on Cooper's algorithm for quantifier elimination in Presburger
arithmetic with a few significant optimizations, and a heuristic extension to
nonlinear constraints. Rast furthermore includes a reconstruction engine so
that most program constructs pertaining the layers of refinements and resources
are inserted automatically. We provide a variety of examples to demonstrate the
expressivity of the language.
