The language Lustre was introduced to design and implement real-time control software, modeling it as a continuous function over treams of datas. A set of equations written in Lustre defines a restricted class of Kahn process networks which can be executed synchronously: all computations can be dated according to a global time scale so that when a value is produced, it is immediately consumed. This restriction is obtained by associating to every stream a clock that defines when a value is present or not according to a global time scale. A dedicated type system — the clock calculus — computes a clock for every expression and checks that its actual clock equals its expected clock and thus that intermediate buffers are not needed.
In these course notes, we present a static and dynamic semantics of synchronous Kahn networks. We consider a first-order functional language of streams reminiscent of Lustre and Lucid Synchrone to which we give several denotational semantics. We show that without imposing restrictions, we get two kinds of bad behavior: some networks may deadlock and some cannot execute without unbounded FIFOs. We introduce a clocked semantics and show that the clocking rules correspond to a type system with dependent types. We then extend the language kernel with an explicit buffer operator to model communication through a FIFO. The clock calculus is extended with a subtyping rule that is applied where the buffer is used and whose size is inferred. To reduce the complexity of the resolution, we present an abstraction of clocks.
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