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Ion is a (heavily experimental) Haskell EDSL for concurrent, realtime, embedded programming. It performs compile-time scheduling, and produces scheduling code with constant memory usage and deterministic execution (i.e. no possibility for divergence).
It interfaces with another, more powerful EDSL, Ivory, to perform code generation. Ivory is responsible for all the code generation to perform the scheduling. One may also embed general Ivory effects in an Ion spec with few restrictions, however, it does very little to enforce constant memory usage or deterministic code here.
Ion generates scheduling code which must be called at regular clock ticks (i.e. from a timer interrupt). The interval of these clock ticks establishes the base rate of the system. All scheduled events in the system take place relative to this base rate, defined in terms of 'period' (interval of repetition) and 'phase' (position within that interval).
This functionality is expressed in the 'Ion' monad - in large part to allow composition and modularity in expressing tightly-scheduled functionality. In addition, it has functions like 'newProc' and 'newArea' which define uniquely-named C functions and globals. The purpose of these is to allow that same composition when working with Ivory definitions that are parametrized and may be instantiated multiple times.
For instance, when dealing with functions that return via asynchronous callbacks or interrupts - a common thing on embedded systems - one must generally work in continuation-passing style. This simplifies the process of creating a reusable pattern for a use-case like:
- Transmit instruction
Iover SPI. Wait to receive 2 bytes.
- In a callback: Check that result for being an error condition. If
an error, call error handler function
E. If successful, transmit instruction
I2and wait to receive 2 bytes.
- In a callback: Check for error and call
Eif needed. If successful, combine result into some composite value, and call success handler
Swith that value.
and then parametrizing this whole definition over instructions
I2, error handler
E, and success handler
S. This definition
then could be parametrized over multiple different instructions, and
all of these chained together (e.g. via
(=<<)) to create a larger
sequence of calls passing control via CPS.
Ion was heavily inspired by another EDSL, Atom. It started as an Atom re-implementation which had other backends, rather than generating C code directly (as Atom does). However, Ion has diverged somewhat, and still does not have many things from Atom, such as synchronous variable access, run-time checks on execution time, various compile-time sanity checks, traces, or most of its standard library.