Zélus is a synchronous language in the style of
Lustre  and Lucid Synchrone  but extended
to model hybrid systems that mix discrete-time and
continuous-time signals. An example is a system that mix a
(discrete-time) model of real-time control software that executes in
closed loop with a model of its physical environment described by a
Ordinary Differential Equations. More intricate interactions between
discrete- and continuous-time behaviors can be expressed, like, for
instance, continuous-time PID controllers or hybrid automata with
several running modes, each of them being defined by an ODEs (the so-called
Zélus provides basic synchronous language constructs—difference and
data-flow equations, hierarchical automata, and stream function
definitions—in the style of Lustre  and
Lucid Synchrone . Continuous-time dymamics are
expressed by ODEs with events defined as zero-crossings.
The expressiveness of the language is deliberately constrained to statically
ensure determinism and the generation of loop-free sequential code that runs
in bounded time and space. Moreover, code is generated identically for both
embedded targets and simulation. For source programs with ODEs, the
generated sequential code is paired with a numerical solver to approximate
the continuous-time dynamics.
Zélus’s main features are:
It is a data-flow language in Single Static Assignment form: every name has
only a single definition in the source code at any instant. A program is a
collection of functions from signals to signals. A signal is a
function from time
to values. A set of signals is defined as
the solution of a set of mutually recursive equations.
- The separation between discrete-time and continuous-time signals and
systems is imposed at the level of function definitions:
A node is a function from discrete-time signals to
discrete-time signals. A discrete-time signal is a sequence of values (a
stream) as in other synchronous languages.
A node is executed consecutively over the elements of a sequence of
inputs to give a sequence of outputs.
Nodes have no other notion of time than this succession of instants.
In particular, there is no a priori ‘distance’ (time elapsed) between
Outputs are produced atomically with triggering inputs, that is,
instantaneously in the same discrete instant.
They may depend on previous inputs; such nodes are termed
- A hybrid node is a function from continuous-time signals to
continuous-time signals. A continuous-time signal is a signal
defined on a sequence of time intervals on the real
A hybrid node is executed on this set of instants. Only hybrid nodes
may contain ODEs and detect zero-crossing events.
discrete-time computations must be executed on a discrete clock.
This is statically enforced by the type system, following the convention:
A clock is termed discrete if it has been so declared or if it
results from the sub-sampling a discrete clock or a zero-crossing.
Otherwise, it is termed continuous.
It is possible to reset a continuous variable defined by an ODE
on a discrete clock.
A zero-crossing occurs when a continuous-time signal crosses zero
from a negative value to a positive one during integration.
Conceptually, a timer is a particular case of a zero-crossing event,
even if the actual implementation is more specific.
- The basic types like integers, floating-point numbers, booleans, and
characters are lifted from the host language OCaml. Abstract types,
product types, record types, and enumerated types can either be
defined directly or imported from the host language. Functions may have
polymorphic types as in ML.
Structured values are accessed via pattern matching.
- Data-flow equations may be composed arbitrarily with hierarchical automata
The compiler ensures determinacy and the absence of infinite loops.
Hierarchical automata are internally rewritten into data-flow equations.
- The compiler is written in OCaml as a series of
source-to-source and traceable transformations that ultimately yield
statically scheduled sequential OCamlcode.
The results of intermediate steps can be displayed.
Continuous components are simulated using an off-the-shelf
numerical solvers (SUNDIALS
and, two built-in basic solvers (based on Matlab’s ode23 and
ode45 solvers ).
Zélus is a research prototype that exhibits a new way of
defining a hybrid systems modeling language based on
the principles and techniques of synchronous languages.
Its expressive power for modeling physics is limited to ODEs, unlike
Modelica which is based on DAEs.
Research papers on the design, semantics and implementation of Zélus are
available at http://zelus.di.ens.fr.
The implementation is written in, and generates programs in OCaml, which
must be installed.
The language is experimental and evolves continuously. Please send
comments or bug reports to Timothy.Bourke@inria.fr or
This software includes the OCaml run-time system, which is
copyrighted INRIA, 2015.
This software is a research prototype that takes considerable time to
If you find it useful, please consider citing our work 
and sending us comments.