# Lustre (programming language)

Short description: Programming language

Lustre is a formally defined, declarative, and synchronous dataflow programming language for programming reactive systems. It began as a research project in the early 1980s. A formal presentation of the language can be found in the 1991 Proceedings of the IEEE.[1] In 1993 it progressed to practical, industrial use in a commercial product as the core language of the industrial environment SCADE, developed by Esterel Technologies. It is now used for critical control software in aircraft,[2] helicopters, and nuclear power plants.

## Structure of Lustre programs

A Lustre program is a series of node definitions, written as:

node foo(a : bool) returns (b : bool);
let
b = not a;
tel


Where foo is the name of the node, a is the name of the single input of this node and b is the name of the single output. In this example the node foo returns the negation of its input a, which is the expected result.

### Inner variables

Additional internal variables can be declared as follows:

 node Nand(X,Y: bool) returns (Z: bool);
var U: bool;
let
U = X and Y;
Z = not U;
tel


Note: The equations order doesn't matter, the order of lines U = X and Y; and Z = not U; doesn't change the result.

### Special operators

 pre p Returns the previous value of p p -> q Set p as the initial value of the expression q

## Examples

### Edge detection

node Edge (X : bool) returns (E : bool);
let
E = false -> X and not pre X;
tel