Constrain Statements

danger

In versions >=0.5.0 use the assert syntax. The constrain statement will be maintained for some time for backwards compatibility but will be deprecated in the future.

Noir includes a special keyword constrain which will explicitly constrain the predicate/comparison expression that follows to be true. If this expression is false at runtime, the program will fail to be proven.

Constrain statement example

fn main(x : Field, y : Field) {
    constrain x == y;
}

The above snippet compiles because == is a predicate operation. Conversely, the following will not compile:

fn main(x : Field, y : Field) {
    constrain x + y;
}

The rationale behind this not compiling is due to ambiguity. It is not clear if the above should equate to x + y == 0 or if it should check the truthiness of the result.