# Constrain Statement

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.