Logging

The standard library provides two familiar statements you can use: println and print. Despite being a limited implementation of rust's println! and print! macros, these constructs can be useful for debugging.

You can print the output of both statements in your Noir code by using the nargo execute command or the --show-output flag when using nargo test (provided there are print statements in your tests).

It is recommended to use nargo execute if you want to debug failing constraints with println or print statements. This is due to every input in a test being a constant rather than a witness, so we issue an error during compilation while we only print during execution (which comes after compilation). Neither println, nor print are callable for failed constraints caught at compile time.

Both print and println are generic functions which can work on integers, fields, strings, and even structs or expressions. Note however, that slices are currently unsupported. For example:

struct Person {
    age: Field,
    height: Field,
}

fn main(age: Field, height: Field) {
    let person = Person {
        age: age,
        height: height,
    };
    println(person);
    println(age + height);
    println("Hello world!");
}

You can print different types in the same statement (including strings) with a type called fmtstr. It can be specified in the same way as a normal string, just prepended with an "f" character:

  let fmt_str = f"i: {i}, j: {j}";
  println(fmt_str);

  let s = myStruct { y: x, x: y };
  println(s);

  println(f"i: {i}, s: {s}");

  println(x);
  println([x, y]);

  let foo = fooStruct { my_struct: s, foo: 15 };
  println(f"s: {s}, foo: {foo}");

  println(15);       // prints 0x0f, implicit Field
  println(-1 as u8); // prints 255
  println(-1 as i8); // prints -1

Examples shown above are interchangeable between the two print statements:

let person = Person { age : age, height : height };

println(person);
print(person);

println("Hello world!"); // Prints with a newline at the end of the input
print("Hello world!");   // Prints the input and keeps cursor on the same line