Logging

The standard library provides a familiar println statement you can use. Despite being a limited implementation of rust's println! macro, this construct can be useful for debugging.

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

It is recommended to use nargo execute if you want to debug failing constrains with println 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). println will not work for failed constraints caught at compile time.

The println statement is unconstrained, so it works for outputting integers, fields, strings, and even structs or expressions. For example:

use dep::std;

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

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

You can print multiple different types in the same statement and string as well as a new "fmtstr" type. A fmtstr can be specified in the same way as a normal string it just should be prepended with an "f" character:

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

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

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

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

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