APPENDIX-7.MD

Appendix 7: Debugging and testing with Motoko.

Debugging

Debug is a module that provides utility functions for debugging and printing informations. See the documentation page.

Print

func print(text : Text)

Debug.print() will print text to the output stream (i.e your terminal).

import Debug "mo:base/Debug";
actor {
    Debug.print("Hello!");
};

The text will be printed in the same tab that is running your replica. Make sure to check out the correct place!

Unfortunately, print can only be used locally. On the mainnet or when using Motoko Playground there is no concept of output stream so Debug.print has no effect.

Debug.print() only accepts a value of type Text as input. If you have any object, you can use the debug_show command to automatically convert it to Text.

let array : [Nat] = [19, 8, 2, 5];
Debug.print(debug_show(array));

You can also use convertion functions

import Nat "mo:base/Nat";
import Debug "mo:base/Debug";
actor {
    let n : Nat = 5;
    Debug.print(Nat.toText(n));
}

Trap

func trap(errorMessage : Text) : None

Debug.trap() will stop the execution and return an error message with it.

import Debug "mo:base/Debug";
actor {
    Debug.trap({errorMessage = "Unexpected reach"});
};

This could be useful as the error message will be visible in the return of the canister call, and not on a separate terminal. Also the behavior will be the same either in Local or Mainnet.

You can use it with debug_show() to output any variable you might deem relevant.

In this example the variable is printed in the same tab as you made the call.