Debug

Utility functions for debugging.

Import from the base library to use this module.

import Debug "mo:base/Debug";

Function print

func print(text : Text)

Prints text to output stream.

When running on ICP, all output is written to the canister log with the exclusion of any output produced during the execution of non-replicated queries and composite queries. In other environments, like the interpreter and stand-alone Wasm engines, the output is written to standard out.

Debug.print "Hello New World!";
Debug.print(debug_show(4)) // Often used with `debug_show` to convert values to Text

Function trap

func trap(errorMessage : Text) : None

trap(t) traps execution with a user-provided diagnostic message.

The caller of a future whose execution called trap(t) will observe the trap as an Error value, thrown at await, with code #canister_error and message m. Here m is a more descriptive Text message derived from the provided t. See example for more details.

Other execution environments that cannot handle traps may only propagate the trap and terminate execution, with or without some descriptive message.

import Debug "mo:base/Debug";
import Error "mo:base/Error";

actor {
  func fail() : async () {
    Debug.trap("user provided error message");
  };

  public func foo() : async () {
    try {
      await fail();
    } catch e {
      let code = Error.code(e); // evaluates to #canister_error
      let message = Error.message(e); // contains user provided error message
    }
  };
}