pub package api doc

Error data structures for fine-grained control over errors. It is possible to use errors as values like in Go


The Err class

   import 'package:err/err.dart';

   // Create an error:
   final err = Err.error("An error has occured");

   // Use with a function as return value 
   Err theFunction() {
     try {
       // something wrong
     } catch (e) {
       return Err.error(e);
     return null;
     // or to not use a null value return an empty error object
     return const Err.nil();

Check the error:

   final err = theFunction();
   if (err != null) {
      // print the error to the console
      // throw an exception from the error
   // or if not using null values
   if (!err.isNil) {
      print("${} ${err.type} : ${err.message}");

Data structure:

   /// An error message for the user
   final String userMessage;
   /// The date of the error
   DateTime get date;
   /// The error type
   ErrType get type;
   /// Get an exception from the message
   Exception get exception;
   /// Get the message
   String get message;
   /// Is the error empty
   bool get isNil;

Available error levels:

   enum ErrType { critical, error, warning, info, debug }


All the constructors accept either a String, an Exception or an Error as input

   /// Critical error
   final err = Err.critical("The error message");

   /// Error
   final err = Err.error("The error message");

   /// Warning
   final err = Err.warning("The warning message");

   /// Info
   final err ="The info message", userMessage: "A nice message for the user");

   /// Debug
   final err = Err.debug("The debug message");   

   // from a type
   final err = Err.fromType("The error message", ErrType.warning);

   // duplicate an error adding a message for the user
   final niceErr = err.copyWithUserMessage("A nice error message")
   print("Message for the developer: ${niceErr.message}");
   print("Message for the user: ${niceErr.userMessage}");

The userMessage parameter is optional for all constructors

Print an error

Print an error to console, similar to console.log in javascript:

   /// print an instance
   /// print from a string or an [Err]
   Err.log("An error");

Error as values

A data structure is available to pass return values of functions with errors: the ÈrrPack class:

   import 'package:err/err.dart';
   ErrPack<int> _someFunctionThatReturnsAnInt() {
     try {
       throw Exception("Oops");
     } catch (e) {
       // return an error and no value
       return ErrPack.err(Err.warning(e));
     // return no error and a integer value
     return const ErrPack.ok(1);
   ErrPack<Null> _someFunctionThatReturnsNull() {
     try {
       // ok
     } catch (e) {
       // return an error and no value
       return ErrPack.err(Err.debug("Something went wrong"));
     // return no error and a null value
     return const ErrPack.nullOk();
   void main() {
     // or
     final res = _someFunctionThatReturnsAnInt();
     if (res.hasError) {
       // print the error
       // throw
     // get the return value
     final int i = res.value;