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Why are there Errors AND Exceptions in Dart and not either Errors OR Exceptions?
Is the reason a historical one oder what?
I can throw an Error, I can throw an Exception. None of them is checked by the Analyzer like in Java (Exception vs. RuntimeException)
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The error indicates trouble that primarily occurs due to the scarcity of system resources. The exceptions are the issues that can appear at runtime and compile time. 2. It is not possible to recover from an error.
Dart Exceptions are the run-time error. It is raised when the program gets execution. The program doesn't report the error at compile time when the program runs internally and if Dart compiler found something not appropriate. Then, it reports run-time error and the execution of program is terminated abnormally.
That error is handled by catchError() . If myFunc() 's Future completes with an error, then() 's Future completes with that error. The error is also handled by catchError() . Regardless of whether the error originated within myFunc() or within then() , catchError() successfully handles it.
An exception is an unexpected issue that occurs when executing a program and disrupts its normal flow. Exceptions can cause the application to terminate abnormally if not handled. Exceptions are intended to be caught and handled so they do not affect the flow of the program.
From this post, quoting Bob Nystrom:
Error and its subclasses are for programmatic errors. If one of those occurs, your code is bad and you should fix your code.
Non-Error exception classes are for runtime errors. Sometimes you can prevent them from being thrown, but often you cannot.
Except in a few special circumstances, idiomatic Dart should throw Errors, but never catch them. They exists specifically to not be caught so that they take down the app and alert the programmer to the location of the bug.
In other words, you should expect (and check for) exceptions (it is intended that you should handle them). If you get an error, then you need to check how you're using the API that's throwing the error - you're probably using it wrong.
If you're writing an API, then you should use the same pattern. Errors are messages to downstream developers about how they are using your API.
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An Exception in Dart should be thrown for regular, expected program flow and is intended to be caught:
An Exception is intended to convey information to the user about a failure, so that the error can be addressed programmatically. It is intended to be caught, and it should contain useful data fields.
TimeoutException A TimeoutException will be thrown "when a scheduled timeout happens while waiting for an async result", which is expected program flow.
If we have a download task for example and that download task is not finished after our specified timeout time of thirty seconds (which can happen), we want to communicate that to our user, hence, we need to catch the Exception.
An Error in Dart should be thrown for unexpected program flow and should not be caught but addressed by the programmer:
An
Errorobject represents a program failure that the programmer should have avoided.
AssertionError An AssertionError is thrown "when an assert statement fails", i.e. it should never happen because we assert that it should not.
If we see such an error, it means that we should change our code and we should definitely not catch the error.
In practice you can catch Errors, but you should not. There is a linter rule to help enforce that.
The fact that Dart allows it can still be useful, e.g. when testing assertions or other errors.
See this answer for a complete example scenario.
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