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The thx.error package contains a bunch of classes to manage errors in your applications. The base class is Error and is a lightweight container for error messages. The nice thing about it is that it separates the message itself from its parameters allowing to format it in a meaningful way:

var error = new Error("invalid import {0:C}", 0.123);
trace(error);
// trace results is: invalid import $0.12

If you provide the needed infrastructure you can also use an instance of ITranslation to deliver your messages:

// Translated Error
var translator = new DictionaryTranslation(ItIT.culture);
translator.addSingular("invalid import {0:C}", "importo invalido {0:C}");
trace(error.translate(translator));
// trace result is: importo invalido € 0,12

I often use to mark my abstract methods so that at least at runtime I have a feedback that the implementation is missing and I do that using the AbstractMethod error class (the NotImplemented error class is functionally equivalent but has a different semantic).

// AbstractMethod / NotImplemented
// in a real world context this will be an instance method and not an inline function

function notImplemented() throw new NotImplemented();
try
{
	notImplemented();
} catch (e : Dynamic) {
	trace(e);
}

The nice thing is that I don’t have to type any message into the constructor because all the info I need are already in there; this is the trace:

method Main.notImplemented() needs to be implemented

Finally the NullArgument error class has a couple of static constructors that helps me check for null arguments. Using macros the argument name is extracted so that you don’t need to type it as a string; that spares you some typing but better yet avoids annoying and error prone repetitions.

// NullArgument
function toUpperCase(text : String)
{
	NullArgument.throwIfNull(text);
	return text.toUpperCase();
}

try
{
	toUpperCase(null);
} catch(e : Dynamic)
{
	trace(e);
}
// trace result is: invalid null argument 'text' for method Main.toUpperCase()

The NullArgument.throwIfNullOrEmpty works similarly but also test for emptiness … this works on Strings, Lists, Arrays (length) and objects (no fields). Note that the type discovery is made at compile time and not at runtime:

// null or empty
function pop(arr : Array) : String
{
	NullArgument.throwIfNullOrEmpty(arr);
	return arr.pop();
}

try
{
	pop([]);
} catch(e : Dynamic)
{
	trace(e);
}

// trace result is: invalid null or empty argument 'arr' for method Main.pop()