[Laszlo-dev] Undefined pattern in AS3

[André Bargull]

I was wrong with the error number, it isn't 1118, but actually 1176:
 > 1176	Comparison between a value with static type %s and a possibly 
unrelated type %s.

> Are you saying that in AS3 if you replace `void 0` with `undefined` you 
> won't get the coercion error?  That would be totally strange, because 
> `===` is not supposed to do any coercion.  It should just return `false` 
> if it is asked to compare unrelated types.
Yes, if I replace "void 0" with "undefined", everything works. But then 
we're back in the JS1-dilemma which you've described in your blog.

> I think you are correct in using `*` as the type declaration when you 
> want to admit `void 0` as a possible value (although that means you open 
> it up to other "primitive" types also, which may or may not be what you 
> want).
My intention was to accept any non-undefined values and to use undefined 
as the marker for non-existing entries in the cache.


Original error output (unfortunately just in German):
>      [java] ERRORS:
>      [java] [(unknown): 45] line unknown: Error: Vergleich zwischen einem Wert mit statischem Typ Object und einem m÷glicherweise nicht verw
> andten Typ void.
>      [java] FAIL: compiler returned 1
>      [java] Done executing compiler
>      [java] Intermediate file c:\DOKUME~1\Admin\LOKALE~1\Temp\lzswf9\lzgen36335\app.swc: does not exist
>      [java] Exception compiling scriptfile: org.openlaszlo.sc.CompilerError: line unknown: Error: Vergleich zwischen einem Wert mit statisch
> em Typ Object und einem m÷glicherweise nicht verwandten Typ void, in line: if ($7 !== void 0) {
>      [java] org.openlaszlo.sc.CompilerException: org.openlaszlo.sc.CompilerError: line unknown: Error: Vergleich zwischen einem Wert mit sta
> tischem Typ Object und einem m÷glicherweise nicht verwandten Typ void, in line: if ($7 !== void 0) {
>      [java]     at org.openlaszlo.sc.Compiler.compile(Compiler.java:397)
>      [java]     at org.openlaszlo.sc.lzsc.compile(lzsc.java:110)
>      [java]     at org.openlaszlo.sc.lzsc.compile(lzsc.java:322)
>      [java]     at org.openlaszlo.sc.Main.main(Main.java:10)
>      [java] Compilation aborted.



On 8/25/2008 4:46 PM, P T Withington wrote:
> I thought I ran into this too, but the error turned out to be something 
> different:
> 
>   http://jira.openlaszlo.org/jira/browse/LPP-6781
> 
> Are you saying that in AS3 if you replace `void 0` with `undefined` you 
> won't get the coercion error?  That would be totally strange, because 
> `===` is not supposed to do any coercion.  It should just return `false` 
> if it is asked to compare unrelated types.
> 
> I think this is really just a bad choice of error message on the part of 
> the compiler.  What it is trying to tell you is: "You declared `old` to 
> be of type `Object`, and on the next line, you are asking if it could 
> possibly be of type `Void`.  That is impossible."
> 
> [So, in AS3, for any class `X`, if you declare something as of type `X` 
> you are implicitly declaring it as `X?` (to use JS2 syntax).  That is, 
> any var that is declared as a subclass of `Object` is also allowed to be 
> of type `Null`... but not of type `Void`.
> 
> Brendan takes full responsibility for the 'Javascript has two nulls 
> mess'.  But it appears that Flex has made it a bit messier, by making 
> `Null` a pseudo-subclass of `Object`, but `Void` not?]
> 
> I think you are correct in using `*` as the type declaration when you 
> want to admit `void 0` as a possible value (although that means you open 
> it up to other "primitive" types also, which may or may not be what you 
> want).
> 
> On 2008-08-25, at 10:04EDT, André Bargull wrote:
> 
>> In my recent changeset for RegExp-support in swf8, you'll get an 
>> error, if you compile the lfc for swf9:
>>
>> The simple cache class I've added accepts "Objects" as values, so I 
>> coded:
>>> function put (key:String, value:Object) :Object {...}
>>> function get (key:String) :Object {...}
>> And in these functions, I've used the "undefined"-pattern described in 
>> [1]:
>>> var old:Object = this.get(key);
>>> if (*old === void(0)*) {
>> But this gave me an AS3 compiler-error [2]:
>>> 1118    Implicit coercion of a value with static type Object to a 
>>> possibly unrelated type void.
>>
>> To get around the compiler error, I've just changed the 
>> type-declaration to "*" (maybe even more reasonable). So far so good, 
>> but we've used the "void 0"-pattern all along in our code. Therefore 
>> do you think it's possible to update the compiler to change "void 0" 
>> to "undefined" for JS2-targets, resp. the other way around, so 
>> "undefined" to "void 0" for JS1-targets (possibly more 
>> straightforward)? Just to ensure the "one code, multiple runtimes" 
>> idea in OpenLaszlo.
>>
>> - André
>>
>>
>> [1] 
>> http://pt.withy.org/ptalk/archives/2005/06/dont_assume_undefined_is_undefined.html 
>>
>> [2] 
>> http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/compilerErrors.html 
>>
> 
>