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From Andy Dufilie <andy.dufi...@gmail.com>
Subject Re: [FALCONJX][FLEXJS] "as" keyword handling
Date Fri, 08 Jan 2016 03:24:07 GMT
On Thu, Jan 7, 2016 at 8:27 PM, Alex Harui <aharui@adobe.com> wrote:

> If "SomeType(somevar)" is not in a try/catch it will throw an exception.
> How often are you relying on that vs just trying to make the compiler
> happy?
>

"function" casting is slower than "as" in ActionScript, so I have stayed
away from it.  When I do use it, there are two cases:

1. If coercing to a primitive type, it actually tries to convert the value
to that type. With String(value) I expect it to give me a String via
toString().  With Number(value) I expect it to converting Strings to
Numbers and undefined -> NaN. I expect it to throw an error if the value
cannot be converted to a Number. Really I would rather have it return NaN
instead of throwing an error, but we can't change that behavior because
that's part of ECMAScript.

2. When coercing to a non-primitive, I mostly use it to get compiler
checking for the member variables or functions I access, but I would still
want it to throw an error even in JavaScript if it is not of the correct
type, because that's what it does in ActionScript and I expect the
cross-compiler to faithfully preserve the behavior.


On Thu, Jan 7, 2016 at 8:32 PM, Alex Harui <aharui@adobe.com> wrote:

>
> On 1/7/16, 5:24 PM, "Andy Dufilie" <andy.dufilie@gmail.com> wrote:
> >if (obj is Thing1)
> >    (obj as Thing1).foo(a,b);
> >else if (obj is Thing2)
> >    (obj as Thing2).bar(x,y);
>
> There are no plans to change "is", just "as".  Are you using "as" as a
> test or just "is".  In the example above, you are using "as" just to make
> the compiler happy.
>
>
It's used above to get compiler checking in case I misspelled foo() or
bar(), or if I pass in bad variable types to either function. In the next
example, I'm relying on "as" to change the value to null without throwing
an exception if it is not of the correct type. The code will break if that
behavior is not preserved:

var thing1:Thing1 = obj as Thing1;
var thing2:Thing2 = obj as Thing2;
if (thing1)
    thing1.foo(a,b);
if (thing2)
    thing2.bar(x,y);

Here is similar real-world code relying on this behavior and it will break
if "as" is removed by the cross-compiler:

https://github.com/WeaveTeam/Weave/blob/13f98136a8cfd8dc4662f6014abcd5aa87baa56d/WeaveJS/src/weavejs/core/SessionManager.as#L335



> >I see "is" and "as" as highly useful features, and I think it would be a
> >mistake to make the code cross-compile into something that behaves
> >differently by default.  IMO if cross-compiled code behaves differently
> >than the original then it's being mangled and can't be trusted.
>
> That's the root of my question: how many folks us "as" to actually convert
> data vs just make the compiler happy.  Again, no plans to change "is".
>


This is not just about making the compiler "happy." It's about getting the
benefit of compiler checking to alert you when you've typed the wrong
member variable / function name, or giving the wrong parameter types. If I
was only concerned about making the compiler happy (preventing it from
yelling at me), I could just use Object(value).whatever() or use untyped
variables everywhere. I may as well use plain JavaScript at that point.

We shouldn't be planning to change anything that would cause the
cross-compiled to code to behave differently than the original source.
This is not something that should require a vote - it is an error to change
the behavior of code during cross-compilation.  If such an option is added
to omit portions of code by default, then the compiler should give you a
warning for every place that occurs so the user knows exactly what is
happening.  Otherwise the cross-compiler cannot be trusted.



> >
> >A related issue is when setting a typed variable or passing in a parameter
> >to a function, it will do type coercion automatically in AS but that
> >behavior is lost when cross-compiling to JS.  For example, I have
> >situations like this where I now have to add manual type casting, and I
> >wish the compiler would do that automatically:
> >
> >var str:String = value_which_may_be_a_number;
>
> What error are you getting?  I thought the auto-conversion worked for both
> JS and AS.
>
> -Alex
>
>
Auto-conversion is currently not done for function parameters or variable
assignment.

AS input:
    public function testfunc(a:String, b:String):Array {
        var c:String = a;
        var d:String = b;
        return [typeof a, typeof b, typeof c, typeof d];
    }

JS output:
    testfunc = function(a, b) {
        var /** @type {string} */ c = a;
        var /** @type {string} */ d = b;
        return [typeof(a), typeof(b), typeof(c), typeof(d)];
    };

This code:
    var foo = testfunc;
    return foo(1,2)
will return ['string', 'string', 'string', 'string'] in ActionScript, but
['number', 'number', 'number', 'number'] in JavaScript.

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