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From Erik de Bruin <>
Subject Re: [FalconJx] Package down to Expression production; JSGoogEmiter prototype
Date Fri, 28 Dec 2012 06:57:36 GMT
Both solutions have been implemented in FalconJx, I'm moving on.


On Fri, Dec 28, 2012 at 1:25 AM, Frank Wienberg <> wrote:
> On Thu, Dec 27, 2012 at 2:33 PM, Erik de Bruin <> wrote:
>> Frank,
>> I did read your blog, I'm not that stubborn ;-)
> Good to hear, I was beginning to suspect you are ;-)
>> I think you're addressing an edge case (passing 'undefined') to make
>> your point, but that is fine as it helps focus me on getting the
>> optimal solution. I say optimal, not perfect, for a reason, which is:
> Sorry, but I don't get your point. Even if you think I discovered an edge
> case that might not make such a big difference in practice, what speaks *
> against* my solution?
>    - It resembles the original ActionScript semantics
>    - It is easy to understand (look for the number of actual parameters,
>    and if there are not enough of them, ...)
>    - It is easy to generate and doesn't need any more generated code than
>    your solution
>    - It is more efficient than your solution
>    - Mike already implemented it
>> As long as we're talking edge cases, I noticed your blog also invokes
>> one to make it's point, namely using an untyped parameter. Your
>> function signature is "insult(s = 'fool')". Only when the parameter
>> you're setting a default for is untyped will it's value be
>> 'undefined', which you are testing for in your blog. In all cases=
>> where you declare a type for the parameter ("insult(s:String =
>> 'fool')"), the value assigned to it when you pass 'undefined' is the
>> initial for that type, i.e 'null' for String and Object, NaN for
>> Number, 0 for int, false for Boolean, etc..
>> I'm not sure how your solution provides for this? Mine doesn't either,
>> mind you, but I gave up perfection for simplicity very early in this
>> thread ;-)
> It does not. I consider this case another problem. That's why I used an
> untyped parameter to demonstrate how ActionScript handles default parameter
> values.
> In ActionScript, it is simply not allowed to stuff a wrong value into a
> typed parameter or variable. And it is not allowed to leave out a required
> (= non-optional) parameter.
> However, there are some cases where implicit type conversion (coercion)
> takes place. Your example of a parameter of type String that is passed
> undefined is an example of coercion and has nothing to do with default
> parameter values. It is the same as assigning undefined to a local variable
> of type String: the variable is null afterwards. Only that the "assignment"
> happens in the function call.
> So far, we do not perform any type checking at run-time. Why? Because we
> rely on the compiler refusing to compile in case of type errors. So we
> wouldn't add any code inside the function that checks whether
> parameter sactually contains a
> String, because we expect the calling code to have been type-checked.
> In the same sense, we should not add code inside the function to check for
> undefined and then assign the initial value for the type (here: null),
> because a variable of type String should never be undefined in the first
> place. Instead, this coercion should be generated in the *calling* code!
> Why? Because if 90% of the calling code can be statically proven to use the
> correct type (ruling out the value undefined for a String), always doing a
> dynamic check for undefined at run-time would be waste and inefficient.
> Note that TypeScript, too, refrains from generating any JavaScript code
> that does type checking; all type checking is done at compile time.
> Coercion of a literal value like undefined to a String can simply be done
> at compile time. However, in case of complex untyped values used as typed
> values, we'd have to do type *coercions* at run-time. E.g. imagine we have
> some veryComplicatedFunction():*, and an ActionScript expression of
> foo(veryComplicatedFunction()), we'd have to translate it to
> foo(coerceToString(veryComplicatedFunction())).
> If we plan to implement type coercions like in ActionScript (at least
> somewhere in the future), here is another argument against checking a
> parameter for undefined to assign its default value: once we have
> implemented coercion, the function body would not "see" undefined, but null,
> and could not distinguish it from a null explicitly given by the caller.
> And handing in null for a String parameter with a default value of 'foo', I
> dare say, is no longer an edge case and thus should behave like in "real"
> ActionScript. Using the arguments.length approach, the called function can
> tell the difference and behave correctly.
> Last but not least, what about coercion if the function is called directly
> from JavaScript? Well, then, we have a problem, anyway, because we also do
> not type-check. We could either mimic ActionScript's ExternalInterface for
> code meant to be called from JavaScript and add type checking and coercion
> code there (in a wrapper function), or live with possible errors at
> run-time for a simpler and more efficient solution.
> Sorry I had to say all that, please please nobody cite XKCD's "someone's
> wrong on the internet" again! ;-)
> Greetings
> -Frank-

Ix Multimedia Software

Jan Luykenstraat 27
3521 VB Utrecht

T. 06-51952295

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