spreading a dynamic value

[gavinking]

The backend needs to support spreading a dynamic value with the * operator. This was something we agreed on with @corbinu.

[gavinking]

This was ceylon/ceylon-spec#800, FTR.

[chochos]

This has a somewhat limited functionality. I can do this:

value tuple = [1,"2",'3'];
somethingDynamic.spread(*tuple); //turns into tuple.get(0), tuple.get(1), tuple.get(2)
somethingDynamic.spread(*someDynamicArray); //stays as someDynamicArray

please check the test code in the related commits @corbinu

[corbinu]

First off I believe the second test should be?

issue270.push(*issue270);
check(issue270.length==10, "dynamic spread #270 [2]");

as the elements already in issue270 would be spread across push so issue270 would go from
{0,0,1,"2",'3'} -> {0,0,1,"2",'3',0,0,1,"2",'3'}

My only other thought is sometimes a dynamic value might be iterable or not. I assume if it is not that the spread will do nothing. So I think there should be a two more tests

dynamic addString = "test string";
issue270.push(*addString);
check(issue270.length==11, "dynamic spread #270 [3]");
dynamic addNull = null;
issue270.push(*addNull);
check(issue270.length==11, "dynamic spread #270 [4]");

[gavinking]

If the value is not Iterable there should be an exception thrown by a runtime type check.

Sent from my iPhone

On 1 Oct 2013, at 8:40 am, Corbin Uselton notifications@github.com wrote:

First off I believe the second test should be?

issue270.push(*issue270);
check(issue270.length==10, "dynamic spread #270 [2]");
as the elements already in issue270 would be spread across push so issue270 would go from
{0,0,1,"2",'3'} -> {0,0,1,"2",'3',0,0,1,"2",'3'}

My only other thought is sometimes a dynamic value might be iterable or not. I assume if it is not that the spread will do nothing. So I think there should be a two more tests

dynamic addString = "test string";
issue270.push(_addString);
check(issue270.length==11, "dynamic spread #270 [3]");
dynamic addNull = null;
issue270.push(_addNull);
check(issue270.length==11, "dynamic spread #270 [4]");
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Reply to this email directly or view it on GitHub.

[corbinu]

Ok that works too. Given the major use case is for varadic JS functions though does seem a bit of a waste having to check if none, one or many args were passed to the wrapper function each and every time.

[chochos]

I know it seems like the second test should add 5 elements to the array, but it's simply impossible to spread that array like I spread the tuple, because I don't know the length of the array at compile time (I can't even know it's an array at compile time; it's a dynamic value).

As for iterables, I can check if the argument is an Iterable, but then what? How can I spread an Iterable to pass its elements to a native js call? I can't.

If I add the runtime type check as @gavinking suggests then the second test will throw an exception, since issue270 is not an Iterable. But like I said before, even if it's an Iterable, there's no way to spread it for a native js call.

[corbinu]

Well maybe I am wrong but my understanding is Ceylon Tuples and Sequences are simplified to a native Array or a generic array-like object correct? couldn't the generated code just look something like this

if (dynamicVal === null) {
     methodSpreadOn();
} else if (Array.isArray(dynamicVal)) {
     methodSpreadOn.apply(null, dynamicVal);
} else {
    methodSpread(dynamicVal);
}

[chochos]

That's quite convoluted for a simple invocation. But the real problem is that we can't really use apply because that only works for top-level methods that have not been wrapped in a JsCallable.

And no, tuples and sequences are not simplified to narive arrays. Ceylon methods with variadic parameters work similar to Java methods with varargs: the variadic parameter is really a Sequential inside the method, and there's a validation that sets the argument to empty if it's undefined (in case it was called with no args). The invocations enclose the listed and/or spread arguments corresponding to a variadic param inside a simple array.

[corbinu]

Well it doesn't sound like there is a simple unconvoluted solution. Could apply not be easily added to JsCallable? or the internal native js function extracted from JsCallable much like how a Ceylon String is unwrapped?

Well I never assumed it would use the Ceylon way of spreading varidic parameters as the dynamic type is not guaranteed to be a Ceylon object. Which I think is a lot of the issue. I defiantly agree this is more complicated to implement then I had anticipated though. I will will see if perhaps I can take a deeper look at JsCallable etc tomorrow

[chochos]

Moving to 1.1

[chochos]

So I guess we could have a native function that receives a Ceylon Iterable (Tuple, Sequence, Array, whatever) and returns a JS array, or checks if the received object is already a native JS array and just returns it; otherwise, throw an Error.

And we can use this function with the apply idea to make this work.

[corbinu]

Sounds good .... Sorry I know my interop work finds the most crazy edge cases ever.

[chochos]

better sooner than later