How about extending functionality of "typeof" to extract type of expression?

[Typhoon Storm]

Hi,

    Currently the keyword "typeof" is used to get type from a class literal. How about extending its functionality to extract type of expression?  E.g.

    val typeof(TheClass.methodWithComplicatedReturnType) result = ...

    val List<typeof(TheClass.methodWithComplicatedReturnType)> results = newArrayList

    

    This idea comes from my experience of an actual project, which is like this:

    def methodWithComplicatedReturnType() {

        return 1 -> "a" -> true -> new ArrayList<String>

    }

    def collectComplicatedResults() {

        val result = methodWithComplicatedReturnType // So far so good; the type of result is automatically inferred from the method

        val List<????> results = newArrayList // Now I want to collect multiple results into a list. 

                                              // What's type of the list......

        (0..<100).forEach[

            results.add(methodWithComplicatedReturnType)

        ]

        results.get(0). ..... // To use the elements of results, I have to know its accurate type, 

                              // which cannot be inferred directly. 

                              // Of course I can figure the type out manually, But that is cumbersome and error prone, 

                              // not to mention it has to be in sync with the return expression

    }

    Actually I figured out a workaround:

    def static <T, L extends List<T>, DUMMY> listOfType(Class<L> listClass, (DUMMY)=>T dummy) {

        return listClass.newInstance as List<T> // I want type of returned list to be List, not ArrayList

    }

    Then I can write:

    

    val results = listOfType(ArrayList) [methodWithComplicatedReturnType]

    Yes it can do the automatic type inferrence trick, but it also has flaws: It is hard to provide arguments to constructor of ArrayList, and I have to write more setOfType/mapOfType... methods for any possible structure needing complicated type as generic parameter.

    So I wonder if we can use already existent keyword "typeof" to extract type of an expression, which can be used like this:

    val typeof(TheClass.methodWithComplicatedReturnType) result = ...

    val List<typeof(TheClass.methodWithComplicatedReturnType)> results = newArrayList

    I think this can address above problem naturally. And I believe it has more power that I just yet to figure out...

[Sven Efftinge]

Just leave out the "List<????>” part and the compiler will do the job for you.

That said, you’ll have to provide a semicolon or parenthesis after newArrayList as otherwise the parser

assumes (0..<100) to be the argument to the newArrayList call.

Sven

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[Typhoon Storm]

Yes it's amazing! I thought type inference only works on the line the variable is defined.

However when I tried some more cases, I found there is still uncovered ones:

    var toReceiveResultLater // the type cannot be derived

    // after a while

    toReceiveResultLater = results.get(0) 

    var toReceiveResultLater2 = null // in this case the inferred type is null, won't it be at least Object?

    // after a while

    toReceiveResultLater2 = results.get(0) // Type mismatch: the [Accurate type of results.get(0)] is not applicable at this loctation, 

                                                            // since the inferred variable type is null, 

So maybe in these cases I have to write down the type manually.