Name of lambda parameters

[Remi Forax]

On 01/08/2013 01:28 AM, Dan Smith wrote:
> On Jan 7, 2013, at 11:04 AM, Remi Forax <forax at univ-mlv.fr> wrote:
>
>> (CC Alex Buckley)
>>
>> As noticed recently on lambda-dev mailing list,
>> there is no way to get the parameter names of a lambda because
>> there is no way to do reflection on a lambda (an object implementing a functional interface) to get the corresponding declared method.
> Note that it _is_ possible to get to the function descriptor:
>
> Method descriptor = lambdaObj.getClass().getInterfaces()[0].getDeclaredMethods()[0]; // horribly oversimplified
>
> What you want, I gather, is to get to the "method" that corresponds to the "lambda object" itself.

I don't want a j.l.r.Method object, just a way to do reflection of 
lambda parameters.

>   But note that there need not exist any such method in general.
>
> Block<String> b1 = (String arg1) -> {};
> Block<String> b2 = (String arg2) -> {};
> assert b1 == b2; // this assertion might succeed
>
> // possible implementation: class EmptyBlock implements Block<String> { void run(String arg3) {} }
>
> Here, what is the "parameter name of" b2?

No, name. That's why I think that we need a marker interface to say to 
the compiler/runtime, I want to keep and be able to do reflection on 
parameter names.

>
> That fact that the current javac implementation has a one-to-one mapping between lambda expressions and compiled methods is an implementation detail.

Yes, for default lambda.
For serializable lambda it's more complicated, you can only share inside 
the same method.
For lambda that expose parameter names, the compiler may have to 
maintain a 1 to 1 mapping.

>
> (High-level point which is getting muddled here: lambdas are not objects that get converted to functional interface types.  Lambdas are expressions that evaluate to instances of functional interfaces.)

Yes, exactly like methods in Java are not object but there a way to 
reflectively access to the parameter names.
The reflection should be able to provide parameter names of the expression.

>
> —Dan

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