Closures prototype equivalence between closures and methodrefs

[Neal Gafter]

Alex-

BGGA's goal was to introduce closures and the function type syntax without
changing the underlying type system, and without requiring changes in the
VM's type system.  Opening up the scope to the possibility of such changes
might affect the shape of the ideal solution.

Regards,
Neal

On Wed, Aug 13, 2008 at 10:45 AM, Alex Buckley <Alex.Buckley at sun.com> wrote:

> I recall you distinguished "function type" from "closure type" many moons
> ago. For my own understanding, here are some notes.
>
> "Ideally" there would be no difference and one could say:
> - A closure literal is a value and has a function type.
> - A variable can store such a value and has a function type.
> - Closure conversion is from a function type to an interface type
>  with a single compatible method.
>
> which would allow:
>
> interface Foo { void m(int i, String s) throws IOException; }
>
> doN(1, "hi", new Foo() {
>  void m(int i, String s) {if (i==3) throw ...; System.out.println(s);}
> });
>
> void doN(int n, String x,
>         {int, String => void throws IOException} b) {...}
>
> Conversion from an arbitary interface type (Foo) to an arbitrary function
> type ({int, String => void throws IOException}) is hard.
> VM-level interface injection could make it less hard.
>
> Conversion from the synthetic and canonical JVM type of a closure literal
> to an arbitrary function type is not hard, since the synthetic type
> ("closure type") can be made a subtype of the target function type.
>
> Therefore, BGGA 0.5 stratifies closure literal terms and variable terms:
> - A closure literal is a value and has a closure type.
> - A variable can store such a value and has a function type.
> - Closure conversion is from a closure type to an interface type
>  with a single compatible method.
>
> Alex
>
> Neal Gafter wrote:
>
>> Subtype relationships among reference types must require no conversion
>> code. For example, converting a List<? extends Number> to a List<? extends
>> Object> requires no code.  If we support more general conversions among
>> function types I believe it ought to be implemented by putting the
>> underlying interfaces in some kind of normal form - for example, using Void
>> for the result type in the interface when the user wrote void in the
>> function type.  All that is possible, but I'm reluctant to take on major
>> changes to the spec at this stage.
>>
>> Regards,
>> Neal
>>
>> On Wed, Aug 13, 2008 at 3:29 AM, Mark Mahieu <
>> mark at twistedbanana.demon.co.uk <mailto:mark at twistedbanana.demon.co.uk>>
>> wrote:
>>
>>    I must admit that I've come close to reporting similar non-bugs,
>>    because even though I *know* - having read the spec far too many
>>    times - that the conversion is only applicable to closures, I've
>>    found it's very easy to slip into thinking that assignment
>>    compatibility works the same way for function types.  At least for
>>    the subtler aspects like boxing or void vs Void.
>>
>>    So I'll ask the question: why is the conversion applicable to
>>    closures only, and not to function types as well?
>>
>>    Presumably if it were allowed for function types then it would be
>>    trivial to write a program which appears to have stable performance
>>    and memory usage but which actually degrades in performance and
>>    eventually blows up with an OutOfMemoryError or StackOverflowError,
>>    but I'm curious to know what the real reasons are.
>>
>>    Regards,
>>
>>    Mark
>>
>>
>>    On 13 Aug 2008, at 04:44, Neal Gafter wrote:
>>
>>     Alex-
>>>
>>>    In your code, doN requires its third parameter of type
>>>
>>>    {int, T => U throws X}
>>>
>>>    but you've supplied something of type
>>>
>>>    {int, String => void throws IOException}
>>>
>>>    In fact, the value you supplied is NOT a closure, it is a variable
>>>    of function type.  The closure conversion is not involved at all.
>>>
>>>    These two function types are unrelated.  There is no conversion
>>>    between them.  So the call is illegal.
>>>
>>>    In the first call, you did supply a closure (a method reference is
>>>    a kind of closure), so the closure conversion was applied.
>>>
>>>    The fact that the diagnostic prints the "required" line in terms
>>>    of javax.lang.function.OIO instead of using the function type
>>>    syntax is a bug.
>>>
>>>    Regards,
>>>    Neal
>>>
>>>    On Tue, Aug 12, 2008 at 2:34 PM, Alex Buckley
>>>    <Alex.Buckley at sun.com <mailto:Alex.Buckley at sun.com>> wrote:
>>>
>>>        Trying to stress the equivalence, I get this interesting error
>>>        on the first line of main but not the second:
>>>
>>>        A.java:13: method doN in class A cannot be applied to given types
>>>        required: int,T,javax.lang.function.OIO<? extends U,? super
>>>        T,? extends X>
>>>        found: int,java.lang.String,{int,java.lang.String => void
>>>        throws java.io.IOException}
>>>           doN(1, "hi" , closure);
>>>           ^
>>>        1 error
>>>
>>>        Seems like the wrong proto-function is being selected.
>>>
>>>        --
>>>        import java.io.*;
>>>
>>>        public class A {
>>>         static {int, String => void throws IOException} closure = {
>>>           int i, String s =>
>>>             if (i==3) throw new IOException(); System.out.println(s);
>>>         };
>>>
>>>         static void method(int i, String s) throws IOException {
>>>           if (i==3) throw new IOException(); System.out.println(s);
>>>         }
>>>
>>>         public static void main(String[] a) {
>>>           doN(1, "hi" , closure);
>>>           doN(2, "bye", A#method(int,String));
>>>         }
>>>
>>>         static <T extends String, U, throws X extends IOException>
>>>         void doN(int n, T x, {int, T => U throws X} b) {
>>>           for (int i = 0; i < n; i++) {
>>>             try {
>>>               b.invoke(i, x);
>>>             } catch (IOException e) { System.out.println("o no!"); }
>>>           }
>>>         }
>>>        }
>>>        --
>>>
>>>
>>>
>>
>>
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