Closures prototype equivalence between closures and methodrefs

[Mark Mahieu]

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> 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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