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Re: enhance an array's static type by a lower length-bound.

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Robert Klemme <shortcutter@googlemail.com> wrote:
> On 17.08.2011 17:04, Andreas Leitgeb wrote:
>> The point of the idea is, that array variables would be added a static
>> bit of information, namely a lower bound (0 by default) for the length
>> of an array that can legally be stored in that variable.
>> I believe such a change would be source-compatible, but might require
>> some severe changes at the bytecode/class-file-format level, to allow
>> to specify such a lower bound in a type signature. Such a change should
>> be possible in a way, that old class-files can still be used. e.g.:
>>   get9th:(]10I)I    (currently without bounds:  get9th:([I)I )
>> Methods could overload others just for different length-bounds.
>> The one with the largest provably satisfyable length-bound would be
>> chosen among those with otherwise matching signature.
>
> Not sure whether this would work - especially since binding of the 
> signature occurs at compile time while you might only learn the real 
> length of a parameter at runtime.

That was the point, that the resolution would still be at compile-time,
and based on the particular statically known length bound, not the actual
length.

e.g.:
   void foo(int[] ia) { ... }
   void foo(int[10] ia) { ... }
   void foo(int[20] ia) { ... }
   ...
   foo(new int[25]) -> third
   int[5] ia = new int[15];
   foo (ia )  -> first, not second!

That would be entirely consistent with 

   void foo (Object o) { ... }
   void foo (List l) { ... }
   void foo (ArrayList al) { ... }
   ...
   foo ( new ArrayList() { ... } ) // (subclass of AL) -> 3rd.
   Collection c = new Vector();
   foo ( c )   -> calls first, not second.

> Also there is probably a whole lot of issues with java.lang.reflect.

Hmm, the Class-instances representing arrays e.g. for a call to 
Class.getMethod() likely would need to be specific for each bound.
There'd need to be a way to get a class-object for, say, int[42].
May be tricky, but doesn't seem impossible to me.
If you know of a specific difficulty here, then I beg you to speak up!

>> Another one could be, that ArrayIndexOutOfBoundsExceptions are
>> not currently perceived as a problem that could be alleviated
>> by static checks, anyway.
> I think this is closer to the real reason why this is probably
> not such a good idea.

I don't refute this. I just like to principially think it through, 
anyway. Maybe until some show-stopper "that can't ever work" is
brought up.

>   What you propose would be useful only if the added 
> property of arrays would allow for *compile time* checking -
Well, that's the intention behind it - to some degree.

> because Java has runtime checks already.  For compile time checks to be 
> reasonable you would need to forbid
> int[] a1 = expr;
> int[10] a2 = a1;

I wrote that in my original post, already. It requires an explicit
cast in the second line, and the cast would mean that a runtime check
on the length occurs.  Principially the same story as with object-type
casts.

> because depending on the complexity of expr it might not be possible for 
> the compiler to determine the guaranteed min length of a1.

The guaranteed min length of a1 is 0 - because the brackets are empty.

> I have the feeling that 
> people would use this as a lazy shortcut for defining a class with n 
> integer fields.

That would be an admittedly entirely unwanted dead giveaway, but just
like the imbalance of cost vs practical applicability, I'd like to 
keep that out of the technical discussion.  It would be relevant, if
it was intended as a serious proposal, but that isn't the case here. 

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Thread

enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-17 15:04 +0000
  Re: enhance an array's static type by a lower length-bound. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-17 12:34 -0500
    Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 13:14 +0000
      Re: enhance an array's static type by a lower length-bound. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-18 10:30 -0500
        Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 18:12 +0000
          Re: enhance an array's static type by a lower length-bound. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-18 20:59 -0500
  Re: enhance an array's static type by a lower length-bound. Robert Klemme <shortcutter@googlemail.com> - 2011-08-17 20:42 +0200
    Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 21:44 +0000
      Re: enhance an array's static type by a lower length-bound. Robert Klemme <shortcutter@googlemail.com> - 2011-08-19 13:24 +0200
        Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-20 17:20 +0000
  Re: enhance an array's static type by a lower length-bound. Roedy Green <see_website@mindprod.com.invalid> - 2011-08-17 22:11 -0700
    Re: enhance an array's static type by a lower length-bound. Lew <lewbloch@gmail.com> - 2011-08-17 23:17 -0700
      Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 10:41 +0000
        Re: enhance an array's static type by a lower length-bound. Patricia Shanahan <pats@acm.org> - 2011-08-18 06:34 -0700
          Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 18:08 +0000
            Re: enhance an array's static type by a lower length-bound. Roedy Green <see_website@mindprod.com.invalid> - 2011-08-18 17:53 -0700
              Re: enhance an array's static type by a lower length-bound. Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-20 17:31 +0000
                Re: enhance an array's static type by a lower length-bound. Lew <lewbloch@gmail.com> - 2011-08-20 10:50 -0700
                Re: enhance an array's static type by a lower length-bound. Robert Klemme <shortcutter@googlemail.com> - 2011-08-21 10:47 +0200
          Re: enhance an array's static type by a lower length-bound. Michal Kleczek <kleku@poczta.onet.pl> - 2011-08-21 20:27 +0200

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