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

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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.

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

> 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.  What you propose would be useful only if the added 
property of arrays would allow for *compile time* checking - 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;

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

And finally the question: how often do you know the array length in 
advance?  How useful is this in practice?  I have the feeling that 
people would use this as a lazy shortcut for defining a class with n 
integer fields.  And then how does the usefulness weight against the 
cost of realizing this?  I think your intuition (that we won't see this) 
was pretty good already. :-)

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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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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