Re: Modelling Disjoint Subtypes

On Mar 26, 12:56 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> David Cressey wrote:
> > "Marshall" <marshall.spi..._at_gmail.com> wrote in message
> >news:1174937535.353341.203170_at_p77g2000hsh.googlegroups.com...
>
> >>On Mar 26, 7:18 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>>But even if the type checking were deferred until runtime in the
> >>>conditional test, the type of x and y is statically known in the block
> >>>under consideration.
>
> >>THAT was the point I was trying to make.
>
> >>Regardless of how we got there, inside the block, the compiler
> >>knows statically that the x and y values are integers, even
> >>though their declared type in the code is rational. There can
> >>exist implementations of rationals such that when the rationals
> >>are integers, the generated code for a multiply is identical to
> >>the generated code for multiplying integers. So within the
> >>context of that block, looking at the generated code, you
> >>would not be able to distinguish those two cases. The
> >>types have been erased.
>
> > OK, now you've lost me. Why is it necessary to distinguish between the case
> > where x and y are integers, and the case where x and y are rationals that
> > are constrained to take on only integer values?
>
> > It seems like a distinction without a difference to me.
>
> Marshall can correct me if I am wrong: I think he was only suggesting
> the optimizer could use faster integer arithmetic for x and y in the
> block under consideration (assuming normalized rationals.)
>
> ie. the compiler knows x and y have more specific types than their
> declared types within the identified block of code.

Yes.

Further, in the case that the compiler does so, the object code will present the appearance of having been derived from source code using the integer type, when in fact it was derived from source code using the rational type. Thus, the object code will embody misleading information. We could perhaps even call it disinformation.

We can say little or nothing with certainty from examining the object code. (In a language with type erasure.)

Marshall Received on Tue Mar 27 2007 - 00:33:04 CEST