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Re: Modelling Disjoint Subtypes

From: David Cressey <cressey73_at_verizon.net>
Date: Sun, 25 Mar 2007 19:53:51 GMT
Message-ID: <jrANh.1318$J21.1047@trndny03>

"Marshall" <marshall.spight_at_gmail.com> wrote in message news:1174834822.194458.248230_at_y80g2000hsf.googlegroups.com...
> On Mar 25, 3:26 am, Joe Thurbon <use..._at_thurbon.com> wrote:
> > Marshall wrote:
> >
> > > Having declared to the system that the type of S1.t is
> > > a unit type (meaning the type has exactly one value)
> > > the system is free not to store anything for the t column.
> >
> > Hi Marshall,
> >
> > To declare the type of the column, you'll need some storage. That is, I
> > think what your suggesting moves some of the space required 'out of the
> > column values' and 'into the column definition'. If reckon that to store
> > one of N types, you'll need at least log2(N) bits.
>
> Yeah, that's a fair point. The column-type storage costs are
> O(1), vs. O(count(R)), but it's still some storage.
>
>
> > > This is possible because the number of bits needed to
> > > represent a value of a given type is the log2 of the number
> > > of possible values. Since T1 has 1 value, the number
> > > of bits needed is log2(1) = 0.
> >
> > But there are now N extra types, and you'll need to store them
somewhere.
>
> As an aside, there exists systems in which the storage cost
> *at runtime* for type information is zero, because the types
> exist only at compile time, and are completely removed
> after.

If you are saying what I think you are saying, then I disagree.

For example, let's I have

   float x, a, b;
   x= a + b

If I look at the variables x, a, and b at runtime, the type is gone. But if I look at the code generated by the compiler, I'm going to find that the plus sign is represented by a floating point addition operation. So, to the extent that operator indicates type of operand, the information is still there at runtime, although buried in the code.

This could be important if you were writing a decompiler. Received on Sun Mar 25 2007 - 14:53:51 CDT

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