# Re: Modelling Disjoint Subtypes

From: Marshall <marshall.spight_at_gmail.com>
Date: 25 Mar 2007 13:55:01 -0700
Message-ID: <1174856101.649290.231830_at_p77g2000hsh.googlegroups.com>

On Mar 25, 11:53 am, "David Cressey" <cresse..._at_verizon.net> wrote:
> "Marshall" <marshall.spi..._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.

if (isInt(x) and isInt(y) {
// consider the code here
} else { ... }

Marshall Received on Sun Mar 25 2007 - 22:55:01 CEST

Original text of this message