Re: Why would anyone want opacity?

[Matthias Blume <blume@CS.Princeton.EDU>]

From: "Guillermo J. Rozas" <gjr@martigny.ai.mit.edu>
Subject: Re: Why would anyone want opacity?
Date: Mon, 13 May 96 15:10:16 -0400

> |   Date: Mon, 13 May 1996 15:02:07 -0400
> |   From: Matthias Blume <blume@CS.Princeton.EDU>
> 
> |   Of course there are times when one needs such things.  My objection is
> |   to making generic numbers the *default*.  Most of the numbers in my
> |   programs are small integers, and I expect this to be true even in
> |   heavy generic math packages.  We can safely assume that loop indices,
> |   array subscripts, etc., are not quaternions most of the time.  And
> |   most of the numbers (at least in my programs) are loop indices of some
> |   sort.
> 
> Why not make it the default?  I can only imagine efficiency being the
> issue, and there are ways to overcome that.  In addition, having to
> detect overflow causes the same level of inefficiency.

Detecting overflow doesn't cause any overhead on many machines,
because they have integer instructions that trap on overflow.

> |   If you really need to use a generic Number type it would be better to
> |   define a signature for the corresponding algebraic structure (pun
> |   intended) and abstract your code over this structure.
> 
> Right, and then I would have had to modify the assembler instead of
> using it verbatim.  Which version is saving me work?

My version saves *you* work.  Your version saves the compiler (or the
linker) some work.

> |   Moreover, the new standard basis for ML defined the structure Int32,
> |   which provides 32-bit integers for those who need them.
> 
> So they've made it even more concrete instead of abstract.  It seems
> like the wrong thing.

It is what users are asking for.  They are writing OS-level code in ML
(see FoxNet).  Is anybody writing OS-level code in R4RS Scheme?

-Matthias