Re: Ada and Java. different behaviour. casting long to int problem.

[Keith Thompson <kst@cts.com>]

D'Arcy Smith <nospam@itools.symantec.com> writes:
> If you're sent into a room that is full of pins sticking up from the
> floor - and you are told that it is full of pins - and you run into
> the room with your eyes closed then it is 100% your fault.

(I took the liberty of correcting some typos in the quoted text.)

If I hire someone to build me a house, and the living room is full of
pins sticking up from the floor -- and the builder clearly tells me
about this feature -- it's my fault if I run into the living room with
my eyes closed.  But I'm going to be a bit upset with the builder for
building it that way in the first place.

No programming language implementation can accurately and efficiently
model arithmetic on the full set of mathematical integers.  (If it
can't handle 10**(10**(10**100)), it's not modeling the full
mathematical set.)  So, some compromises are necessary, typically by
guaranteeing mathematical correctness only for a limited range (32
bits, 64 bits, whatever).  My favorite compromise is the one that
says:

    For any arithmetic operation on integers, you will get the
    mathematically correct result if it fits in the representation;
    otherwise, you will be clearly informed that the implementation
    was unable to do so (via an exception or some similar mechanism).

For most purposes, I believe this is *better* than a compromise that
says,

    For any arithmetic operation on integers, you will get the
    mathematically correct result if it fits in the representation;
    otherwise, you will get a mathematically incorrect (but perhaps
    well-defined) result, with no straightforward way to detect that
    it's mathematically incorrect.

This assumes, of course, that the abstraction you're trying to
(partially) model is the mathematical set of integers -- which it
usually is.  Some people have argued that wraparound semantics, if
they're well-defined, are as "correct" as anything else.  Sorry, but I
don't buy it.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center  <http://www.sdsc.edu>                 <*>
One of the great tragedies of ancient history is that Helen of Troy
lived before the invention of the champagne bottle.