[M3devel] subranges 0..-1?

[Tony Hosking]

Sure, they make sense.  Just like arrays with no elements.  I don't understand your fearfulness, unless it is just C poisoning your thinking.

On 4 Oct 2010, at 13:16, Jay K wrote:

> 
> It worries me somewhat that we allow zero-sized types.
> Including TYPE Foo = RECORD END;
> It is reassuring to me that C and C++ doesn't have these.
> I don't want to think about what they mean.
> Do they make sense to others?
> 
> C requires a field. C++ adds a dummy one internally, that it can/must try
> to optimize away when inheritance is involved but still.
> 
> 
> C++ is interested in the property of pointers not being equal, like:
> 
> TYPE T1 = RECORD END;
> TYPE T2 = RECORD a,b:T1 END;
> 
> VAR a:T1;
> ADR(a.b) # ADR(a.a.);
> 
> But in Modula-3 they would have the same address.
> What does it mean to take the address of something that doesn't exist?
> Perhaps it isn't allowed?
> 
> As well this kind of thing worries me because Modula-3 truly is
> mostly trivially isomorphic with C, which means it is easy to imagine
> the gcc backend can handle it. This is an exception.
> Sure, there probably is a transform to C, it just isn't obvious to me.
> (Granted as well, the gcc backend handles Ada, and Pascal,
> so it isn't so limited. Ada at least superficially looks like
> Modula-3, I was surprised to see.)
> 
>  - Jay
> 
> ________________________________
>> Subject: Re: [M3devel] subranges 0..-1?
>> From: hosking at cs.purdue.edu
>> Date: Mon, 4 Oct 2010 00:08:50 -0400
>> CC: jay.krell at cornell.edu; m3devel at elegosoft.com
>> To: ttmrichter at gmail.com
>> 
>> It's an array with no elements, since the subrange [0..-1] is empty.
>> 
>> On 3 Oct 2010, at 22:23, Michael Richter wrote:
>> 
>> What does an ARRAY [0..-1] OF actually look like?
>> 
>> On 3 October 2010 20:51, Jay K
>>> wrote:
>> 
>> ../src/Common/X.i3: In function 'X_I3':
>> ../src/Common/X.i3:4845:0: internal compiler error: in
>> m3cg_declare_subrange, at m3cg/parse.c:3567
>> 
>> 
>> gcc_assert (min <= max);
>> 
>> XClientMessageEvent = RECORD
>> type: int;
>> serial: unsigned_long; (* # of last request processed by server *)
>> send_event: Bool; (* true if this came from a SendEvent
>> request *)
>> display: DisplayStar; (* Display the event was read from *)
>> window: Window;
>> message_type: Atom;
>> format: int;
>> alignment: ARRAY [0..-1] OF long;
>> data: ARRAY [0..MAX (20 * BYTESIZE (char),
>> MAX (10 * BYTESIZE (short),
>> 5 * BYTESIZE (long))) - 1] OF char; END;
>> 
>> 
>> alignment: ARRAY [0..-1] OF long;
>> 
>> 
>> typedef struct {
>> int type;
>> unsigned long serial; /* # of last request processed by server */
>> Bool send_event; /* true if this came from a SendEvent request */
>> Display *display; /* Display the event was read from */
>> Window window;
>> Atom message_type;
>> int format;
>> union {
>> char b[20];
>> short s[10];
>> long l[5];
>> } data;
>> } XClientMessageEvent;
>> 
>> 
>> 
>> eh?
>> 
>> 
>> I'll remove the assert.
>> "Clearly"? the Modula-3 should just use an array of long though for the
>> union?
>> 
>> 
>> - Jay
>> 
>> 
>> 
>> 
>> 
>> --
>> "Perhaps people don't believe this, but throughout all of the
>> discussions of entering China our focus has really been what's best for
>> the Chinese people. It's not been about our revenue or profit or
>> whatnot."
>> --Sergey Brin, demonstrating the emptiness of the "don't be evil" mantra.
>> 
>