🚀 go-pugleaf

Message-Id:   <01.23294@ame.nbg.sub.org>

EEPJM%WOMBAT.NEWCASTLE.EDU.AU@USENET.ZER wrote on Tue, 12-15-1992 at 23:47 h
under the subject 'Re: Generic stack implementation':

> I know that Wirth's original idea was that uses of SYSTEM should be
> rare, so that importation of SYSTEM should act as a marker that
> something suspect is being done; but IMO this has failed because:
>  (a) there are some non-portable features which were (accidentally?)
>      left out of SYSTEM - e.g. address arithmetic, type BITSET,

Both problems solved by ISO_M2: 

1. Address arithmetic can only be done by SYSTEM.(ADDADR, SUBADR, DIFADR).

2. A new keyword PACKEDSET in addition to SET was introduced in ISO_M2
  to be able to declare a type such as BITSET. 
  
  TYPE BITSET = PACKEDSET OF [0..SIZE(BITSET)*8-1];
  
 The type BITSET is still pervasive and is not in SYSTEM (as in PiM).

>      type casting - so that it's possible to write non-portable code

Solved by ISO_M2's SYSTEM.CAST (T, expr). 
PiM's form T(expr) cast is no longer allowed.

>      without ever importing from SYSTEM.  This is especially evident
>      among programmers who learnt C before learning Modula-2.

You shouldn't learn C before Modula-2! This has to be guaranteed by
those who are responsible for programmers' education. :-)

>  (b) SYSTEM is too big, and contains some things that don't belong
>      there.  Every man and his dog ends up importing from SYSTEM,
>      so that uses of SYSTEM no longer stand out as being something
>      unusual.

> The solution, I believe, is to break up SYSTEM into a set of smaller
> low-level modules, to give the reader of a source listing a better
> idea of what low-level features are being used.  (Many M2 implementations
> already do this to a limited extent.  For example, it's become common
> to put TRANSFER/IOTRANSFER in a module other than SYSTEM.  This is a
> good example of something which didn't belong in SYSTEM in the first
> place.)  The low-level features of Modula-2 are certainly worth
> keeping (otherwise you drive people like me towards C and possible
> suicide), but their uses need to be flagged in a more visible way.

Already done in ISO_M2. SYSTEM is now very small, although individual
implementations are allowed to extend it. ISO_M2 has several
additional system modules for 

  -library exceptions,
  -language exceptions, 
  -termination, and
  -coroutines.

> One obvious approach, by the way, is to have a compiler option to
> generate warning messages for things which might be non-portable
> - e.g. type-casting.  Strangely, I've never seen a M2 compiler

No warnings needed in ISO_M2.

Guenter Dotzel, 
ModulaWare GmbH, Wilhelmstr. 17A, D-W 8520 Erlangen/F.R.Germany
Tel. +49 (9131) 208395, Fax +49 (9131) 28205.
E-mail/Internet: 100023.2527@compuserve.com
                 g_dotzel@ame.nbg.sub.org