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From: new@ee.udel.edu (Darren New)
Newsgroups: comp.lang.modula3
Subject: Questions from a new advocate of Modula-3
Message-ID: <44046@nigel.ee.udel.edu>
Date: 7 Feb 91 21:10:46 GMT
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Maybe somebody out there can answer a couple of questions for me about
Mod-3, especially if you have a running version (since all I have is
the report):

(Note: I can FTP to gatekeeper, so a pointer to the appropriate month were
this was discussed would be enough. Thanks!)

1) The type of an expression is the set of all values the expression can
take on.  By definition, a constant expression can only take on one value.
So if I declare something as

PROCEDURE x(y := 6);

does this not imply that the type of y is [6..6]?  What is the type really,
and why is it INTEGER instead of CARDINAL (if it is)?  Should the
report not say something about "base type" instead in this case?

2) The NEW expression's behaviour is not defined in the case that not
enough memory is left.  Is this a checked runtime error, or does it
return NIL?  If it returns NIL, then it isn't returning a "new" value.
If it is a checked runtime error, how do you allocate all available
memory without crashing?

3) A new variable is initialized to one of the members of the type.
Does this mean that (say) a UNTRACED REF REAL could be initialized to point
to any REAL anywhere in memory, or only to a REAL that has been
actually allocated via NEW?  I.e., if REALs must be on 4-byte boundries,
could a newly-declared but otherwise uninitialized UNTRACED REF REAL point
to *any* 4-byte-aligned quantity, or to any in the untraced heap, or
to any previously-initialized real, or what? Does an initialized nonNIL
REF necessarily point to initialized data?

4) BIT n OF t <: t    -- I can accept that
   t <: BIT n OF t    -- What about BIT 2 OF [0 .. 255]? Is this a static
      error, a checked runtime error, or implementation dependant?

5) Why try for generic packages given that objects can support generic
   types (like the Queue and IntQueue example)?

6) I would think that REAL <: LONGREAL, but that doesn't appear to be the case.

7) Has an "assert" capability been considered?  Maybe preconditions and
   postconditions could be included.  It seems like this would make
   for a much "safer" situation where an unsafe module exports a safe
   interface.  Asserts in interfaces could help document the semantics
   of the interface.

------------

Then some comments:

REFs should be defined to be initialized to NIL (unless explicitly
initialized). This would catch more errors because a pointer that
missed being initialized could not point to valid data possibly being
used by some other module/thread/whatever. This would also prevent
code which assumed that pointer are initialized to NIL (probably the
vast majority of cases) from failing when it isn't (Holy VAXisms, batman!).

An interface should be defined which specifies the ranges of integers,
floats, etc.  (Like LIMITS.H in ANSI C)


--- Darren New --- Grad Student --- CIS --- Univ. of Delaware ---
----- Network Protocols, Graphics, Programming Languages, 
      Formal Description Techniques (esp. Estelle), Coffee, Amigas -----
              =+=+=+ Let GROPE be an N-tuple where ... +=+=+=
-- 
--- Darren New --- Grad Student --- CIS --- Univ. of Delaware ---
----- Network Protocols, Graphics, Programming Languages, 
      Formal Description Techniques (esp. Estelle), Coffee, Amigas -----
              =+=+=+ Let GROPE be an N-tuple where ... +=+=+=