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In article <2740@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
.In article <7161@sol.ARPA> crowl@cs.rochester.edu (Lawrence Crowl) writes:
.>[Text by Niklaus Wirth]
.>With the proliferation of basic types, a relaxation of compatibility rules
.>between them becomes almost mandatory.  To this end, the notion  of type
.>inclusion is introduced: a type T includes a type T', if the values  of T'
.>are also values of type T. Oberon postulates the following hierarchy:
.>	LONGREAL > REAL > LONGINT > INTEGER > SHORTINT
.
.The basic problem is that the values of a 32 bit real do not include the
.values of a 32 bit integer.  If one is going to be meticulous enough about
.assignments to define type inclusion, one should only make types be included
.when they really are.

This depends on what is meant by value.  Including precision, you are
right; n bit reals cannot exactly represent all the n bit integers.  But,
if you allow the precision to 'slack off', n bit reals CAN represent
all the n bit integers.  But the mapping is not 1:1; it is many:1 (from
ints to reals); this means that in the following case:

VAR IX, IY : INTEGER;	{integers are 32 bits}
    RZ : REAL;		{reals are 32 bits}
...
{Somethings sets IX}
...
RZ := IX;
IY := INTEGER(RZ);

IX and IY are not (necessarily) equal.

But I wonder:  in this circumstance, is it possible for the IY assignment
statement to ever fail (ie, produce a run-time error)?  I would hope not.


A lot of people have praised Pascal and Modula-2 BECAUSE there were no
implicit type conversions.  Me, I'm not sure whether this is good or bad.
Personally, I make most of my type conversions explicit anyway in most of the
languages I use.  Does it really add all that much complexity to the
compiler; or, more importantly, does the compiler generate better code if
the conversions are implicit (Personally, I can't see this happening)?
-- 
 _ __			NEVIN J. LIBER	..!ihnp4!ihlpf!nevin1	(312) 510-6194
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