Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!decvax!ima!cfisun!stellar!keith
From: keith@stellar.UUCP (Keith Crews @stellar)
Newsgroups: comp.lang.fortran
Subject: extensions to logical variables
Keywords: vax,logicals
Message-ID: <31660@stellar.UUCP>
Date: 26 Apr 89 12:53:24 GMT
Organization: Stellar Computer Inc. Newton, MA
Lines: 76

I have some questions about common fortran extensions to logical variables.
Some fortrans (such as vms) have extended logicals so that they behave a lot
like integers in most contexts.  For example, you can assign them values
other than 0 and -1 (or whatever the conventional values for false and true
are) and you can do arithmetic on them.  However if you have assigned
values to them other than 0 or -1 you then get some strange results.
You get logical variables that are neither true nor false (kind of like
ieee nans) and you can have 2 non-zero logicals variables that don't have
the same truth value.  

The basic problem I have with the vax implementation is that I think that 
a logical variable should always be true or false.  You can do this by 
forcing a conversion to 0 or -1 when assigning to them (although this can 
be defeated by using equivalences) or you can treat all non-zero logicals 
as being true and only 0 as being false.  However, the vax and other compilers 
seem to have extended logicals by allowing them to act like integers in 
many contexts while leaving the underlying implementation alone and the 
result is that using extensions to logicals can result in some surprising 
results.

My questions are:  
    1)  Do people depend on having logicals that are neither true nor false?  
    2)  What would people consider a consistent set of semantic rules for 
these extensions to logicals?  
    3)  Should .eq. and .eqv. behave identically with logical operands 
or should .eq. do integer comparisons and .eqv. check for both zero or 
both non-zero?

If you allow 2 distinct non-zero logicals to not be equal (.eq. different from
.eqv. in 3 above) then you have another problem where the 2 expressions below 
may evaluate differently:

	l1 = 3
	l2 = .true.
	if (l1 .eq. .true.) ...
	if (l1 .eq. l2) ...

However, if you evaluate them the same then you are incompatible with vms
fortran which seems like a dubious idea.

I guess the real issue is whether I have missed a unifying idea under-
lying the vms implementation.

Here are a couple of examples:

This prints 0 on the vax:

	logical l1
	l1 = 3
	i = 0
	if (l1 .eq. .true.) then
	    i = 1
	endif
	if (l1 .eq. .false.) then
	    i = 2
	endif
	print *, i
	end

This prints 2 on the vax and 1 on the sun:

	logical l1, l2
	l1 = 3
	l2 = 4
	i = 0
	if (l1 .eqv. l2) then
	    i = 1
	endif
	if (l1 .neqv. l2) then
	    i = i + 2
	endif
	print *,i
	end
-- 
Keith Crews			Stellar Computer Inc.
				95 Wells Avenue, Newton, MA 02159