assumed shape dummy arrays and implicit interfaces

[Richard E Maine]

On Sep 13, 2005, at 11:52 PM, THOMAS Paul Richard 169137 wrote:

> Why do we not always pass array descriptors?  Come to that, why do we 
> not
> store arrays as structures, rather than recreating descriptors all the 
> time?

I'm not sure what you mean by the question about structures, so I'll 
ignore it. As for why you don't always pass array descriptors, well... 
I can't testify as to why gfortran doesn't. I'm almost sure you could 
make such an implementation work and conform to the standard. But I 
know why the standard went to a lot of trouble... make than an *AWFUL 
LOT* of trouble to distinguish cases where you need to pass descriptors 
from those where you don't need to. Of course, the standard doesn't use 
that kind of implementation terminology, but the implications are 
there.

The standard doesn't *REQUIRE* an f95 compiler to be compatible with 
libraries compiled by f77, C, or anything other than itself. And your 
users wouldn't even bitch much if you didn't have such 
compatibility.... but that's only because you wouldn't have enough 
users to bitch much.  :-)

Since f77 compilers (with rare exceptions) didn't pass array 
descriptors, you had better not pass one when calling a routine that 
might have been compiled by an f77 compiler. An awful lot of the 
complication in distinguishing between what does and doesn't require an 
explicit interface has to do with compatibility with f77 (and somewhat 
with C, as a side effect, but f77 was the objective) compilers. You 
don't think that it would have been such a "mess" if it were designed 
from scratch without compatibility needs, do you? I don't.  Anything 
that could possibly have been written in f77 has to work with an 
implicit interface (since that's all f77 had) and has to not pass 
descriptors because f77 compilers (mostly) didn't. This has to work in 
both directions - an f90/f95/f2003 call to an f77 routine, or an 
f90/f95/f2003 routine being called from f77. But some new features need 
descriptors or something similar in order to work reasonably. The 
explicit interface lets you know whether you need the descriptor for 
new features or need to not have one for f77 compatibility.

-- 
Richard Maine                |  Good judgment comes from experience;
Richard.Maine@nasa.gov       |  experience comes from bad judgment.
                             |        -- Mark Twain