Re: ada and robots

[gwinn@res.ray.com (Joe Gwinn)]

In article <5mv984$7kn@news.emi.com>, jejo@empireone.net (Jesse Johnson) wrote:

> Ada was designed by the DOD as a systems programming language
> for embedded systems in military vehicles, while C was designed as an
> application programming language to be run on top of systems.  This
> makes Ada inherently better suited for robot programming.  However,
> for someone like me who has done a number of years of application
> programming and is now trying robotics, it's easier to use C because I
> already know it.

Umm, C was written as a systems (that is, operating systems) programming
language, riding right on the metal.  The comparison was assembly code.  C
was intended to be in effect a machine-independent assembly language, so
that an OS (UNIX) written in C would be portable from platform to
platform.  

Read the Introduction section of K&R to get the short history and original
intent of C.  For a non-Ada comparison, read the Introduction section on
page 133 of the "Pascal User Manual and Report", 2nd ed, Jensen and Wirth,
Springer-Verlag 1974.  Recall that Pascal was the base language upon which
Ada was based.  Reading these side by side is quite illuminating.

So, one cannot conclude on the basis of original intent alone that Ada is
more suited to robot programming, as both were intended from the start to
ride the metal.  

In practice, C was much more successful than Ada83 at riding metal. 
Experience with Ada83 shows that it is very bad at direct control of
hardware, especially I/O hardware, and simply does not handle shared
memory correctly.  Ada95 is claimed to be better, but I don't have any
direct experience with it.  Actually, not many people do just yet.  I
suspect that most people are using C (perhaps called from Ada) for direct
control of I/O hardware and the like.

Most "Ada systems" I have seen built recently are actually mixed-language
systems, being a mix of C, C++, and Ada, with the Ada being in the
minority (maybe 25%), if one counts all the purchased COTS code in the
system.  Typically, the operating system, middleware, and GUI stuff are
all in C, and the application code is mostly in Ada (with C bindings, and
no Ada runtime).

Not all Ada compilers offer adequate support for such mixed-language
systems, especially when it comes time to debug on target (vice host)
systems.  This can be a severe problem, because it's generally impossible
to build a competitive offering without extensive use of COTS components,
which are almost always in C/C++, and impossible to get working soon
enough to matter without good debugging support.  Whatever set of
compilers and tools you plan to use should be demonstrated to do *exactly*
what you need done, with sufficient performance, before purchase.  Don't
assume that anything works until proven.  (A year ago, we very nearly
bought an Ada that couldn't even link a mixed-language (C and Ada83)
VxWorks system, let alone execute the code and debug.  Insisting on a true
and accurate demo saved our butts.)

I would also comment that DoD's recent recinding of the Ada Mandate will
likely cause the Ada compiler and tools market to shrink to perhaps as
little as one tenth of its prior size, and that significant added
investment in Ada compilers and tools is therefore unlikely, at least
until the size of the remaining market becomes clear.  A significant
shrink is widely expected because the many DoD customers who chose Ada
because of the mandate will now immediately vanish, and DoD's big push
towards COTS is in effect a push to C/C++.  The Ada market used to be at
most one tenth the size of the C market, and things took forever to get to
the Ada world.  Now, it will be one hundredth.  So, if something Ada is
not in hand today, don't wait.  It could be a long time coming.  

Joe Gwinn