Re: Why .ads as well as .adb?

[djakoogler <djakoogler@verizon.net>]

John,

There is one point in this discussion no has raised yet: There
may be several different implementations (bodies) for a given
specification.

In VHDL, a hardware design language which based itself on
Ada83 concepts, there is a specification for logic function.
The language allows for "architecture behavioral" and
"architecture structural" implementations against the
specification. The "behavioral" implementation defines the
defines the behavior of the function as an algorithm (much
as you write the function in Ada), while the "structural"
implementation gives a very precise gate-level or netlist
description of the function (how you actually do the function
with digital components). I believe there may be a third
such "architecture" available to describe an analog
(transistor) level description.

While Ada does not explicitly provide for parallel
implementations, you may find many GNAT based projects the
use the naming facilities of a GNAT Project File (.gpr) to
select different bodies to match certain configurations.
For instance, you may want to have one implementation which
is a stub and maybe has some tracing statements, and another
implementation which has the complete algorithm. Another
common variation is one implementation for Unix and another
for Windows. By using environment variables when launching
gprbuild, the user selects which bodies to use. To be
complete, you can use the same naming technique for specs
as well as bodies.

If you look at it in the right way, you can see where Java
and Ada interfaces are to do something similar except
interfaces allow the user to make a run-time selection not
a build-time selection. For my work with small machines,
I almost always need just one implementation at run-time
so build-time selection is the way I go.

Dave Koogler
Engineer