Re: Side-Effects in Functions [Rosen Trick]

[dewar@gnat.com (Robert Dewar)]

pete@nospam <pete_member@newsguy.com> wrote in message news:<9s5eub02j61@drn.newsguy.com>...
> functions, in the mathemtical sense, do not accept an IN OUT paramters,
> and do not have side effects.
> 
> y=f(x), x here can never be modified by the function f().
> 
> so, Ada function is more like a math function.

Yes of course, that is the naive view that might be taught
in a beginning course, but it is bogus. Sure, Ada functions
can be used to model mathematical functions, that's true
in any case, but Ada functions are not a bit like math
functions. Mathematical functions cannot take pointer
arguments allowing modification of the calling variable,
mathematical functions cannot generate output on files,
mathematical functions cannot modify global variables.

Mathematical functions are very restrictive, and the language makes no
attempt to limit the semantics to
functions that have some sensible meaning as mathematical
functions.

That's the whole point here. Functions in Ada DO allow
side effects quite freely, and what seems odd is the
Ada rule in this case:

Functions are allowed to have side effects, providing that
there is no trace of this in the function specification :-)

The real issue here, and the point at which this becomes
significant, is that we use functions in Ada for interfacing to
foreign languages and there the restriction
is quite onerous, as anyone who has designed bindings to
C knows. Yes, access parameters help, but the requirement
that access parameters be non-null means that this usage
often is inapplicable.

For an example of functions that modify their parameters,
which is in fact possible in GNAT in some cases, using
implementation dependent features, see gnat.spitbol.patterns
(g-spipat.ads). Here the goal was
to model SNOBOL-4 style as closely as possible, and in
some cases, functions modifying their parameters lead to
the most natural usage from a SNOBOL-4 point of view. 
Since the point of this package is to assist in translation
of existing SNOBOL-4 programs, stylistic considerations
are significant. 

If (important criterion) you are familiar with SNOBOL4,
feel free to suggest other approaches that would avoid
this usage, I didn't find anything as convenient, and
would have preferred being able to program this in a more
direct manner.