Procedure Parameters in Ada

[drm1@vrdxhq.UUCP (Donn Milton)]

Many people seem to be unaware of a major problem entailed by
procedures as parameters.  That is, when procedures are allowed
as parameters, strong type-checking becomes generally undecidable
(not just hard) UNLESS recursive mode-type declarations (a la Algol 68)
are introduced.  For example, in languages like Fortran, Algol 60,
PL/1, and C, strong type-checking cannot be done.  This was proved
formally by Langmaack in "On Correct Procedure Parameter
Transmission in Higher Programming Languages," Acta Informatica,
Vol. 2, 1973, pp. 110-142.

With the declaration mechanism available in Algol 68, it is possible
to declare "a procedure that has two parameters: an integer and a
procedure that has two parameters: an integer and a procedure that
has two parameters ..." ad infinitum, which is required for strong
type-checking of procedure parameters.  Understand that
type equivalence in Algol 68 means structural equivalence,
while type equivalence in Ada means name equivalence -- thus
type-checking is much simpler in Ada.

However, I do not understand why such a declaration mechanism, which
would allow procedures as parameters with strong type-checking,
was not included in Ada.  Algol 68 had demonstrated its
feasibility (with some elegance), and, as far as I know,
this capability is not a source of significant compile-time
or run-time inefficiencies.

Donn Milton