Re: Enumerated "procedural" parameters

[milne@ICSE.UCI.EDU (Alastair Milne)]

    Using an enumeration-indexed case to choose the procedure to be called
    will work if all the procedures to be called are known and available at
    the time that case is compiled.  It will therefore work if all the
    enumeration-passing calls are in the same program unit as the case, or if
    they are exported by a package which will permit only "procedures" known
    to it (its own, or others from packages used in the specification) to be 
    passed in.

    It will not work in a library package exporting routines to be applied to 
    routines from units not known to the package.  Routines that come to mind 
    immediately are in the field of numeric analysis: curve fitting, best fits,
    adaptive quadrature, etc. .  In these cases what one needs is a routine 
    which, given some routine x to produce original function values, calls 
    that routine as it needs for input to its own algorithm.  Such packages
    would be essential parts of libraries for numeric analysis.  Their routines
    could be applied to any data-producing routine a user supplied,
    with algorithms 50 years old or produced yesterday.  But in Ada, the old 
    approach of simply passing that routine as another parameter is 
    not available.

    A good point has already been made on this board that in using Ada, one
    can find that the way one used to do something is no longer available,
    only to discover that it can be done in new and superior, more secure ways.
    I hope that will turn out to be so of this case.


    Alastair Milne