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From: rjh@cs.purdue.edu (Bob Hathaway)
Newsgroups: comp.lang.ada
Subject: Re: Garbage Collection
Message-ID: <5696@medusa.cs.purdue.edu>
Date: 27 Dec 88 22:24:22 GMT
References: <4195@enea.se>
Sender: news@cs.purdue.EDU
Reply-To: rjh@cs.purdue.edu (Bob Hathaway)
Organization: Department of Computer Science, Purdue University
List-Id: <comp.lang.ada>

>According to Bill the data allocated to Reference should be freed when 
>we exit this procedure, but if Insert does what we think this is of
>course a severe error. Where did I go wrong? Did I plead guilty to

I think Bill meant deallocation should only occur if an object is no
longer accessible at the end of a subprogram or block, and Reference is 
accessible.  Subprogram and block exit is a very convenient time to perform 
clean-up of access types.  If Ada had an Adt construct for the "completion of
the Adt paradigm", we could specify a termination procedure for Adts which
could also be called at scope exit.  I agree that Adts/objects are an important
enough programming development to justify a new language construct.  I've 
worked with over 200,000 lines of code which used the older data-structure
oriented design and found the greatest problem in understanding and working
with these programs was caused by the use of global variables and the 
indescriminant access and modification of data-structures which were not 
implemented as Adts.  Such data-structures are poorly defined and lead to
code which is hard to read.

For my own opinion on the garbage collection discussion, I think a destroy 
procedure is desirable.  Adts need to be explicitly deallocated for long 
running daemons or loops even while objects are still accessible.  While 
calling a procedure or declaring objects in a local block from within a loop
allows automatic deallocation of objects, I'd rather use scope rules and 
subprograms for creating environments and performing actions and not for 
storage (de)allocation.  Also, I think Adt operations should include a destroy
procedure for completeness. If space management is not a problem the destroy 
procedure does not have to be called.  Concerning working sets, if a page is 
not accessed it will not reside in memory for long and should not cause a 
virtual space management problem.

I strongly agree with assignment operator overloading, and another
powerful extension is to allow any name/operator to be overloadable.

For yet another Ada 9X extension, I propose procedural variables.  As in 
Modula procedural variables can be limited to top level procedures but formal
parameter names must be included for named parameter association.  Here is an
example declaration:
    type insertNode_procedure = procedure (adt : in out structure,
					   node : in node);
Variables of type insertNode_procedure can be assigned to any procedure with
the same parameter type structure (type domain).  Procedural variables can
avoid the redundant use of case statements by allowing an operation to be 
stored within an Adt.  It also allows individually parameterizable and 
reprogrammable Adts since operations can be provided to alter the Adts actions
or structure.  Generic subprogram parameters can only allow Adts to be set 
once for any instantiation.  I use procedural variables and function pointers
in Adts frequently when programming in languages other than Ada and am 
convinced they are an elegant way to model Adt actions.

                                         Bob Hathaway
                                         rjh@purdue.edu