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TEAM-ADA  November 1998

TEAM-ADA November 1998

Subject:

"Classes" as packages in Ada

From:

Samuel Mize <[log in to unmask]>

Reply-To:

Samuel Mize <[log in to unmask]>

Date:

Tue, 24 Nov 1998 13:18:13 -0600

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (264 lines)

This message concerns object-oriented design, how that interacts with
the Ada language's design philosophy, and some thoughts on additions
to Ada that would better support object-oriented programming.

I'm cross-posting to comp.object, comp.lang.ada and the Team Ada
mailing list, hoping to get some input on this specific suggestion.
I've seen good comments between those groups in the past.

If you just flat think Ada 95 is bad for object-oriented work, or if
you think object-oriented development is nonsense, I hope you'll find
another thread in which to express that.  Thanks.

The particular mechanism I'm proposing is a set of "pragma"
(non-executing) statements.  These would tell the compiler about the
developer's intent for code structures surrounding a tagged type
(Ada's object-oriented programming construct).  This is explained in
detail below.

I've tried to state things so that a non-Ada programmer can
understand and contribute.


- - - - -
Ada's tagged types provide (in my opinion) excellent facilities for
object-oriented programming.  However, some people find it disturbing
that Ada doesn't provide a "class" wrapper to encapsulate an object's
data type and operations.

Each class can be "wrapped" with Ada's "package" -- its encapsulation
construct.  However, this is not required, and nothing indicates the
difference between a class and any other package.

I find this to be a partial and unsatisfactory answer.

It is not in keeping with Ada's general philosophy of expressing your
design intent in the code.  Doing so helps others, later, to
understand it.  It also lets the compiler help you find errors
earlier -- for instance, by telling you that a "positive" number has
just been assigned a negative value.

Note that Ada already provides a tremendous amount of such support
for object-oriented development.  For example, it isn't possible to
send a message to an object that has no method for that message.  I
believe that Ada provides as much automated checking of an
object-oriented design as C++, probably more (a different design
philosophy is at work there).

However, I want to express some checkable object-oriented semantics
that go beyond Ada's current expressive ability.

And -- again, because of the Ada 95 "toolbox" philosophy -- we can do
so in an orderly way.  A tool vendor can define a specific set of
semantic limitations, which we can then apply to selected parts of a
program.  This lets us define how object-oriented programming should
be done, and lets the compiler warn us when we fail to follow the
paradigm correctly.

The mechanism is Ada's "pragma" facility.  This lets the programmer
tell the compiler something about the program that is outside the
code.  The general theory is that a pragma shouldn't alter the
semantics of a legal program, but it may cause a program to become
illegal.  For example, there exists

    pragma Restrictions (...);

where one restriction may be that the program has no internal
multi-tasking.  The compiler can make simplifying assumptions to
generate more efficient code.  The compiler should reject a program
that both states and violates a restriction, although the program
would be legal without the pragma.

The "Restrictions" pragma is language-defined, but a compiler vendor
is allowed to define any other pragmas he wants.  (A different
compiler may ignore another vendor's pragmas, although it should warn
the user if a program contains a pragma that it does not recognize.)

I have thought of some pragmas that would let compiler help us
enforce our own restrictions when we are doing object-oriented
development, with an eye toward catching errors in coding or design.
I list them below.  My questions to you-all are:

1. Would checking for these semantic items be helpful to catch design
   or coding errors in an object-oriented design?

2. What other things might we look for?

3. Is there a better technical approach?


The specific list of pragmas follows.

- - - - -
1. pragma Class;

Legal directly inside the declarative region of a package (Ada's
module-building construct), plain or generic.

The package must export exactly one tagged type.  It may only export
subprograms in one of these categories:

- has one or more parameters of that tagged type
- has one or more parameters of the tagged type's class-wide type
- follows a Class_Subprograms pragma (see below)

This assures the package's user that this is a complete definition of the
class, and nothing else.

Ideally, the compiler would refuse to allow this type to be extended
except in another "Class" package, but I'm not sure if that kind of
restriction is possiible.

2. pragma Class_Subprograms;

Legal directly inside the declarative region of a "Class" package (plain
or generic), after the tagged type has been defined.

Subprograms following a "pragma Class_Subprograms" may have no tagged type
or class-wide parameters.  This allows for selectors and operations on the
class itself, such as a function to query how many objects of that class
exist, without requiring them to have dummy parameters of the tagged type
or its class-wide type.

Note that none of the class's methods can follow pragma Class_Subprograms.


3. pragma Inherits;
   pragma Inherits (subprogram_name...)

   pragma Inherits_Function (function_name, return_type, parameter_type...)
   pragma Inherits_Procedure (procedure_name, parameter_type...)

   pragma In_Fun (function_name, return_type, parameter_type...)
   pragma In_Pro (procedure_name, parameter_type...)

Legal directly inside the declarative region of a "Class" package (plain
or generic, although it is unlikely to be used in a generic).

With no parameters, "pragma Inherits" states that, for each potentially
dispatching (polymorphic) subprograms that this class's tagged type
inherits, this class either overrides that subprogram or names it in an
"Inherits" or "Hides" pragma.

With parameters, "pragma Inherits" must list subprograms that are
inherited.  Like "pragma Inline," all subprograms of that name are
inherited, and multiple "pragma Inherits" may be used.

The pragmas Inherits_Function and Inherits_Procedure let you specify the
parameter profile of the subprogram being inherited, in case subprograms
with the same name are distinguished by their profiles.

Pragmas In_Fun and In_Pro are abbreviations for Inherits_Function and
Inherits_Procedure, respectively.  (They are not intended to suggest that
functions are more enjoyable, or that procedures are more remunerative :-)

A subprogram listed in an Inherits pragma may not also be over-ridden.

All of these pragmas imply the semantics of a plain pragma Inherits.

This lets you enumerate, in a given class package, all the methods for
that class, even the ones it inherits.  It also helps catch spelling or
typing errors, where you intend to over-ride a parent class's method but
instead create a new message by mistake (e.g., trying to over-ride
"Initialize" with a procedure named "Initialise").


4. pragma Hides (subprogram_name...)

   pragma Hides_Function (function_name, return_type, parameter_type...)
   pragma Hides_Procedure (procedure_name, parameter_type...)

Legal directly inside the declarative region of a "Class" package (plain
or generic, although it is unlikely to be used in a generic).

"pragma Hides" must list subprograms that are inherited.  Like "pragma
Inline," all subprograms of that name are hidden, and multiple "pragma
Hides" may be used.

A subprogram listed in an Inherits pragma may not also be over-ridden.

All of these pragmas imply the semantics of a plain pragma Inherits.

If a subprogram listed in a Hides pragma is called, a new exception will
be raised.  If the compiler detects at run-time that this exception will
be raised, the compilation should fail.  (This obviously needs some work
to get it semantically precise.)

This breaks Ada's guarantee that no object can receive a message that it
doesn't have a method for, but it does so in an orderly way and in a way
that is visible in the package specification.  It is preferable, in a few
cases, to do so, rather than to inherit an irrational method.

"pragma Hides" implicitly over-rides the subprogram's body with one that
just raises the exception, and that is what any child of this class
inherits.  It may un-hide the subprogram by over-riding it.  However, it
cannot re-inherit the subprogram of the class's parent.


5. pragma Object_First;

Legal directly inside the declarative region of a "Class" package (plain
or generic).

In all parameter lists for the tagged type's subprograms, the controlling
tagged-type parameters will come first.  If there are none, the class-wide
parameters will come first.  If the subprogram is a function, "first" may
be either the return value, or the first parameter.

This enforces a regimen that, like an "object.method" syntax, helps the
reader to immediately identify the object that is receiving the message.

(Note that we can't really enforce the concept of "recipient" in Ada, but
this will at least help the user easily locate the right package.)

Ideally, the compiler would refuse to allow this type to be extended
except in another "Object_First" package, but I'm not sure if that kind of
restriction is possiible.


6. pragma Mixin;

Legal directly inside the declarative region of a generic "Class" package.

This generic must have exactly one tagged-type input parameter, and extend
it as its exported "class" tagged type.

This simply clarifies the intent of the programmer.


7. pragma Categorized;
   pragma Category (Identifier, Subprogram_Identifier...);

Legal directly inside the declarative region of a "Class" package (plain
or generic).  "pragma Category()" must follow the declarations of the
subprograms in its parameter list.

"pragma Categorized" causes compilation to fail unless all methods of
the class (subprograms that precede any "pragma Class_Subprograms")
are listed in a "pragma Category".

"pragma Category" indicates that the listed subprograms fall into the
named category.

This is intended to communicate with analysis tools.  For instance, a
tool may draw a line for each message that objects of one class send
to objects of another; all the subprograms in a category could be
lumped into one line, so that instead of showing 20 control functions
the diagram would show only one line labelled "control".

The identifier is arbitrary, and more than one "pragma Category" in a
given compilation may use the same identifier.

"pragma Category" may be used without "pragma Categorized" to indicate
categories for a subset of the class's methods.

- - - - -
That ends the pragmas I have thought of to date.

Best,
Sam Mize

--
Samuel Mize -- [log in to unmask] (home email) -- Team Ada
Fight Spam: see http://www.cauce.org/ \\\ Smert Spamonam

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