[Oberon] objects and jewels

Felix Friedrich felix.friedrich at inf.ethz.ch
Tue Feb 19 19:37:23 CET 2013

Dear Frans-Pieter,

procedure types are types such as INTEGER or REAL. If a variable is of 
procedure type T, then it means that it is capable of holding a pointer 
to executable code of a procedure that has the interface desribed by T. 
It is actually not more and not less. (It is slightly different in 
ActiveOberon with Delegates -- different story.)

This means that a variable, field or parameter of procedure type is not 
automatically bound to executable code, but the binding has to be 
provided by "the user". In your example, this could have been done via 
an assignment aLed.init := init

An attempt to call a non-initialized procedure variable usually results 
in a trap.


> Thanks Jorg,
> this was the interview I remembered. However the interview mentions
> extention of record types, but not the introduction of procedure types.
> I could not find simple examples of using procedure types, probably
> because of the introduction of type bound procedures with Oberon-2.
> The original oberon system uses procedure types a lot as part of the
> message concept. I wonder if the concept of message handling together with
> the task loop can be transfered to microcontrollers.
> Procedure Types are supported in Oberon07.
> To start simple, I was thinking of something like this
> MODULE ExampleProcType;
>   Led = RECORD
>           pin, port : INTEGER;
>           init : PROCEDURE(led : Led; Pin,Port:INTEGER)
>          END;
> VAR aLed : Led;
> PROCEDURE init (led: Led; Pin,Port: INTEGER);
> (*some code *)
> END init;
>    aLed.init(aLed,1,5)
> END ExampleProcType.
> This compiles in Astrobe. But i do not know what is happening here. For
> instance, if I change the name of the PROCEDURE init to PROCEDURE onit the
> module still compiles. So it seems that the procedure type init not binds
> with the procedure init.
> Could someone explain me a little more about procedure types?
> Greets,
> Frans-Pieter
>> Frans-Pieter
>>> I remember reading an argument of Wirth against object orientation.
>> However
>>> the object way, "myservo.atttach(9)" seems more elegant than the
> modular way. Did I overlook something?
>> You might consider this syntax as more elegant, but basically
>> object-oriented programming (OOP) does not add one single new concept to
> traditional procedural programming (PP). Read Wirth's argumentation
> below.
>> Whether you write in OOP "myservo.attach(9)" or write in PP
>> "SERVO.Attach(myservo, 9)" is the same.
>> br
>> Jörg
>> Wirth:
>>      "Many people tend to look at programming styles and languages like
>> religions: if you belong to one, you cannot belong to others. But this
> analogy is another fallacy. It is maintained for commercial reasons
> only. Object-oriented programming (OOP) solidly rests on the principles
> and concepts of traditional procedural programming (PP). OOP has not
> added a single novel concept, but it emphasizes two concepts much more
> strongly that
>> was done with procedural programming. The fist such concept is that of
> the procedure bound to a composite variable called object. (The binding
> of the procedure is the justification for it being called a method). The
> means for
>> this binding is the procedure variable (or record field), available in
> languages since the mid 1970s. The second concept is that of
> constructing a
>> new data type (called subclass) by extending a given type (the
>> superclass).
>>      It is worthwhile to note that along with the OOP paradigm came an
>> entirely new terminology with the purpose of mystifying the roots of
> OOP. Thus, whereas you used to be able to activate a procedure by
> calling it, one
>> now sends a message to the method. A new type is no longer built by
> extending a given type, but by defining a subclass which inherits its
> superclass. An interesting phenomenon is that many people learned for
> the first time about the important notions of data type, of
> encapsulation, and (perhaps) of information hiding when introduced to
> OOP. This alone would have made the introduction to OOP worthwhile, even
> if one didn't actually make use of its essence later on.
>>      Nevertheless, I consider OOP as an aspect of programming in the large;
>> that is, as an aspect that logically follows programming in the small
> and requires sound knowledge of procedural programming. Static
> modularization is
>> the first step towards OOP. It is much easier to understand and master than
>> full OOP, it's sufficient in most cases for writing good software, and
> is sadly neglected in most common languages (with the exception of Ada).
>>      In a way, OOP falls short of its promises. Our ultimate goal is
>> extensible programming (EP). By this, we mean the construction of
> hierarchies of modules, each module adding new functionality to the
> system.
>> EP implies that the addition of a module is possible without any change
> in the existing modules. They need not even be recompiled. New modules
> not only
>> add new procedures, but - more importantly - also new (extended) data
> types.
>> We have demonstrated the practicality and economy of this approach with the
>> design of the Oberon System."
> --
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> https://lists.inf.ethz.ch/mailman/listinfo/oberon

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