[Oberon] Protocols (interfaces) in Oberon-2
Charles Perkins
chuck at kuracali.com
Tue Oct 27 16:35:16 CET 2020
Hi Jörg,
I think Luca is looking for the same thing I am, which is to separate the
users of an interface from the implementers of the interface more fully
than it is in Oberon currently.
I have been exploring your proposed syntax for interfaces and it almost
does what I want but Lucas has identified the gap in semantics between your
syntax and what I want to do.
An important part of interfaces is compatibility of assignment, in addition
to method invocation. I wish to be able to refer to anything that satisfies
an interface as well and call the compatible procedures of that interface.
It may be the case that assignment capability is not possible without the
introduction of ANYREC and ANYPTR as Luca suggests.
Gonna have to think on that...
Thanks for the very interesting conversation!
Cheers,
Chuck
On Tue, Oct 27, 2020 at 8:09 AM Jörg <joerg.straube at iaeth.ch> wrote:
> Why?
>
> Jörg
>
>
>
> *Von: *Oberon <oberon-bounces at lists.inf.ethz.ch> im Auftrag von Luca
> Boasso <luke.boasso at gmail.com>
> *Antworten an: *ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> *Datum: *Dienstag, 27. Oktober 2020 um 16:08
> *An: *ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> *Betreff: *Re: [Oberon] Protocols (interfaces) in Oberon-2
>
>
>
> The problem is not the syntax but the semantic.
>
> This should be allowed:
>
>
>
> VAR i: I; r: R;
> inc: Incrementer;
>
> inc := i;
> inc := r;
>
>
>
>
>
> On Tue, Oct 27, 2020 at 10:02 AM Jörg <joerg.straube at iaeth.ch> wrote:
>
> Luca
>
>
>
> I fully agree that with your proposed INTERFACE syntax the setup of the
> environment is much easier.
>
> My proposed MODULE structure is more cumbersome to setup.
>
>
>
> After you set up everything (implemented all needed interface
> procedures…), is there a big difference whether you write
>
> in your syntax
>
> NEW(i);
> i.h := 3;
>
> inc := i;
> inc.IncBy2;
>
>
>
> or in my syntax
>
> New(i);
> i.h := 3;
>
> i.inc.IncBy2(i);
>
>
>
> br
>
> Joerg
>
>
>
> *Von: *Oberon <oberon-bounces at lists.inf.ethz.ch> im Auftrag von Luca
> Boasso <luke.boasso at gmail.com>
> *Antworten an: *ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> *Datum: *Dienstag, 27. Oktober 2020 um 15:37
> *An: *ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> *Betreff: *Re: [Oberon] Protocols (interfaces) in Oberon-2
>
>
>
> Although this is a solution to the problem, it doesn't buy you much
> because all generic signatures must have the type *Data.Any*.
>
> To lift the limitation that the clients must know about the module *Data*
> we could extend Oberon-07 to support Component Pascal's ANYPTR type,
> defined as
>
>
>
> ANYREC and ANYPTR
> Each base record is implicitly regarded as an extension of the new
> abstract standard type ANYREC,
> even if it is declared without explicit base type. ANYREC is an empty
> record that forms the root of all
> record type hierarchies. ANYPTR is a new standard type that corresponds to
> a POINTER TO
> ANYREC.
> These new types make it easier to achieve interoperability between
> independently developed
> frameworks, by allowing completely generic parameters.
>
>
>
> The following pseudo definitions can be assumed:
> ANYREC = ABSTRACT RECORD END;
> ANYPTR = POINTER TO ANYREC;
> PROCEDURE (a: ANYPTR) FINALIZE-, NEW, EMPTY;
>
>
>
> But regardless, the beauty of interfaces is to be able to do something
> like the following:
>
>
>
> MODULE Demo;
> VAR W: Texts.Writer;
> TYPE
> I* = POINTER TO IDesc;
> IDesc* = RECORD
> h: INTEGER
> END ;
>
> R* = POINTER TO RDesc;
> RDesc* = RECORD
> h: REAL
> END ;
>
> Stringer* = INTERFACE
> PROCEDURE ToString* (VAR a: ARRAY OF CHAR) ;
> END ;
>
> Incrementer* = INTERFACE
> PROCEDURE IncBy2*();
> END ;
>
> PROCEDURE ( i : I ) ToString* (VAR a: ARRAY OF CHAR) ;
> BEGIN a := "integer"
> END ToString;
>
> PROCEDURE ( r : R ) ToString* (VAR a: ARRAY OF CHAR) ;
> BEGIN a := "real"
> END ToString;
>
> PROCEDURE ( i : I ) IncBy2*;
> BEGIN i.h := i.h + 2
> END IncBy2;
>
> PROCEDURE ( r : R ) IncBy2*;
> BEGIN r.h := r.h + 2.0
> END IncBy2;
>
> PROCEDURE CallStringer(s: Stringer);
> VAR t: ARRAY 32 OF CHAR
> BEGIN
> s.ToString(t);
> Texts.WriteString(W,t);
> END CallStringer;
>
> PROCEDURE Test*;
> VAR i: I; r: R; ;
> s: Stringer;
> inc: Incrementer;
> BEGIN
> NEW(i); NEW(r);
> i.h := 3;
> r.h := 7.5;
> (*
> The assignment below allows for structural type compatibility
> between a
> record (i) and an interface (s). Interfaces can only have
> PROCEDUREs in
> their definition, if a record implements the set of PROCEDUREs of
> a
> interface, it can then be assigned to it.
> Any record implements the empty interface: INTERFACE END;
> This is how the Go language works.
> *)
> inc := i;
> inc.IncBy2;
> inc := r;
> inc.IncBy2;
> CallStringer(i); (* this is an assignment to the formal parameter*)
>
> CallStringer(r); (* this is an assignment to the formal parameter*)
>
> (*
> Alternatively we could still allow for nominal type compatibility
> forcing the user to specify that a record implements a set of
> interfaces:
>
> I* = POINTER TO IDesc;
> IDesc* = RECORD(Stringer, Incrementer)
> h: INTEGER
> END ;
>
> The fact that a record implements an interface is explicit. This is
> how Java does it. Extending from multiple interface types does not
> bring the complexity and pitfalls of multiple inheritance.
> To simplify the language in both cases, structural or nominal type
> compatibility, we should drop the ability to extend RECORDs with
> other
> records. Only INTERFACEs are allowed because we want only to
> extend
> behavior not data. If you also need the data simply add compose
> records as fields.
> *)
> END Test;
> END Demo.
>
>
>
>
>
> On Mon, Oct 26, 2020 at 2:29 PM Charles Perkins <chuck at kuracali.com>
> wrote:
>
> Hmm...
>
>
>
> Everything that implements an interface must be a descendant of Module
> Data but Data is empty, therefore its signature never needs to change. The
> use of an interface is checked in the CASE statement and cast there to the
> interface needed by the client. I think multiple interfaces can be
> implemented by successively extending Data. This could do it!
>
>
>
> I am going to try to use this, thank you Jörg for your patience with me.
>
>
>
> Cheers,
>
> Chuck
>
>
>
> On Mon, Oct 26, 2020 at 11:42 AM Jörg <joerg.straube at iaeth.ch> wrote:
>
> Chuck
>
>
>
> Okay. My proposal in standard Oberon-07 for your new requirement is this:
>
>
>
> MODULE Data;
>
> TYPE
>
> Any* = POINTER TO Empty;
>
> Empty* = RECORD END;
>
> END Data.
>
>
>
> MODULE Jsonify;
> IMPORT Data;
> TYPE
> Methods* = POINTER TO MDesc;
> MDesc* = RECORD
> toJSON: PROCEDURE(this: Data.Any);
> END;
> END Jsonify.
>
> MODULE myData;
>
> IMPORT Data;
>
> TYPE
>
> Tree* = POINTER TO TreeDesc;
>
> TreeDesc* = RECORD (Data.Empty)
>
> left, right: Tree;
>
> j: Jsonify;
>
> END;
>
> VAR t: Tree;
>
> PROCEDURE J(t: Data.Any);
>
> BEGIN
>
> CASE t OF Tree: (* implement here your Tree-specific JSON routine *)
>
> END
>
> END J;
>
> PROCEDURE New*(VAR t: Tree); BEGIN NEW(t); Jsonify.New(t.j); t.j.toJSON :=
> J END New
>
> BEGIN
>
> New(t); t.j.toJSON(t)
>
> END myData.
>
>
>
> br
>
> Jörg
>
>
>
> *Von: *Oberon <oberon-bounces at lists.inf.ethz.ch> im Auftrag von Charles
> Perkins <chuck at kuracali.com>
> *Antworten an: *ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> *Datum: *Montag, 26. Oktober 2020 um 17:03
> *An: *ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> *Betreff: *Re: [Oberon] Protocols (interfaces) in Oberon-2
>
>
>
> Hi Jörg and other interested people,
>
>
>
> There is just one piece missing I think for the usage of Interfaces that I
> am looking for. I would like to be able to write a routine that doesn't
> know what Data is (in the provided example) but can still hold a pointer to
> it or to anything else that implements the JSONify interface (and not
> anything that doesn't implement the JSONify interface) and can call it's
> ToJSON procedure.
>
>
>
> In my previous reply I incorrectly thought that a record in Oberon can be
> an extension of two base records, which is not right, and not what Jörg is
> presenting.
>
>
>
> Apologies for my confusion,
>
> Chuck
>
>
>
>
>
> On Mon, Oct 26, 2020 at 7:37 AM Charles Perkins <chuck at kuracali.com>
> wrote:
>
> Hi Jörg,
>
>
>
> With this example I think I understand how Oberon can in fact already do
> what I want in composing Interfaces. In Wirthian fashion the mechanism is
> explicit rather than implicit. I had not made the mental jump to how a
> record may be based on more than one base record... with that, everything
> else falls into place.
>
>
>
> I observe that this means that interfaces are 'opt in' rather than
> automatic based on just matching a subset of method names, but that's
> typical and arguably more safe anyway.
>
>
>
> I'm going to think on this some more.
>
>
>
> Thank you very much for taking the time to illustrate this to us.
>
>
>
> Best,
>
> Chuck
>
>
>
>
>
> On Sun, Oct 25, 2020 at 11:55 PM Jörg <joerg.straube at iaeth.ch> wrote:
>
> Chuck
>
> > Andreas, in your scheme can you create separate protocols, for
> example
> > "Jsonify" with the method ToJSON and a different protocol
> "Persistify" with
> > the methods "Store" and "Load", and have other records implement
> one or the
> > other or neither or both?
>
> Here my proposal for doing this in standard Oberon-07.
> In the previous mail, I separated interface/protocol and implementation
> for clarity. The same should be done here. For brevity, I combined the two
> here.
>
> MODULE Data;
> (* Definition of whatever your internal data structure looks like. Here
> just an example *)
> TYPE
> Tree* = POINTER TO TreeDesc;
> TreeDesc* = RECORD val*: ARRAY 15 OF CHAR; left*, right*: Tree END
> END Data.
>
> MODULE Jsonify;
> IMPORT Data;
> TYPE
> Methods* = POINTER TO MDesc;
> MDesc* = RECORD
> toJSON: PROCEDURE(this: Data.Tree);
> END;
> (* empty or default implementation *)
> PROCEDURE J(this: Data.Tree); END J;
> PROCEDURE New*(VAR m: Method); BEGIN NEW(m); m.toJSON := J END;
> END Jsonify.
>
> MODULE Persistify;
> IMPORT Data;
> TYPE
> Methods* = POINTER TO MDesc;
> MDesc* = RECORD
> Load: PROCEDURE(VAR this: Data.Tree);
> Store: PROCEDURE(this: Data.Tree)
> END;
> (* empty or default implementation *)
> PROCEDURE L(VAR this: Data.Tree); BEGIN this := NIL END L;
> PROCEDURE S(this: Data.Tree); END S;
> PROCEDURE New*(VAR m: Methods); BEGIN NEW(m); m.Load := L; m.Store := S
> END Init;
> END Persistify.
>
> Here now a module using both interfaces/protocols and overwrite even one
> persist procedure with an own version, if wanted.
> MODULE Usage;
> IMPORT Data, Jsonify, Persistify;
> TYPE
> User = RECORD (Data.Tree)
> j: Jsonify;
> p: Persistify
> END;
> VAR u: User;
> PROCEDURE myLoad(this: Data.Tree); (* implement your version of persist
> Load *) END myLoad;
> PROCEDURE New*(VAR u: User);
> BEGIN NEW(u); Jsonify.New(u.j); Persistify.New(u.p); u.p.Load := myLoad
> END New;
>
> BEGIN
> New(u); u.j.toJSON(u); u.p.Load(u) (* this calls my version *)
> END Usage.
>
> Adding the qualifiers "j" and "p" circumvents the ambiguity in case the
> two interfaces defined methods with the same name.
>
> br
> Jörg
>
>
>
> --
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