Re: [BLACKBOX] SYSTEM.VAL, WITH, IS, and type casting

From: [at]} <Bernhard>
Date: Tue, 23 Aug 2011 00:06:02 +0200

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Hi Doug,

> By writing SYSTEM.VAL(BYTE, schar) one is essentially doing
> schar := byte; and by passing the type checking mechanism.

yes, it is a type transfer (or in C terminology a type cast) ...

> One can write
> wr.WriteStore(m);
> rd.ReadStore(SYSTEM.VAL(Store, m));
> which is asymmetric in the treatment of writing and reading.

who writes it this way?

I still do not get the point. If I look at the Stores docu I find:

    Reader = RECORD
        rider-: Files.Reader;
        cancelled-: BOOLEAN;
        readAlien-: BOOLEAN;
        (VAR rd: Reader) ConnectTo (f: Files.File), NEW;
...
        (VAR rd: Reader) ReadStore (OUT x: Store), NEW;
...
    END;

    Writer = RECORD
        rider-: Files.Writer;
        writtenStore-: Store;
        (VAR wr: Writer) ConnectTo (f: Files.File), NEW;
...
        (VAR wr: Writer) WriteStore (x: Store), NEW;
...
    END;

which is completely symmetric. If there is a variable of type Store it can
be handed as parameter to wr.WriteStore and rd.ReadStore without any
need for SYSTEM.VAL ...

and in the source I find

PROCEDURE (VAR rd: Reader) ReadSChar* (OUT x: SHORTCHAR), NEW;
BEGIN
    rd.rider.ReadByte(SYSTEM.VAL(BYTE, x))
END ReadSChar;

and

PROCEDURE (VAR wr: Writer) WriteSChar* (x: SHORTCHAR), NEW;
BEGIN
    wr.rider.WriteByte(SYSTEM.VAL(BYTE, x))
END WriteSChar;

which is using SYSTEM.VAL but completely symmetric. It redirects the IO to
the corresponding rider (i.e. Files.Reader/Files.Writer). This pattern
(carrier/rider)
is best described in Clemens Szyperski's PhD Thesis page 38ff (available,
but not
searchable, since I did not have time for OCRing, on his pages at MicroSoft:
http://research.microsoft.com/en-us/um/people/cszypers/books/Insight-EthOS.pdf).

As I understand it, many design principles of BlackBox stem from the
research
documented in "Insight ETHOS".

regards
     Bernhard

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Received on Tue Aug 23 2011 - 00:06:02 UTC