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  • Semantics of CASE statement

    From Lance Schwerdfager@21:1/5 to [email protected] on Thu Dec 28 21:53:17 2017
    On Thu, 7 Sep 2017 13:04:17 +0200, August Karlstrom
    <[email protected]> wrote:

    Let's consider the following example:

    MODULE test;

    IMPORT Out;

    TYPE
    Animal = RECORD END;
    Cat = RECORD (Animal) END;
    Dog = RECORD (Animal) END;

    VAR
    x: Cat;
    y: Dog;

    PROCEDURE P(VAR x: Animal);
    BEGIN
    CASE x OF
    Animal: Out.String("animal"); Out.Ln
    | Cat: Out.String("cat"); Out.Ln
    | Dog: Out.String("dog"); Out.Ln
    END
    END P;

    BEGIN
    P(x);
    P(y)
    END test.

    What should be the output from this module?


    Regards,
    August

    Since the intent of the CASE statement here is to distinguish the
    subclass of an Animal instance (and not the value of the Animal
    instance), it shouldl fail to compile. Using the Oberon-2 WITH
    statement to perform type testing (in the style of a CASE statement)
    is one way to get around this.

    Regards,
    Lance

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  • From [email protected]@21:1/5 to Lance Schwerdfager on Fri Dec 29 03:48:11 2017
    On Friday, December 29, 2017 at 1:23:22 PM UTC+10:30, Lance Schwerdfager wrote:
    On Thu, 7 Sep 2017 13:04:17 +0200, August Karlstrom
    <[email protected]> wrote:

    Let's consider the following example:

    MODULE test;

    IMPORT Out;

    TYPE
    Animal = RECORD END;
    Cat = RECORD (Animal) END;
    Dog = RECORD (Animal) END;

    VAR
    x: Cat;
    y: Dog;

    PROCEDURE P(VAR x: Animal);
    BEGIN
    CASE x OF
    Animal: Out.String("animal"); Out.Ln
    | Cat: Out.String("cat"); Out.Ln
    | Dog: Out.String("dog"); Out.Ln
    END
    END P;

    BEGIN
    P(x);
    P(y)
    END test.

    What should be the output from this module?


    Regards,
    August

    Since the intent of the CASE statement here is to distinguish the
    subclass of an Animal instance (and not the value of the Animal
    instance), it shouldl fail to compile. Using the Oberon-2 WITH
    statement to perform type testing (in the style of a CASE statement)
    is one way to get around this.

    Regards,
    Lance

    I disagree. The CASE statement above is legal as it stands. WITH in Oberon-2 behaves exactly the same as CASE in Oberon-07.

    WITH
    x: Cat DO Out.String("cat"); Out.Ln
    | x: Dog DO Out.String("dog"); Out.Ln
    | x: Animal DO Out.String("animal"); Out.Ln
    END

    gives the output

    cat
    dog

    whereas

    WITH
    x: Animal DO Out.String("animal"); Out.Ln
    | x: Cat DO Out.String("cat"); Out.Ln
    | x: Dog DO Out.String("dog"); Out.Ln
    END

    gives the output:

    animal
    animal

    Regards,
    Chris Burrows
    CFB Software
    http://www.astrobe.com

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