A minor(?) problem I need help with

The Arrow <arrow@trelleborg.mail.telia.com>
16 May 1999 15:09:04 -0400

          From comp.compilers

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A minor(?) problem I need help with arrow@trelleborg.mail.telia.com (The Arrow) (1999-05-16)
Re: A minor(?) problem I need help with Maffu@Kindy.net (1999-05-22)
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From: The Arrow <arrow@trelleborg.mail.telia.com>
Newsgroups: comp.compilers
Date: 16 May 1999 15:09:04 -0400
Organization: Compilers Central
Keywords: parse, types, question

I have a little problem with a compiler I am writing. The language it
compiles is completely typeless, and variables can be assigned
anything. The problem is that I have no way of telling wether a
variable is assigned to a class or an object (an instance of a class)
or a function, or checking for property calls or assignments at
compile time.

The following code snip shows illegal and legal stuff in my language:

--- begin snip ---

    /* define two classes, properties in classes can not be used without
      * the classes being instanciated
    class Class1 =
            prop1 = "This is a string";
            prop2 = { return "This is another string"; }

            /* prop1 can get assigned any value, while prop2 can not get
              * assigned at all

    class Class2 =
            prop1 = 5;
            prop2 = "foo";

    /* define an instance of a class, this one with inheritance */
    object Instance : Class2 =
            prop1 = 4;

    function fun1(a, b) = return a + b;

    function fun2 =
            var obj1, obj2, obj3;
            var tmp, fun. cls;

            obj1 = new Class1;
            obj2 = new Class2;
            obj3 = Instance;

            tmp = Instance.prop1; /* tmp is assigned 4 */
            Instance.prop1 = 7; /* Instance.prop1 is assigned 7 */
            obj3.prop1 = 1; /* Instance.prop1 should be assigned 1 */

            obj1.prop1 = "bar"; /* what class is obj1? */
            tmp = obj2.prop2; /* what class is obj2? */

            tmp = obj1.prop2; /* legal */
            obj1.prop2 = tmp; /* illegal */

            fun = fun1; /* how can I know fun is now a function? */
            tmp = fun(1, 2); /* tmp should be assigned 3 (1 + 2) */
            tmp = fun1(2, 3); /* tmp is assigned 5 (2 + 3) */

            cls = Class1;
            tmp = cls.prop1; /* illegal, not an instanciated object */
            obj3 = new cls; /* creating a new instance */
            tmp = obj3.prop1; /* this is ok, obj3 is an instance */

--- end snip ---

Is there any way of doing these checks at compile time, without
special cases in assignments to keep track of these things?

The compiler is recursive-descent, allthough I have been thinking
about using operator precedence for my expression parsing (but it
wouln't help much on solving this problem). All functions are stored
in a global table, as are all classes/objects. Variables are stored
in a table in the function it is defined in, and global variables is
stored in a global no-name-not-callable function. Properties are
really a kind functions, and are stored in tables local for the class.

The language itself is pretty generic (I think), but for now it is to
be used in a multi-player Internet text-game. Both to define the
world, and to handle game specific things.

All ideas or thoughts (good or bad) are more than wellcome! :)

I hope you excuse any spelling misstakes... ;)

/ Joachim
The Arrow Email: arrow@trelleborg.mail.telia.com
Joachim Pileborg WWW: [Under construction]

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