Runtime access to property values.

• §1. Whether Provides
• §2. Scan Property Metadata
• §3. Read Property
• §4. Write Property
• §5. Printing Property Names

§1. Whether Provides. This routine defines the phrase "if O provides P": there are three tests to pass, and if any of the three fail, we return false. (The issue_rtp flag, causing RTPs to be issued depending on which test fails, is never set when the routine is simply testing the condition.)

Firstly, P has to be a property known to I7. Secondly, there has to be permission either for this individual object to have it, or for its kind to have it, or its kind's kind, and so on. Thirdly, the object has to actually have the property in question at an I6 level — having permission to have it doesn't mean it actually does have.

[ ProvidesProperty K obj q issue_rtp  a l holder;
    if (K == OBJECT_TY) {
        if (ScanPropertyMetadata(obj, q, 4)) jump PermissionFound;
        if (obj provides KD_Count) {
            l = obj.KD_Count;
            while (l > 0) {
                a = l*2;
                if (ScanPropertyMetadata(KindHierarchy-->a, q, 4)) jump PermissionFound;
                l = KindHierarchy-->(a+1);
            }
        }
        if (ScanPropertyMetadata(K0_kind, q, 4)) jump PermissionFound;
        if (issue_rtp) {
            IssueRTP("UnprovidedProperty",
                "Attempt to use a property without permission.", BasicInformKitRTPs);
            print "*** Since ", (the) obj, " is not allowed the property ~",
                (PrintPropertyName) q, "~, it can't be used.^";
        }
        rfalse;
    }

    .PermissionFound;
    if (K >> obj provides q) rtrue;

    if (issue_rtp) {
        IssueRTP("UnsetProperty",
            "Attempt to use a property with no value.", BasicInformKitRTPs);
        print "*** Although ", (the) obj, " is allowed to have the property ~",
            (PrintPropertyName) q, "~, no value was ever given.^";
    }
    rfalse;
];

§2. Scan Property Metadata. The supplied metadata table is a series of zero-terminated lists of objects (or class objects, representing I7 kinds). Each such list corresponds to a single property; the position in the table is called the "offset" for the property. The following searches from a given offset.

[ ScanPropertyMetadata obj property_metadata off i;
    for (i=off: property_metadata-->i >= 0: i++)
        if (obj == property_metadata-->i) rtrue;
    rfalse;
];

§3. Read Property. Some value properties belong to other values (those created in tables), and these are stored indirectly through a pseudo-object V — an I6 object which is not part of the world model, and not a valid I7 "object" value, but which is used in order that properties belonging to values are still I6 property numbers. V.P is the table column address for this property; obj is then a value for the kind of value created by the table, so it is used as an index into the table column to get the address of the memory location storing the property value.

[ GProperty K V pr err holder val;
    if (ProvidesProperty(K, V, pr, 1-err)) {
        return (K >> V . pr);
    }
    return 0;
];

§4. Write Property. This routine's name must consist of the read-value-property routine's name with the prefix Write, as that is how a reference to such a property is converted from an rvalue to an lvalue.

[ WriteGProperty K V pr val holder;
    if (ProvidesProperty(K, V, pr, true)) {
        K >> V . pr = val;
    }
];

§5. Printing Property Names. Inform doesn't print property names prettily; it more or less prints them only as decimal numbers.

[ PROPERTY_TY_Say v;
    print "property ", v;
];

[ PrintPropertyName  p  textual;
    if (p<0) p = ~p;
    textual = p-->3;
    print (string) textual;
];