8 The Use of this(P) for Component Objects

Consider the following example: 
P: (#  A: (# X: ^P; (* reference to item qualified by P *)
       B: ^|P (* reference to component qualified by P *)
   do this(P)[] -> X[];   (* legal use of this(P)[] *)
      this(P)[] -> R[];   (* illegal use of this(P)[] *)
   #)

The compiler assumes that this(P)[] is a reference to an item object. Since an item reference cannot be assigned to a component reference the evaluation this(P)[] -> R[] is illegal.

It is, however, possible to use a run-time routine to convert an item reference to a component reference, provided that the item is part of a component. Consider: 

(# P: (# B: |^ P
      do this(P)[] -> objectToComponent -> B[] 
      #);
   X: |@ P;
   Y: @ P;
do X;  (* OK *)
   Y; (* a run-time error will happen *)
#)
When X is executed, the P-object is part of the component X and objectToComponent will return a reference to X.

When Y is executed, the P-object is not part of a component and objectToComponent will fail.

It is also possible to get a reference to the item-part of a component by using the pattern componentToObject as shown in: 

X[] -> componentToObject

The patterns objectToComponent and componentToObject are placed in the library: 

sysutils/objinterface.bet

BETA Language Modifications - Reference Manual
 © 1999-2002 Mjølner Informatics
[Modified: Sunday October 22nd 2000 at 0:53]