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Content-type: text/plain
Hi:
I am not sure I understand the reason for the error message
generated by the following test module. We wanted to implemented
a few list-management procedures from the textbook by Reiser and Wirth.
The procedures do not compile under BlackBox because the pointer
to the daughter class cannot be passed as a VAR parameter.
I am somewhat confused why. The variable "anchor" is assignment-compatible
with the formal VAR parameter "first", see the Language Report Appendix A,
Assignment compatible, rule 3:
3. Te and Tv are pointer types and Te is an extension of Tv;
It follows that it should be possible to assign a variable of type
HistNode to the VAR parameter of type Node, because HistNode is
an extension of Node. However, the program does not compile
with the error message that the types are not *identical*. They do not
need to be identical, they only need to be assignment-compatible.
What am I missing?
Wojtek Skulski skulski{([at]})nowhere.xy
MODULE WorkTest;
TYPE
Node = POINTER TO EXTENSIBLE RECORD
next: Node;
END;
HistNode = POINTER TO RECORD (Node)
hist: INTEGER
END;
VAR
anchor: HistNode;
last: HistNode;
PROCEDURE InsertLast (VAR first:Node; new:Node);
BEGIN
END InsertLast;
PROCEDURE AddNew;
BEGIN
NEW(anchor);
NEW(last);
InsertLast (anchor, last) (*incompatible?*)
END AddNew;
END WorkTest.
--------------------------------------------
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Received on Fri Jun 20 2003 - 00:25:27 UTC