Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!gatech!rutgers!bellcore!faline!thumper!ulysses!andante!alice!dbl
From: dbl@alice.UUCP (David Levine, for Kim Bruce)
Newsgroups: comp.lang.eiffel
Subject: constrained generic problem
Keywords: constrained generics, inheritance
Message-ID: <9091@alice.UUCP>
Date: 23 Mar 89 16:00:37 GMT
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 82



	I have been having problems with constrained generics.  Can anyone
give me some advice.  I have a class called SIMPLE_NODE which handles
requests of the form: get_info and print_node  (all it does is package
up a variable name and simple operations on it).  I also have a subclass of 
SIMPLE_NODE called COMPLEX_NODE (what else!) which adds an attribute
phone_number and modifies get_info and print_node to include getting
and printing phone_number as well as name.  NAME_TREE[T] inherits from
BIN_SRCHTREE[T], redefining node_action to call print_node.
	I also have a class tree_menu[T -> SIMPLE_NODE] (see below) which is
constrained generic, and which also has a local variable new_node : T.
---------------------------------------

class tree_menu[T -> SIMPLE_NODE]

inherit
	STD_FILES

feature
	...;
	my_tree : NAME_TREE[T] ;

	create is
	local
		new_node : T
	do
		from done:=false
		until done
		loop
			if .. then...;
			elsif cmd='a' then
				new_node.create;
				next_line ;
				new_node.get_info;
				if my_tree.void then
					my_tree.create(new_node)
				else
					my_tree.insert(new_node)
				end;
			elsif ...;
			else ...;
			end
		end; 
	end;

	getcommand : CHARACTER is
	do ...
	end ;

end --tree_menu

----------------------------------
The problem is that no matter how I instantiate the T, I always get the
operations in SIMPLE_NODE on new_node.  E.g. the call new_node.getinfo
above always calls the operation defined in SIMPLE_NODE even if I define
a variable, complex_menu : TREE_MENU[COMPLEX_NODE], and then write:
	complex_menu.Create
Interestingly if I remove the generics from TREE_MENU and just replace
T everywhere by COMPLEX_NODE, it works fine.  So the problem clearly
has to do with the use of generics.
	Can anyone help me on this?  Is this a compiler error that has
arisen since constrained generics are so new?  
	I have found a work-around based on eliminating the use of 
constrained generics (basically write TREE_MENU using SIMPLE_NODE, 
including a dummy anchor, and then declaring my tree to be of 
class NAME_TREE[like dummy_anchor], and then defining a new
subclass of tree_menu that redefines the class of dummy_anchor to be 
COMPLEX_NODE.  However, I consider this to be a much less elegant
solution to the problem.  The notion of constrained genericity (really
the "bounded quantification" of Cardelli and Wegner) is a much better
way of retaining control of the types of variables than the rather
ad hoc techniques of using anchors.
	Kim Bruce
	Williams College
	kim@cs.williams.edu

P.S.  Is anyone else having the problem of getting '5's all over the place
when trying to use the interactive debugger?