From: Albert Wagner <alwagner@...>
Date: 2002-08-04T04:35:12+09:00
Subject: Subtle bug in Ruby/Tk? (Attn: Hidetoshi Nagai)

In the sample code below, a subtle bug appears in either Ruby/Tk or Tk itself.
Within the class Node, when a recursive call is made to method descendTree, 
the to_s method is apparently invoked within the Tk.mainloop, only if to_s 
does not return a string.  I know that to_s should return a string, but why 
is to_s even being called?  And why is it being called only when descendTree 
is called recursively?  The code can be made to work correctly if (1) in 
to_s, comment out ---puts "to_s"---and replace with ---"to_s"--- OR  (2) 
comment out  the addChild statement so that descendTree is not called 
recursively.

----------------------------------------------------
require 'tk'

class Node
	def initialize(name)
		@name = name
		@children = Array.new
	end

	def addChild(aNode)
		@children.push(aNode)
	end
	attr_reader(:name, :children)

	def to_s
		puts "to_s" #comment this out and
		#"to_s"		# uncomment this
	end
end

def aProc
	showTree
end

def showTree
	descendTree(0, @tree)
end

def descendTree(level, aNode)
	puts "#{"\t"*level} #{aNode.name}"
	aNode.children.each do |node|
		descendTree(level + 1, node)
	end
end

@tree = Node.new('top')
@tree.addChild(Node.new('A')) #comment this out

root = TkRoot.new
menuFrame = TkFrame.new(root).pack
menuButton = TkMenubutton.new(menuFrame,'text'=>'Button').pack
menu = TkMenu.new(menuButton)
menuButton.configure('menu'=>menu)
menu.add('command','label'=>'Item','command'=>proc{aProc})
Tk.mainloop
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