[R] Difficulties with R.oo (static fields, etc.)

[Gabriel Baud-Bovy]

I would like to use R.oo and tcltk to implement a Turtle World. I have 
encountered
many problems because:

1) I am not sure how to implement static fields with R.oo
2) I am not sure how to implement a constructor that would
     call a function only for the first instance of a class (i.e., to 
initialize
     value of static fields only once)
3) I am not sure how to remove/delete cleanly existing instances. In 
particular,
     I don't know how to reset tcktk when the last instance is deleted.

Here is the sort of behavior I would like to get:

t1<-TurtleBasic()   # a window (the Turtle world) with a turtle appears
forward(t1,50)       #  the turtle moves (turtle's state is also updated)
turn(t1,pi/2)
forward(t1,50)
t2<-TurtleBasic()  # a second turtle in the Turtle world appears
turn(t2,pi/2)  # second turtle moves
forward(t1,50)
delete(t1) # first turtle disappears
delete(t2) # second turtle and the world disappears

If possible, I would like to keep this syntax. In fact, I am using R.oo to
create mutable objects so as to avoid syntax like  tu<-forward(tu,10) which
would be more typical of a functional language like R. From the doc/help 
files of R.oo,
I believe that one could do it but I don't find many examples to guide me.

I pasted my current code with comments below. Thank you for your help.

Gabriel Baud-Bovy

#---------------------------------------------------------------------------------------------------------------------

Description of Turtle Basic Class:

private fields:
   .x, .y, .a (turtle position and heading)
   .turtle: ID of canvas item representing turtle
static fields:
   .canvas: tcltk canvas widget
   .top: tcktk toplevel widget
methods:
    TurtleBasic: constructor
    plot: display turtle in Turtle World
    forward, turn: move turtle
    delete: delete turtle (not implemented)

Note: My current code is very buggy:
  - Can't see turtle movements
  - Does not define method to delete turtles
  - I get an tcltk error when I try to recreate a turtle world after having 
tried to destroy it
    (for the moment, I need to detach tcltk to reset it)

#---------------------------------------------------------------------------------------------------------------------

library("R.oo")

setConstructorS3("TurtleBasic",function() {
   new<-extend(Object(),"TurtleBasic",.x=100,.y=100,.a=0,.turtle=NA)
   ## Create a new Turtle World if necessary
   require(tcltk) || stop("tcl/ library not available")
   if(!is.tkwin(new$.canvas)) { # check to see if it is first instance or not
     cat("Create Turtle World\n")
     top <- tktoplevel()
     tktitle(top) <- "Turtle World"
     canvas <- tkcanvas(top, relief="raised", width=200, height=200)
     tkpack(canvas, side="top", fill="both",expand="1")
     new$.canvas<-canvas 	 			# store canvas widget in static field
     new$.top<-top
   }
   plot.TurtleBasic(new)					# plot Turtle
   new
})

setMethodS3("plot","TurtleBasic",function(turtle) {
     ## delete old item (if it exists)
     if(is.tkwin(turtle$.canvas) && is.tclObj(turtle$.turtle)) 
tkdelete(turtle$.canvas,turtle$.turtle)
     ## a new canvas item representing the turtle is created
     ## (note: it's necessary to make new item because turtle's heading can 
change)
     x <- c(-10,10)
     y <- c(-10,10)
     aux<-   cos(turtle$.a)*x+sin(turtle$.a)*y + turtle$.x
     y  <-  -sin(turtle$.a)*x+cos(turtle$.a)*y + turtle$.y
     x  <- aux
     turtle$.turtle <- tkcreate(turtle$.canvas, "line", 
x[1],-y[1],x[2],-y[2],width=1)
})

setMethodS3("forward","TurtleBasic",function(turtle,length){
   aux       <- turtle$.x + length*cos(turtle$.a)
   turtle$.y <- turtle$.y + length*sin(turtle$.a)
   turtle$.x <- aux
   plot(turtle)
})

setMethodS3("turn","TurtleBasic",function(turtle,angle){
    turtle$.a<-turtle$.a+angle
    plot(turtle)
})


if(0) {

t1<-TurtleBasic()  # a window (the Turtle world) with a turtle appears
forward(t1,50)
turn(t1,pi/2)
forward(t1,50)
t2<-TurtleBasic()  # a second turtle in the Turtle world
turn(t2,pi/2)

# manually destroy first turtle (would be great to define some sort of 
method for it)
tkdelete(t1$.canvas,t1$.turtle)
rm(t1)
# destroy second turtle and world
tkdelete(t2$.canvas,t2$.turtle)
tkdestroy(t2$.top)
rm(t2)

}
--------------------------------------------------------------------
Gabriel Baud-Bovy
Assistant Professor
UHSR University
via Olgettina, 58	tel:  (+39) 02 2643 4839
20132 Milan, Italy	fax: (+39) 02 2643 4892