[R] tcl/tk return problem

[Duncan Murdoch]

Prof Brian Ripley wrote:
> On Wed, 31 Aug 2005, Philippe Grosjean wrote:
> 
> 
>>deggle wrote:
>>
>>>Hello,
>>>
>>>I'm very new in working with tcl/tk in R and have a problem which will
>>>probably
>>>sound silly to most of you.
>>>Here is the code I have problems with:
>>>
>>>readcelfiles <- function()
>>>{
>>>   require(tcltk)
>>>   tt <- tktoplevel()
>>>   tkgrid(tklabel(tt,text="Choose a directory!"))
>>>
>>> OnOK <- function()
>>> {
>>>    fileDir<-tclvalue(tkchooseDirectory())
>>>    data.raw <- ReadAffy(celfile.path=fileDir)
>>>    #return(data.raw)
>>> }
>>>
>>>   OK.but <- tkbutton(tt,text="OK",command=OnOK)
>>>   tkgrid(OK.but)
>>>   tkfocus(tt)
>>>}
>>>
>>>So after clicking on my "OK" button, I choose my directory and read the
>>>cel-files.
>>>But now I want to return the object to my workspace ... "return" doesn't
>>>work here.
>>
>>I suppose you mean in the User Workspace. Your OnOK function should look
>>like that:
>>
>>OnOK <- function() {
>>      fileDir<-tclvalue(tkchooseDirectory())
>>      data.raw <<- ReadAffy(celfile.path=fileDir)
>>}
>>
>>Note that the function overwrites any existing 'data.raw', so this could
>>be dangerous. Writting directly in the User Workspace is not advised
>>from inside a function, but here, it is the simplest way to return a
>>result from a tk widget action.
> 
> 
> Maybe simplest, but not a very good way.  See
> R_SOURCES/src/library/tcltk/R/utils.R for ideas on how to write a modal
> dialog box that returns the value selected.
> 
> One problem with <<- is that it does not necessarily write in the
> workspace.  You need
> 
>    assign("data.raw", ReadAffy(celfile.path=fileDir), envir=.GlobalEnv)
> 
> to be sure of that.  (The example code I quote does use <<- but in a
> controlled way.)

This works, and you weren't suggesting it as a good style, but I'd like 
to say it's really a bad style to write to .GlobalEnv.  The controlled 
use of <<- as in tk_select.list from the file you quoted is really the 
best way to solve this problem.  As a general rule, you shouldn't stomp 
on something you don't own, and functions don't own variables in 
.GlobalEnv.

What tk_select.list does is define OnOK locally, and use <<- to write to
the tk_select.list environment.  Then the result can be manipulated and 
returned politely, without stomping on anything anywhere.

Duncan Murdoch