[R] tcl/tk return problem

[Prof Brian Ripley]

On Wed, 31 Aug 2005, Duncan Murdoch wrote:

> 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.

It was the question asked!  If you use a non-modal tcltk dialog you
have little choice, as the parent function will have returned and vanished
long ago.  Now, having a widget firing off R commands independently of the
command line is not a great idea (R's evaluator is not multithreaded and
this could be interspersed in another computation) so there is a lot to be
said for using dialog boxes modally.

> 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.

One could argue the user who pressed the button does: it is the user
workspace.

> 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.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595