[R] Feature or bug?

[peter dalgaard]

On 21 May 2015, at 16:26 , Bert Gunter <gunter.berton at gene.com> wrote:

> Well..
> 
> "Because the global variable is changed before input is evaluated.  R
> has lazy argument evaluation, arguments are only evaluated once they
> are needed.  You are essentially getting bitten by R's lazy evaluation
> plus "pass by value" syntax."
> 
> While I may be either wrong or just picking on semantics, I don't
> think so. It is merely what you stated previously: input was assigned
> a value in the local server function environment, and that assignment
> was not affected by the subsequent assignment to the global
> environment. So it is a matter of R's semantics -- where it looks for
> the values bound to symbols -- rather than lazy evaluation.
> 
> Obviously then, a simple way to do what the OP seemed to want would be
> to simply assign the updated value in the local function environment,
> rather than the global. I get nervous whenever I see constructs with
> eval(substitute...)) or global assignments from within a function.
> Both have their place, of course, but (the latter especially) can be
> dangerous, and my experience both on the list and with my own code, is
> that if you think you need them, you probably should rethink what you
> need to do.
> 
> Corrections and/or criticism of these comments are welcome.
> 

Berwin is right, or rather: There are two issues. The "input" argument to server() is evaluated when first used. The difference between the two examples is whether this happens before or after update(), and that is the effect of lazy evaluation. The fact that it in the first case the value is unchanged by the update is due to scoping: Once evaluate "input" becomes a local variable  an is unchanged by assignment to the global variable.

-pd


> Best,
> Bert
> 
> 
> 
> Bert Gunter
> Genentech Nonclinical Biostatistics
> (650) 467-7374
> 
> "Data is not information. Information is not knowledge. And knowledge
> is certainly not wisdom."
> Clifford Stoll
> 
> 
> 
> 
> On Thu, May 21, 2015 at 3:50 AM, Berwin A Turlach
> <Berwin.Turlach at gmail.com> wrote:
>> G'day Sigbert,
>> 
>> long time no see :)
>> How is Berlin these days?
>> 
>> On Thu, 21 May 2015 11:45:26 +0200
>> Sigbert Klinke <sigbert at wiwi.hu-berlin.de> wrote:
>> 
>> It is a feature.
>> 
>>> if I run
>>> 
>>> update <- function (newtime) { ginput <<- list(time=newtime)}
>>> 
>>> server <- function (input) {
>>>  print(paste("Before", input$time))
>>>  update(1)
>>>  print(paste("After:", input$time))
>>> }
>>> 
>>> ginput <- list(time=0)
>>> server(ginput)
>>> 
>>> then I get as result
>>> 
>>> [1] "Before 0"
>>> [1] "After: 0"
>> 
>> The first print command evaluates input and after this the function
>> server has an object named "input" in its local environment.  The
>> second print command reuses this object and extracts the component time
>> from it (which has not changed).  The change of the global variable has
>> no effect.
>> 
>>> If I uncomment the first print
>>> 
>>> update <- function (newtime) { ginput <<- list(time=newtime) }
>>> 
>>> server <- function (input) {
>>>  #print(paste("Before", input$time))
>>>  update(1)
>>>  print(paste("After:", input$time))
>>> }
>>> 
>>> ginput <- list(time=0)
>>> server(ginput)
>>> 
>>> then I get
>>> 
>>> [1] "After: 1"
>> 
>> Because the global variable is changed before input is evaluated.  R
>> has lazy argument evaluation, arguments are only evaluated once they
>> are needed.  You are essentially getting bitten by R's lazy evaluation
>> plus "pass by value" syntax.
>> 
>>> Even when I use a side effect (by assign some new value to a global
>>> variable) I would have expected the same behaviour in both cases.
>> 
>> To get the behaviour that you expect, you would have to write your code
>> along the following lines:
>> 
>> R> update <- function (newtime) { ginput <<- list(time=newtime)}
>> R> server <- function(input){
>> +     inp <- as.name(deparse(substitute(input)))
>> +     print(paste("Before", eval(substitute(XXX$time, list(XXX=inp)))))
>> +     update(1)
>> +     print(paste("After:", eval(substitute(XXX$time, list(XXX=inp)))))
>> + }
>> R> ginput <- list(time=0)
>> R> server(ginput)
>> [1] "Before 0"
>> [1] "After: 1"
>> 
>> 
>> A cleaner way is perhaps to use environments, as these are passed by
>> reference:
>> 
>> R> update <- function(env, newtime) env$time <- newtime
>> R> server <- function(input){
>> +     print(paste("Before", input$time))
>> +     update(input, 1)
>> +     print(paste("After:", input$time))
>> + }
>> R> ginput <- new.env()
>> R> ginput$time <- 0
>> R> server(ginput)
>> [1] "Before 0"
>> [1] "After: 1"
>> 
>> HTH.
>> 
>> Cheers,
>> 
>>        Berwin
>> 
>> ========================== Full address ============================
>> A/Prof Berwin A Turlach               Tel.: +61 (8) 6488 3338 (secr)
>> School of Maths and Stats (M019)            +61 (8) 6488 3383 (self)
>> The University of Western Australia   FAX : +61 (8) 6488 1028
>> 35 Stirling Highway
>> Crawley WA 6009                     e-mail: Berwin.Turlach at gmail.com
>> Australia                http://www.maths.uwa.edu.au/~berwin
>>                         http://www.researcherid.com/rid/A-4995-2008
>> 
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> 
> ______________________________________________
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
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