Feature or bug?

update <- function (newtime) {

server <- function(input, doFirstPrint) {

ginput <- list(time=0)
server({cat("# Evaluating server's 'input' argument\n"); ginput},

ginput <- list(time=0)
server({cat("# Evaluating server's 'input' argument\n"); ginput},

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

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