Feature or bug?

G'day Sigbert,

long time no see :)
How is Berlin these days?

On Thu, 21 May 2015 11:45:26 +0200

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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