Despite reading many tutorials, it's still fundamentally unclear to me what is being passed back from a reactive expression/code block, especialy when code blocks inside reactive become longer. I did some basic tests, assuming reactiveConsole(TRUE) is a proper way of testing these things.
Is the following summary accurate?
return statement, reactive always returns the value produced by the last statement within its expression, regardless of whether that value was assigned to a local variable (but see "unexpected" behavior in examples below)return statement(s) to prevent returning the result of the final statement, for example to avoid returning a debugging print statement at the end of the expression, to build in if/else logic on what to return, etc.If there are other important variations on or exceptions to this behavior worth mentioning, please do.
library(shiny)
reactiveConsole(TRUE)
#------single line-----
#direct assignment
x = reactive({5})
x() #-> 5
ans = x()
ans #-> 5
#assign value to local variable, return
x = reactive({tmp = 5
return(tmp)})
x() #-> 5
ans = x()
ans #-> 5
#assign value to local variable, don't return
x = reactive({tmp = 5})
x() #-> [no response]
ans = x()
ans #-> 5 [value unexpectedly returns]
#-----multi-line---
#direct assignment: final line is retained
x = reactive({5
10})
x() #-> 10
ans = x()
ans #-> 10
#return requested local var, tmp
x = reactive({tmp = 5
tmp2 = 10
return(tmp)})
x() #-> 5
ans = x()
ans #-> 5
#return requested local var, tmp2
x = reactive({tmp = 5
tmp2=10
return(tmp2)})
x() #-> 10
ans = x()
ans #-> 10
#without return
x = reactive({tmp = 5
tmp2 = 10})
x() #-> [no response]
ans = x()
ans #-> 10 [value unexpectedly returns]
I don't know reactive(), but the behaviour you saw is exactly the same as a standard function or block of code in braces. If your function is
f <- function() {
x <- 5
}
then calling f() doesn't print anything, but it returns the value 5, because the statement x <- 5 returns the value 5 "invisibly". In R, things returned invisibly don't auto-print, and the value of the last statement executed in the body of the function is the default return value of the function.
The same is true of the code block
{
x <- 5
}