R Shiny: test existence of reactive value which is optional after req() - r

Question

I have code like this
myfx <- reactive({
req(
isTruthy(input$value),
isTruthy(data1()) || isTruthy(data2())
)
...
if(exists(data2())) {
# do some stuff
}
## do this other stuff regardless
})
The if(exists()) piece is where I am stuck. Exists is not the right function here, nor does validate(need(data2())) work. How can I conditionally execute some code if one of the optional reactives (from a group where at least one is required) exists?
EDIT 1: To make explicit the problem, see the issue illustrated by the debug prints below:
myfx <- reactive({
req(
isTruthy(input$value),
isTruthy(data1()) || isTruthy(data2())
)
print("I am printed, and data2() has not been uploaded by user")
print(isTruthy(data2()))
print("I am never printed")
if(isTruthy(data2())) {
# do some stuff
}
## do this other stuff regardless
})
Edit 2: ok I see the reason. And I also see that I'm not observing the behavior in my first req() call because of short-circuiting (||). But now I'm stuck how to achieve the behavior I want. Bascially I don't calculate data2() until the user uploads something (use a req() here too). So that is where this is hanging. But if I remove the req() from the top of this, then I get errors due to trying to work on input that doesn't exist. See the definition for data2() below. How can I fix this?
data2 <- reactive({
req(input$data2)
read.csv(input$data2$datapath) %>%
as_tibble() %>%
return()
})

Answer 1

You can re-use isTruthy. Since it still returns true for 0 rows, you may want to add a check for non-zero row count:
myfx <- reactive({
req(
isTruthy(input$value),
isTruthy(data1()) || isTruthy(data2())
)
...
if (isTruthy(data2()) && nrow(data2()) > 0) {
# do some stuff
}
## do this other stuff regardless
})
Or you can capture the attempt in a try/tryCatch and react accordingly:
myfx <- reactive({
req(
isTruthy(input$value),
isTruthy(data1()) || isTruthy(data2())
)
...
res2 <- tryCatch({
# do some stuff with data2()
}, error = function(e) NULL)
## do this other stuff regardless
})

Answer 2

Ok, here is the final working solution.
myfx <- reactive({
req(
isTruthy(input$value),
isTruthy(data1()) || isTruthy(data2())
)
print("I am printed, and data2() has not been uploaded by user")
print(isTruthy(data2()))
print("I am never printed")
if(isTruthy(data2())) {
# do some stuff
}
## do this other stuff regardless
})
data2 <- reactive({
if(isTruthy(input$data2)) {
read.csv(input$data2$datapath) %>%
as_tibble() %>%
return()
}
})

Answer 3

raise an error if dat2 doesn't exit by checking number of rows
chk will become NULL if dat2 doens't exist
use chk in if statement to determine what to do
chk <- tryCatch({ nrow(dat2()) > 0}, error = function(e) NULL)
if(is.null(chk)){
dat1
} else{
cbind(dat1, dat2)
}