Unexpected behavior of req() - r

Question

I experienced an unexpected behavior of my R shiny code and I'm asking myself if this is bug or if I don't understand how req() works.
I have an app where the user first uploads a *.txt file containing data with a location id, a date, and other data. Then the user has to choose two numerical values. The data is checked for NAs in the col date. If there are no NAs a text should appear telling the user everything is fine.
Below are two versions of my code. In output$txt <- renderText({ I use req() to test if all inputs are set and the file is loaded.
The differences between the two codes are the ordering of the last two arguments in req. Whereas in the first version, the green text appears already when both numeric inputs are set even the file is not uploaded, the second code behaves as expected; the user has to choose the numeric value and has to choose a file before the green bar with text appears.
Why makes the ordering of the arguments in req() a difference?
Code 1:
library(shiny)
library(lubridate)
# UI
ui <- fluidPage(
fluidRow(
column(4,
wellPanel(
fileInput("file1", label = h4("file upload")),
numericInput("in1", label = h4("first input"),value = 2.5, step=0.5),
numericInput("in2", label = h4("second input"),value = NULL, step=0.5)
)
),
column(8,
h4(textOutput("txt"), tags$style(type="text/css", "#txt {vertical-align:top;align:center;color:#000000;background-color: #4cdc4c;}")),
h2(""),
textOutput("fileinfo"),
tableOutput("tab_data")
)
)
)
# SERVER
server <- function(input, output) {
output$txt <- renderText({
req(input$in1, input$in2, fl_data(), input$file1)
"your data is ok and you have chosen input 1 and 2"
})
fl_data <- reactive({
validate(
need(input$file1 != "", "upload data and choose input 1 and 2...")
)
inFile <- input$file1
if (is.null(inFile)) {
return(NULL)
} else {
dd <- read.table(inFile$datapath, sep=";", stringsAsFactors=FALSE, header=TRUE)
dd[,2] <- ymd(dd[,2])
if (sum(is.na(dd[,2]))>0) dd <- NULL
}
})
output$tab_data <- renderTable({head(fl_data()[,1:4])})
output$fileinfo <- renderPrint({input$file1})
}
# Run the application
shinyApp(ui = ui, server = server)
Code 2:
library(shiny)
library(lubridate)
# UI
ui <- fluidPage(
fluidRow(
column(4,
wellPanel(
fileInput("file1", label = h4("file upload")),
numericInput("in1", label = h4("first input"),value = 2.5, step=0.5),
numericInput("in2", label = h4("second input"),value = NULL, step=0.5)
)
),
column(8,
h4(textOutput("txt"), tags$style(type="text/css", "#txt {vertical-align:top;align:center;color:#000000;background-color: #4cdc4c;}")),
h2(""),
textOutput("fileinfo"),
tableOutput("tab_data")
)
)
)
# SERVER
server <- function(input, output) {
output$txt <- renderText({
req(input$in1, input$in2, input$file1, fl_data())
"your data is ok and you have chosen input 1 and 2"
})
fl_data <- reactive({
validate(
need(input$file1 != "", "upload data and choose input 1 and 2...")
)
inFile <- input$file1
if (is.null(inFile)) {
return(NULL)
} else {
dd <- read.table(inFile$datapath, sep=";", stringsAsFactors=FALSE, header=TRUE)
dd[,2] <- ymd(dd[,2])
if (sum(is.na(dd[,2]))>0) dd <- NULL
}
})
output$tab_data <- renderTable({head(fl_data()[,1:4])})
output$fileinfo <- renderPrint({input$file1})
}
# Run the application
shinyApp(ui = ui, server = server)

Answer 1

req short-circuits, just like the && and || operators. As soon as it comes across an unavailable value (args evaluated left-to-right), it stops the reactive chain and doesn't care about any further args.
In the second example, input$file1 prevents fl_data() from ever executing if missing, so the validation in fl_data never occurs. But rather than order the req args like in the first example, I would just remove the check for input$file1 in output$txt, as it's already being checked in fl_data.