example.R

server <- function(input, output, server) {
  output$out <- renderText({ 
    validate(
      need(input$x < 0, "x can not be negative"),
      need(input$trans %in% c("log", "square-root"), "trans blah blah")
    )    
    
    switch(input$trans,
      square = input$x ^ 2,
      "square-root" = sqrt(input$x),
      log = log(input$x)
    )
  })
}