Processing input data, step by step, using Shiny modules

modular-processing.R

# -------------------------------------------------------------------
# ------------------ PROCESSING DATA MODULARLY----------------------
# -------------------------------------------------------------------

# This app processes some input text through three modules, passing 
# the result of each module down to the next module. We're also 
# displaying to the user the (reactive) result of each module, but
# usually we'd just be interested in the last one... In those cases,
# the modules' ui functions would be empty (except for the last 
# module, of course).

library(shiny)

# This module is responsible for getting the input and for the first
# step of processing (turning the input text into a header 2)
mod1UI <- function(id){
  ns <- NS(id)
  textInput(ns("txtIn"), "Enter text:", "Hello"),
  uiOutput(ns("txtOut"))
}
mod1 <- function(input, output, session){
  processedTxt <- reactive({ h2(input$txtIn) })
  output$txtOut <- renderUI( processedTxt() )
  return(processedTxt)
}

# This module is responsible for the second step of processing 
# (italicizing the previously processed text)
mod2UI <- function(id){
  ns <- NS(id)
  uiOutput(ns("txtOut"))
}
mod2 <- function(input, output, session, prev){
  processedTxt <- reactive({ tags$em(prev()) })
  output$txtOut <- renderUI( processedTxt())
  return(processedTxt)
}

# This module is responsible for the third step of processing 
# (coloring the previously processed text)
mod3UI <- function(id){
  ns <- NS(id)
  uiOutput(ns("txtOut"))
}
mod3 <- function(input, output, session, prev){
  processedTxt <- reactive({ tags$div(prev(), style = "color: red") })
  output$txtOut <- renderUI( processedTxt() )
  return(processedTxt)
}

# This is the main app that calls the three modules consecutively
ui <- fluidPage(
  mod1UI("myMod1"),
  mod2UI("myMod2"),
  mod3UI("myMod3")
)

server <- function(input, output, session) {
  mod1return <- callModule(mod1, "myMod1")
  mod2return <- callModule(mod2, "myMod2", mod1return)
  mod3return <- callModule(mod3, "myMod3", mod2return)
}

shinyApp(ui = ui, server = server)

Hi, in the first module the code is missing a tagList in the UI part.