library(teal)

data <- within(teal_data(), {
  iris <- iris
  mtcars <- mtcars
})

transformator_iris_scale <- teal_transform_module(
  label = "Scaling transformator for iris",
  ui = function(id) {
    ns <- NS(id)
    uiOutput(ns("scaled_columns_container"))
  },
  server = function(id, data) {
    moduleServer(id, function(input, output, session) {
      ns <- session$ns

      scalable_columns <- names(Filter(is.numeric, isolate(data())[["iris"]]))

      output$scaled_columns_container <- renderUI({
        selectInput(
          inputId = ns("scaled_columns"),
          label = "Columns to scale",
          choices = scalable_columns,
          selected = input$scaled_columns,
          multiple = TRUE
        )
      })

      reactive({
        data() # no-op for simplicity
      })
    })
  }
)

app <- init(
  data = data,
  modules = teal::example_module(transformators = list(transformator_iris_scale))
)

shinyApp(app$ui, app$server)