In reply to https://groups.google.com/forum/?fromgroups=#!topic/shiny-discuss/jM6ORFOXuXg

gistfile1.rebol

# global.R
dists <- list(
  normal = list(
    label = "Normal",
    func = rnorm,
    inputs = tagList(
      sliderInput("mean", "mean: ", min=0, max=400, value=0),
      sliderInput("sd", "st. dev.: ", min=0.1, max=20, value=1, step=.1)
    ),
    params = c("mean", "sd")
  ),
  beta = list(
    label = "Beta",
    func = rbeta,
    inputs = tagList(
      sliderInput("shape1", "shape1: ", min=0.1, max=10, value=1, step=0.05),
      sliderInput("shape2", "shape2: ", min=0.1, max=10, value=1, step=0.05)
    ),
    params = c("shape1", "shape2")
  )
)
distChoices <- names(dists)
names(distChoices) <- sapply(dists, function(x) {x$label})


# ui.R
selectInput("popDist", "Population", distChoices),
lapply(names(dists), function(name) {
  conditionalPanel(condition = sprintf("input.popDist == '%s'", name),
                   dists[[name]]$inputs)
})

# server.R
params <- reactive(function() {
  values <- lapply(dists[[input$popDist]]$params, function(param) {
    input[param]
  })
  names(values) <- dists[[input$popDist]]$params
  values
})
rdist <- reactive(function() {
  dists[[input$popDist]]$func
})