In a shiny app (by RStudio), on the _server_ side, I have a reactive that returns a list of variables by parsing the content of a `textInput`. The list of variables is then used in `selectInput` and/or `updateSelectInput`.

I can&#39;t make it work. Any suggestions?

I have made two attempts. The first approach is to use the reactive `outVar` directly into `selectInput`. The second approach is to use the reactive `outVar` in `updateSelectInput`. Neither works.

### server.R

    shinyServer(
      function(input, output, session) {
    
        outVar &lt;- reactive({
            vars &lt;- all.vars(parse(text=input$inBody))
            vars &lt;- as.list(vars)
            return(vars)
        })
    
        output$inBody &lt;- renderUI({
            textInput(inputId = &quot;inBody&quot;, label = h4(&quot;Enter a function:&quot;), value = &quot;a+b+c&quot;)
        })
    
        output$inVar &lt;- renderUI({  ## works but the choices are non-reactive
            selectInput(inputId = &quot;inVar&quot;, label = h4(&quot;Select variables:&quot;), choices =  list(&quot;a&quot;,&quot;b&quot;))
        })
    
        observe({  ## doesn&#39;t work
            choices &lt;- outVar()
            updateSelectInput(session = session, inputId = &quot;inVar&quot;, choices = choices)
        })

    })


### ui.R

    shinyUI(
      basicPage(
        uiOutput(&quot;inBody&quot;),
        uiOutput(&quot;inVar&quot;)
      )
    )

A short while ago, I posted the same question at shiny-discuss, but it has generated little interest, so I&#39;m asking again, with apologies, https://groups.google.com/forum/#!topic/shiny-discuss/e0MgmMskfWo


__Edit 1__

@Ramnath has kindly posted a solution that appears to work, denoted _Edit 2_ by him. But that solution does not address the problem because the `textinput` is on the `ui` side instead of on the `server` side as it is in my problem. If I move the `textinput` of Ramnath&#39;s second edit to the `server` side, the problem crops up again, namely: nothing shows and RStudio crashes. I found that wrapping `input$text` in `as.character` makes the problem disappear.

__Edit 2__

In further discussion, Ramnath has shown me that the problem arises when the server attempts to apply the dynamic function `outVar` before its arguments have been returned by `textinput`. The solution is to first check whether `is.null(input$inBody)` exists.

_**Checking for existence of arguments is a crucial aspect of building a shiny app**_, so why did I not think of it? Well, I did, but I must have done something wrong! Considering the amount of time I spent on the problem, it&#39;s a bitter experience. I show after the code how to check for existence.

Below is Ramnath&#39;s code with `textinput` moved to the `server` side. It crashes RStudio so don&#39;t try it at home. (I have used his notation)

    library(shiny)
    runApp(list(
      ui = bootstrapPage(
        uiOutput(&#39;textbox&#39;),  ## moving Ramnath&#39;s textinput to the server side
        uiOutput(&#39;variables&#39;)
      ),
      server = function(input, output){
        outVar &lt;- reactive({
          vars &lt;- all.vars(parse(text = input$text))  ## existence check needed here to prevent a crash
          vars &lt;- as.list(vars)
          return(vars)
        })
    
        output$textbox = renderUI({
          textInput(&quot;text&quot;, &quot;Enter Formula&quot;, &quot;a=b+c&quot;)
        })
    
        output$variables = renderUI({
          selectInput(&#39;variables2&#39;, &#39;Variables&#39;, outVar())
        })
      }
    ))

The way I usually check for existence is like this:

    if (is.null(input$text) || is.na(input$text)){
      return()
    } else {
      vars &lt;- all.vars(parse(text = input$text))
      return(vars)
    }

Ramnath&#39;s code is shorter:

    if (!is.null(mytext)){
      mytext = input$text
      vars &lt;- all.vars(parse(text = mytext))
      return(vars)
    }

Both seem to work, but I&#39;ll be doing it Ramnath&#39;s way from now on: maybe an unbalanced bracket in my construct had earlier prevented me to make the check work? Ramnath&#39;s check is more direct.

Lastly, I&#39;d like to note a couple of things about my various attempts to debug. 

In my debugging quest, I discovered that there is an option to &quot;rank&quot; the priority of &quot;outputs&quot; on the server side, which I explored in an attempt to solve my problem, but didn&#39;t work since the problem was elsewhere. Still, it&#39;s interesting to know and seems not very well known at this time:

    outputOptions(output, &quot;textbox&quot;, priority = 1)
    outputOptions(output, &quot;variables&quot;, priority = 2)

In that quest, I also _tried_ `try`:

    try(vars &lt;- all.vars(parse(text = input$text)))

That was pretty close, but still did not fix it.

The first solution I stumbled upon was: 

    vars &lt;- all.vars(parse(text = as.character(input$text)))

I suppose it would be interesting to know why it worked: is it because it slows things down enough? is it because `as.character` &quot;waits&quot; for `input$text` to be non-null?

Whatever the case may be, I am extremely grateful to Ramnath for his effort, patience and guidance.

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R shiny passing reactive to selectInput choices