Correct way to integrate Elm with existing web-app service

i have a backend service that I wrote in Golang, something like this:

func PageA_RequestHandler(ctx *W.Context) {
  // init things
  if is_ajax {
    // handle the API request, render the JSON
    return
  }
  // query the initial rows
  values := M.SX{
    `rows`: model1.GetRows(10),
    `columns`: model1.GetColumns(),
  }
  // render the html
  ctx.Render(`page_a_template`,values)
}

then the template file page_a_template.html (that loaded and cached at the first time it rendered), is a html file, with content something like this:

<div id="grid"></div>
<script>
  var rows = {/* rows */};
  var cols = [/* columns */]
  new GridBuilder('grid',cols,rows);
</script>

Where's:

{/* rows */} and [/* columns */] are my javascript-friendly template syntax, there are some other syntax like: /*! bar */ or #{yay}

new GridBuilder is my custom javascript component that creates something like datatables.net or editablegrid.net

The question is, if I use Elm, what's the correct way to inject the {/* rows */} into the compiled html?

Use Html.programWithFlags (supposing your main module is named App):

<div id="grid"></div>
<script>
  var rows = {/* rows */};
  var cols = [/* columns */];
  var node = document.getElementById('grid');
  Elm.App.embed(node, { rows: rows, cols: cols });
</script>

This is the way you can initialize your Elm app with available data just like you are initializing GridBuilder.

To be able to read those values, you should create a Flags type alias and your init function will receive it.

type alias Flags =
    { rows : [ ... ]
    , cols : [ ... ]
    }

init : Flags -> ( Model, Cmd Msg )
init flags =
    ...

Your main function will look like this:

main =
    Html.programWithFlags
        { init = init, ... }

I highly recommend that you read the JavaScript Interop section of the Elm Guide. It explains the programWithFlags and other approaches to receive/send data from/to JavaScript.