Unfortunately, it is not a bug. This is a limitation of Drab.Live’s local variables scope && the way how Drab is replacing parts of the page with the evaluated expression with new assign values.

When you’re poking the new version of assign @page_id, Drab tries to update it in every expression on a page, which contains that value. In this case, it is if page.id == @page_id, do:.... But in the particular moment it does not know the value of page variable - that’s why it shows undefined function page() while evaluating this expression.

It is done by purpose. I could search expressions for assigns in their do..end blocks, and re-render the the whole for comprehension while changing @page_id. And actually it was made like this before, but I realized that in the real world it provides to full reload of almost the whole page and more issues, like #42.

In this particular situation, when you poke @page_id, you really want to update the whole for compehension. One of the solution would be to put @page_id assign in the comprehension filter:

<%= for page <- @holder.pages, page.id == @page_id do %>
  <tr>
    <td><%= page.title %></td>
  </tr>
<% end %>

This moves @page_id to the for expression, and now, after you poke it, it re-evaluates the whole one.

I know it is not very intuitive, but so far I couldn’t find the better solution.

Anyway, please create an issue on github, this situation needs more explanation/docs and/or at least the better error description. Probably it should not even compile.

Docs: Drab.Live.EExEngine – drab v0.10.5