Forms and masked input

A complete greeting form #

On GET /, the handler returns a form. Activating its submit control sends POST /greet with ctx.fields.name. The handler validates the field and returns a personalized page. Paste the whole example into app.lua, then validate and preview.

return function(ctx)
  local P = ctx.page
  if ctx.method == "POST" and ctx.path == "/greet" then
    local name = ctx.fields.name
    if type(name) ~= "string" or not name:match("%S") or #name > 60 then
      return {kind="error", status=400, message="Enter a name, up to 60 characters."}
    end
    return {kind="page", page=P.document("Welcome", {
      P.heading{text="Hello, " .. name .. "!"},
      P.link{text="Try again", href="/"}
    })}
  end
  if ctx.method ~= "GET" or ctx.path ~= "/" then
    return {kind="error", status=404, message="Page not found"}
  end
  return {kind="page", page=P.document("Say hello", {
    P.form{action="/greet", children={
      P.input{name="name", text="Your name"},
      P.submit{text="Say hello"}
    }}
  })}
end

Field behavior #

Give each field in a form a distinct name using letters, digits, underscores or hyphens (up to 40 characters). Put all field controls inside form.children. Nested forms are invalid. A form can submit only to a path on its own site.

Use Tab and Enter, or click a control. Input opens a text prompt, checkboxes toggle, and select controls choose an option. Navigating clears form values. Always validate incoming values: another client can submit fields without using your form.

Password masking is not authentication #

password=true hides typing and the displayed value in an updated Lantern browser. The site operator still receives the field. Masking does not create users, sessions, password hashing, authorization or a password-reset flow. Never ask visitors to reuse real passwords in a demo.

The official Observatory uses the public code glow. It checks that code and returns a response without saving or echoing it. This demonstrates conditional server logic, not a private account system.

P.input{name="code", text="Demo code", password=true}

Retries and side effects #

The Hub records request IDs to prevent silent repeated execution. Reusing an ID with different content is rejected. A request that is still running or has failed is not automatically rerun. If a submission fails, check the current state before asking someone to submit it again.

Do not use ctx.requestId as proof of identity. Requests do not include a built-in authenticated site visitor. The Hub creator’s GitHub session is separate from visitors to your Lantern pages.