Compile once, render many times

app_root = Path.expand("..", __DIR__)

deps =
  if File.exists?(Path.join(app_root, "mix.exs")) do
    [{:letterpress, path: app_root}]
  else
    [{:letterpress, "~> 0.1"}]
  end

Mix.install(deps)

unless Letterpress.Compiler.available?() do
  {:ok, _pid} = Letterpress.Compiler.Supervisor.start_link(pool_size: 1)
end

Letterpress separates authoring from delivery. Authoring validates a typed schema and compiles source into an immutable artifact. Delivery renders that artifact without Node or MJML.

Compile an email

The schema declares each value and its output context. Contexts are enforced rules, not labels for editor display.

schema = %{
  "version" => 1,
  "variables" => %{
    "html_name" => %{"type" => "string", "context" => "html_text"},
    "subject_name" => %{"type" => "string", "context" => "subject"},
    "text_name" => %{"type" => "string", "context" => "text"},
    "action_url" => %{"type" => "url", "context" => "url"}
  }
}

source = """
<mjml>
  <mj-body>
    <mj-section>
      <mj-column>
        <mj-text>Hello {{ html_name }}</mj-text>
        <mj-button href="{{ action_url }}">Open account</mj-button>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>
"""

{:ok, artifact, diagnostics} =
  Letterpress.compile("email/mjml-liquid@1", source, schema,
    subject: "Welcome, {{ subject_name }}",
    text: "Hello {{ text_name }}. Open {{ action_url }}"
  )

{artifact.profile, diagnostics}

{"email/mjml-liquid@1", []}

Persist the complete artifact, not only its HTML. The artifact carries the profile, compiler identity, schema, source map, and content hash needed for safe decoding and later rendering.

Render at delivery time

All channels render atomically. A subject, HTML, or text failure returns an error instead of a partial message.

{:ok, rendered} =
  Letterpress.render(artifact, %{
    "html_name" => "Taylor",
    "subject_name" => "Taylor",
    "text_name" => "Taylor",
    "action_url" => "https://example.test/account"
  })

Map.take(rendered, [:subject, :text])

%{subject: "Welcome, Taylor", text: "Hello Taylor. Open https://example.test/account"}

The consumer now passes rendered.subject, rendered.html, and rendered.text to its chosen delivery library. Letterpress does not own the provider or send operation.