Harlock

A pure-Elixir TUI framework for Unix terminals. TEA-style model / update / view loop on top of OTP, with first-class focus traversal, layout constraints, ANSI cell-diff rendering, and a small termios NIF for direct /dev/tty control.

defmodule Counter do
  use Harlock.App     # imports the view DSL (box/1, text/2, vbox/1, …)

  def init(_), do: %{n: 0}

  def update({:key, {:char, ?+}, []}, m), do: %{m | n: m.n + 1}
  def update({:key, {:char, ?-}, []}, m), do: %{m | n: max(0, m.n - 1)}
  def update({:key, {:char, ?q}, []}, _), do: :quit
  def update(_, m), do: m

  def view(m) do
    box(
      title: "Counter",
      border: :rounded,
      child: text("count: #{m.n}")
    )
  end
end

Harlock.run(Counter)

A more realistic app wires focus traversal, a selectable table, a scrollable viewport, and a side-effect via Cmd — all together. Tab moves focus between the two boxes; the focused widget owns its keys. Full source: examples/overview.exs.

defmodule Overview do
  use Harlock.App
  alias Harlock.{Cmd, Focus}

  def init(_) do
    %{
      tasks: [
        %{id: 1, name: "compile", state: "done"},
        %{id: 2, name: "test", state: "running"},
        %{id: 3, name: "dialyzer", state: "queued"},
        %{id: 4, name: "credo", state: "queued"},
        %{id: 5, name: "publish", state: "blocked"}
      ],
      selected: 1,
      log: for(i <- 1..40, do: "[#{i}] event line #{i}"),
      log_offset: 0
    }
  end

  def update({:key, {:char, ?q}, []}, _), do: :quit

  def update({:key, {:char, ?r}, []}, m) do
    cmd =
      Cmd.from(fn -> Enum.map(1..3, &"[refresh] new line #{&1}") end)
      |> Cmd.map(fn lines -> {:refreshed, lines} end)

    {m, cmd}
  end

  def update({:refreshed, lines}, m), do: %{m | log: lines ++ m.log}

  # The runtime auto-routes scroll keys to the focused viewport and
  # delivers this message; the app just writes where the offset lives.
  def update({:harlock_scroll, :log, new_offset}, m), do: %{m | log_offset: new_offset}

  def update({:key, _, _} = ev, m) do
    case Focus.current() do
      :tasks -> update_tasks(ev, m)
      _ -> m
    end
  end

  def update(_, m), do: m

  defp update_tasks({:key, :up, _}, m), do: %{m | selected: max(1, m.selected - 1)}

  defp update_tasks({:key, :down, _}, m),
    do: %{m | selected: min(length(m.tasks), m.selected + 1)}

  defp update_tasks(_, m), do: m

  def view(m) do
    here = Focus.current()

    vbox(
      constraints: [fill: 1, length: 1],
      children: [
        hbox(
          constraints: [percentage: 40, fill: 1],
          children: [
            box(
              title: "Tasks",
              border: :rounded,
              border_style: border_style(here == :tasks),
              focusable: :tasks,
              child:
                table(
                  columns: [
                    column(title: "#", width: {:length, 3}, render: &Integer.to_string(&1.id)),
                    column(title: "name", width: {:fill, 1}, render: & &1.name),
                    column(title: "state", width: {:length, 8}, render: & &1.state)
                  ],
                  rows: m.tasks,
                  row_id: & &1.id,
                  selection: {:single, m.selected}
                )
            ),
            box(
              title: "Log",
              border: :rounded,
              border_style: border_style(here == :log),
              child:
                viewport(
                  focusable: :log,
                  offset: m.log_offset,
                  content_height: length(m.log),
                  child:
                    vbox(
                      constraints: List.duplicate({:length, 1}, length(m.log)),
                      children: Enum.map(m.log, &text/1)
                    )
                )
            )
          ]
        ),
        text("Tab focus  arrows/PgUp/PgDn scroll  r refresh  q quit", style: [dim: true])
      ]
    )
  end

  defp border_style(true), do: [fg: :cyan, bold: true]
  defp border_style(false), do: [dim: true]
end

Harlock.run(Overview)

Installation

def deps do
  [{:harlock, "~> 0.4"}]
end

Harlock builds a small C NIF (c_src/termios.c, ~250 LOC of POSIX) for termios access — elixir_make handles this automatically. Requires a C compiler and make available at install time. macOS, Linux, and *BSD are supported; Windows native is not (WSL works).

Why Harlock

If you've written a Phoenix LiveView app you already know how to use Harlock — init / update / view, message-passing for events, side-effects as Cmd values. The runtime is a single OTP supervision tree: terminal owner → IO → cmd executor → TEA loop, with terminal restoration guaranteed on any crash path via the supervisor's rest_for_one strategy.

Compared to alternatives:

• Owl is a styled-output library ("println but pretty"). Harlock is a full interactive runtime — focus, layout, dirty-flag rendering, async cmds, resize handling.
• Ratatouille wraps termbox via a C port. Solid, but the C dep is bigger and the runtime model is its own thing. Harlock is pure-Erlang for rendering with a small in-process NIF only for termios — closer to "Elixir all the way down" if that matters to you.
• ratatui-via-port approaches (Rust binary speaking a wire protocol to BEAM) ship as two artifacts: your Elixir release plus a separately-compiled Rust binary that has to be on PATH at runtime. Harlock ships as one mix release — no extra binary, no version-skew between BEAM and renderer. The element tree is also ordinary Elixir data, which makes testing and composition easier than a wire-protocol boundary.

Status

Harlock is v0.4. The API is intentionally narrow and stable for the primitives it ships; widgets and ergonomics are still landing. Anything @moduledoc false is internal and free to change.

See ROADMAP.md for the full plan through v1.0.

Examples

./scripts/run.sh counter    # simplest possible app — count up/down
./scripts/run.sh sysmon     # live BEAM process monitor
./scripts/run.sh contacts   # contact manager: search, list, modal forms, async save
./scripts/run.sh showcase   # tabs, viewport, widgets, modified keys

The scripts/run.sh wrapper is in the GitHub repo — clone the repo to run the examples. The hex package itself is the library; apps depend on :harlock and build their own runtime entry point (see the Counter snippet above).

contacts exercises most of the core primitives: tab focus traversal, text_input fields, an overlay with focus_trap, async save via Cmd.from, custom theme, status bar with current-focus indicator.

showcase is a four-tab tour of everything that landed in v0.3 — a 200-row scrollable log viewer with viewport + scrollbar, a long form that uses scroll-into-view to keep the focused field visible, a widget gallery with animated progress/spinner/statusbar/keybar, and a key-event inspector you can use to try out modified arrows (Ctrl-Up, Shift-Right, etc.).

Testing your app

Harlock.Test boots an app under a headless backend — no /dev/tty required — and exposes synchronous helpers:

test "Tab cycles focus through the form" do
  h = Harlock.Test.start_app(MyApp, init_arg)

  Harlock.Test.send_key(h, :tab)
  assert Harlock.Test.focused(h) == :email

  Harlock.Test.send_key(h, :tab)
  assert Harlock.Test.focused(h) == :submit

  Harlock.Test.stop(h)
end

Same code path as the real runtime — only the bytes-in / bytes-out boundary is mocked.

Smoke tests

A handful of scripts in priv/*_smoke.exs exercise the real runtime + termios NIF via script(1):

./scripts/smoke.sh

Picks the right flag syntax for BSD vs util-linux script automatically.

Contributing

Issues and PRs welcome at https://github.com/thatsme/harlock. The codebase is small enough (~3k LOC of Elixir + ~250 LOC of C) to read in an afternoon. Start with lib/harlock/app/runtime.ex — everything else is reachable from there.

License

MIT. See LICENSE.