diff --git a/.formatter.exs b/.formatter.exs
new file mode 100644
index 0000000..d2cda26
--- /dev/null
+++ b/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..735f29d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,25 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+commandex-*.tar
+
+.iex.exs
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..7b819db
--- /dev/null
+++ b/README.md
@@ -0,0 +1,21 @@
+# Commandex
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `commandex` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:commandex, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at [https://hexdocs.pm/commandex](https://hexdocs.pm/commandex).
+
diff --git a/lib/commandex.ex b/lib/commandex.ex
new file mode 100644
index 0000000..89d025a
--- /dev/null
+++ b/lib/commandex.ex
@@ -0,0 +1,81 @@
+defmodule Commandex do
+  @moduledoc """
+  Documentation for Commandex.
+  """
+
+  @doc false
+  defmacro __using__(_opts) do
+    prelude =
+      quote do
+        # @after_compile Commandex
+        Module.register_attribute(__MODULE__, :struct_fields, accumulate: true)
+        Module.put_attribute(__MODULE__, :struct_fields, {:success, false})
+        Module.put_attribute(__MODULE__, :struct_fields, {:error, nil})
+        Module.put_attribute(__MODULE__, :struct_fields, {:halted, false})
+      end
+
+    postlude =
+      quote unquote: false do
+        params = for key <- Module.get_attribute(__MODULE__, :params), into: %{}, do: {key, nil}
+        data = for key <- Module.get_attribute(__MODULE__, :data), into: %{}, do: {key, nil}
+
+        Module.put_attribute(__MODULE__, :struct_fields, {:params, params})
+        Module.put_attribute(__MODULE__, :struct_fields, {:data, data})
+        defstruct @struct_fields
+        import Commandex
+      end
+
+    quote do
+      unquote(prelude)
+      unquote(postlude)
+
+      def new(opts) do
+        Commandex.parse_params(%__MODULE__{}, opts)
+      end
+
+      def run(command) do
+        pipeline()
+        |> Enum.reduce_while(command, fn fun, acc ->
+          case acc do
+            %{halted: false} -> {:cont, fun.(acc, acc.params, acc.data)}
+            _ -> {:halt, acc}
+          end
+        end)
+        |> Commandex.maybe_mark_successful()
+      end
+    end
+  end
+
+  def maybe_mark_successful(%{halted: false} = command), do: %{command | success: true}
+  def maybe_mark_successful(command), do: command
+
+  @doc false
+  def parse_params(%{params: p} = struct, params) when is_list(params) do
+    params = for {key, _} <- p, into: %{}, do: {key, Keyword.get(params, key)}
+    %{struct | params: params}
+  end
+
+  def parse_params(%{params: p} = struct, %{} = params) do
+    params = for {key, _} <- p, into: %{}, do: {key, get_param(params, key)}
+    %{struct | params: params}
+  end
+
+  defp get_param(params, key) do
+    case Map.get(params, key) do
+      nil -> Map.get(params, to_string(key))
+      val -> val
+    end
+  end
+
+  def put_data(%{data: data} = command, key, val) do
+    %{command | data: Map.put(data, key, val)}
+  end
+
+  def put_error(command, error) do
+    %{command | error: error}
+  end
+
+  def halt(command) do
+    %{command | halted: true}
+  end
+end
diff --git a/lib/register_user.ex b/lib/register_user.ex
new file mode 100644
index 0000000..d88c6eb
--- /dev/null
+++ b/lib/register_user.ex
@@ -0,0 +1,27 @@
+defmodule Commandex.RegisterUser do
+  @params ~w(email password)a
+  @data ~w(user auth)a
+
+  use Commandex
+
+  def pipeline do
+    [
+      &create_user/3,
+      &record_auth_attempt/3
+    ]
+  end
+
+  def create_user(command, %{password: nil} = _params, data) do
+    command
+    |> put_error(:no_password)
+    |> halt()
+  end
+
+  def create_user(command, %{email: email} = _params, data) do
+    put_data(command, :user, %{email: email})
+  end
+
+  def record_auth_attempt(command, _params, _data) do
+    put_data(command, :auth, true)
+  end
+end
diff --git a/mix.exs b/mix.exs
new file mode 100644
index 0000000..36000bd
--- /dev/null
+++ b/mix.exs
@@ -0,0 +1,21 @@
+defmodule Commandex.MixProject do
+  use Mix.Project
+
+  @version "0.1.0"
+
+  def project do
+    [
+      app: :commandex,
+      deps: [],
+      elixir: "~> 1.9",
+      start_permanent: Mix.env() == :prod,
+      version: @version
+    ]
+  end
+
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+end
diff --git a/test/commandex_test.exs b/test/commandex_test.exs
new file mode 100644
index 0000000..0e527ce
--- /dev/null
+++ b/test/commandex_test.exs
@@ -0,0 +1,4 @@
+defmodule CommandexTest do
+  use ExUnit.Case
+  doctest Commandex
+end
diff --git a/test/test_helper.exs b/test/test_helper.exs
new file mode 100644
index 0000000..869559e
--- /dev/null
+++ b/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()