# `BoxTurtle.Runtime.Application.Helper`
[🔗](https://github.com/twajjo/box_turtle/blob/v0.1.0/lib/runtime/application/helper.ex#L1)

Utilities for building an application's supervision tree.

Import the module in your `Application` and build the children list step by step:

    defmodule MyApp.Application do
      use Application

      import BoxTurtle.Runtime.Application.Helper

      @impl true
      def start(_type, _args) do
        children =
          [MyApp.Repo]
          # A boolean, e.g. from config: config :my_app, start_web?: false
          |> conditional_append(Application.get_env(:my_app, :start_web?, true), MyAppWeb.Endpoint)
          # A function, called when the tree is built
          |> conditional_append(&MyApp.Features.workers_enabled?/0, [MyApp.Worker, MyApp.Scheduler])
          # A function with arguments
          |> conditional_append(&Code.ensure_loaded?/1, MyApp.Metrics, [Telemetry.Metrics])

        Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
      end
    end

# `condition`

```elixir
@type condition() :: boolean() | function()
```

A boolean, or a function returning one when applied to the `args` list.

# `conditional_append`

```elixir
@spec conditional_append(list(), condition(), term(), list()) :: list()
```

Appends `conditional` (a single child or a list of children) to `current` when `condition`
holds.

`condition` is either a boolean or a function. A function is applied to `args`
(as with `Kernel.apply/2`) and must return a boolean.

## Examples

    iex> conditional_append([:a], true, :b)
    [:a, :b]

    iex> conditional_append([:a], false, [:b, :c])
    [:a]

    iex> conditional_append([:a], fn x, y -> x < y end, [:b], [1, 2])
    [:a, :b]

---

*Consult [api-reference.md](api-reference.md) for complete listing*