with chains in Elixir

A launch checklist: with walks the happy path of ok tuples and else catches the first miss.

defmodule Launch do
  defp fuel(:full), do: {:ok, 100}
  defp fuel(_tank), do: {:error, :fuel_low}

  defp weather("clear"), do: {:ok, "clear"}
  defp weather(sky), do: {:error, {:sky, sky}}

  # with chains steps that must all match; else handles any miss.
  def go?(tank, sky) do
    with {:ok, pct} <- fuel(tank),
         {:ok, _} <- weather(sky) do
      "launch with #{pct}% fuel"
    else
      {:error, why} -> "scrubbed: #{inspect(why)}"
    end
  end
end

IO.puts(Launch.go?(:full, "clear"))
IO.puts(Launch.go?(:full, "storm"))

How it works

1. fuel and weather each return {:ok, ...} or {:error, ...}.
2. with matches each <- step in order; passing them all reaches the do block.
3. The first failed match falls to else, which reports why the launch scrubbed.

Keywords and builtins used here

• IO
• Launch
• _
• def
• defmodule
• defp
• do
• else
• end

The run, in numbers

Lines

20

Characters to type

490

Tokens

151

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 129 seconds.

Type this snippet

Step 2 of 3 in Control flow, step 20 of 27 in Language basics.

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