@spec compile(String.t()) ::
  {:ok, (-> {:ok, TuningFork.Score.t()} | {:error, String.t()})}
  | {:error, String.t()}

The source compiled once into a function that gives a fresh score every time it is called.

iex> {:ok, body} = TuningFork.Part.Source.compile("part(bpm: 120) |> play(:c3, 1)")
iex> {:ok, %TuningFork.Score{}} = body.()

Compile once and call the function as often as needed: each call evaluates the source again, so TuningFork.Tick counters advance and TuningFork.State values are read as they stand at that call. TuningFork.Stage calls it once per round.

The function returns {:ok, score} or {:error, message}; source that compiles can still fail on a later call.

@spec fault(String.t()) :: String.t() | nil

Why source will not read, in one line, or nil when it reads.

parse/1 said the other way round, for a caller that only wants to report.

@spec parse(String.t()) :: {:ok, TuningFork.Score.t()} | {:error, String.t()}

The score source describes, or why it will not read.

Returns {:ok, %TuningFork.Score{}} or {:error, message}, the message trimmed to one line so it fits under the loop it belongs to. Empty source is an empty score rather than an error, so a loop being written from scratch reports nothing until there is something to report.

@spec reference() :: [String.t()]

What a loop may be written with, as lines of text.

The vocabulary parse/1 evaluates against. TuningFork.LoopsApp.reference/0 puts the terminal's keys around it; the Livebook board shows it behind ?.

@spec template() :: String.t()

Source a new, empty loop opens on: a four-beat drum part that plays as it stands.

iex> {:ok, score} = TuningFork.Part.Source.parse(TuningFork.Part.Source.template())
iex> TuningFork.Score.duration(score)
2.0

What mix tuning_fork.loops gives a loop made with Ctrl+N, and what the Livebook board's + loop gives a new one.