Sentinel indicating a computation was cancelled.

Like Throw, Cancelled goes through the leave_scope chain, allowing effects to clean up resources (close connections, release locks, etc.) when a computation is cancelled.

Usage

When cancelling a suspended computation, use Skuld.Comp.cancel/2:

# Computation yielded a Suspend
{%Suspend{} = suspend, env} = Comp.run(my_comp)

# Cancel it (invokes leave_scope chain for cleanup)
{%Cancelled{reason: :user_cancelled}, final_env} =
  Comp.cancel(suspend, env, :user_cancelled)

Effect Cleanup

Effects can detect cancellation in their leave_scope handler:

def my_leave_scope(result, env) do
  case result do
    %Cancelled{reason: reason} ->
      cleanup_resources(reason)
      {result, env}

    _ ->
      {result, env}
  end
end