DeriveT

newtype DeriveT m a = DeriveT (DeriveStack m a)
    deriving (Functor, Monad, Trans.MonadIO, Error.MonadError DeriveError)
run_derive_t (DeriveT m) = m

type DeriveStack m = Error.ErrorT DeriveError
    (Monad.State.StateT State
        (Log.LogT m))

data State = State {
    -- Environment.  These form a dynamically scoped environment that applies
    -- to generated events inside its scope.

    -- | Derivers can modify it for sub-derivers, or look at it, whether to
    -- attach to an Event or to handle internally.
    state_controls :: Score.ControlMap
    -- | Absolute pitch signal currently in scope.
    , state_pitch :: PitchSignal.PitchSignal
    , state_instrument :: Maybe Score.Instrument
    , state_attributes :: Score.Attributes
    , state_warp :: Warp
    -- | This is the call stack for events.  It's used for error reporting,
    -- and attached to events in case they want to emit errors later (say
    -- during performance).
    , state_stack :: [Warning.StackPos]
    , state_log_context :: [String]

    -- | Remember the warp signal for each track.  A warp usually a applies to
    -- a set of tracks, so remembering them together will make the updater more
    -- efficient when it inverts them to get playback position.
    , state_track_warps :: [TrackWarp]

    -- | Constant throughout the derivation.  Used to look up tracks and
    -- blocks.
    , state_ui :: State.State
    , state_lookup_deriver :: LookupDeriver
    , state_control_op_map :: Map.Map Operator ControlOp
    , state_pitch_op_map :: Map.Map Operator PitchOp
    , state_call_map :: CallEnv
    -- | This is set if the derivation is for a signal deriver.  Signal
    -- derivers skip all special tempo treatment.  Ultimately, this is needed
    -- because of the 'add_track_warp' hack.  It might be 'add_track_warp' is
    -- too error-prone to allow to live...
    , state_ignore_tempo :: Bool
    }