# Playing an Expression
This sequence diagram illustrates the fundamental interactions between components when playing an expression in Melrose.
```mermaid
sequenceDiagram
actor User
participant REPL as REPL (ui/cli)
participant Evaluator as Evaluator (dsl)
participant Play as play() (control)
participant Device as DeviceRegistry (midi)
participant Output as OutputDevice (midi)
participant Timeline as Timeline (core)
participant MIDI as MIDI Stream
User->>REPL: play(note('C'))
REPL->>Evaluator: EvaluateProgram("play(note('C'))")
Evaluator->>Play: NewPlay(...)
Play-->>Evaluator: playable
Evaluator->>Play: playable.Evaluate()
Play->>Device: Play(sequence, bpm, now)
Device->>Output: Play(sequence, bpm, now)
Output->>Timeline: Schedule(noteOn)
Output->>Timeline: Schedule(noteOff)
Timeline-->>MIDI: WriteShort(NoteOn)
Timeline-->>MIDI: WriteShort(NoteOff)
```
