Ask HN: What are your music prototyping solutions? 15 years ago
If you have an iPad, check out: http://symphonypro.net.
The app allows you to create and edit sheet music. There's a built in keyboard and a real-time recording feature. If you have a Core MIDI compatible device, you connect it to your iPad and input notes in real time from that as well.
Disclaimer: I'm one of the developers