I took up the violin just a few years ago---my midlife crisis: neither cheaper nor less noisy than a motorbike---and it's been fascinating to dive into the maths of music in a way that my school music teachers were never able to satisfactorily explain.

At first I too was tempted to build an app just like this, but when I mentioned this to my teacher, she made it very plain that the only way to learn intonation is: hear, move fingers, repeat. Anything that gets in the way of making that feedback loop as tight as possible---devices, tuners, even conscious thought---is a distraction. In hindsight I think she's right. Though I too am still a barely endurable amateur, my intonation has improved to the extent that it feels like I have grown a new organ in my brain that connects my ears directly to my fingers.