Your mockup reminds me of Mathcad[0]. It's used to create documents that look like math papers, except the formulas and code there can be executed "in situ". I had a semester of numerical methods on it at my university, and one thing I realized while using it is that, while cool for math expressions, it starts to get problematic when you start implementing more complex algorithms. Programming is a slightly different mental exercise than maths, and the code doesn't lend easily to that kind of representation.

Not to say it's a thoroughly bad idea; I think there's a lot room for improvement over our flat, plaintext canvas we work with.

[0] - https://en.wikipedia.org/wiki/Mathcad