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You should check out IDEs :)

IDE takes plain old text as input, and lets the coder to interact with the model of the program.

1. Various graphical hints in text (highlighting keywords, selections, …)

2. Visualisation tools (draw database and class diagrams)

3. Navigation capabilities (go to implementation, definition etc)

4. Refactoring capabilities (“rename, “extract variable”, …)

In my view, IDE is already “book with diagrams”. (“Diagrams” can still be improved, but concept is really there)

True, but IDEs are currently severely limited by nature of textual sources (e.g. macros are notoriously tricky to deal with). It's much easier to start from a good model for IDE and derive textual reprentation and input method from that.

No, it is not the case that if the thing I said was true, then textbooks wouldn't contain diagrams.

If GPs statement were true, textbooks would be called text books instead of diagram books.