Well, the expression problem is a clearly formulated question: how can we have three desirable things at the same time (no need to modify existing code, no need

Well, the expression problem is a clearly formulated question: how can we have three desirable things at the same time (no need to modify existing code, no need for code repetition and no need for runtime type errors [1])? See also the project management triangle: if you need something fast, good and cheap, then fast, good and expensive won't be a satisfactory answer, no matter how many millionaires are satisfied with the latter. [1] https://link.springer.com/chapter/10.1007/978-3-540-24851-4_6 https://link.springer.com/chapter/10.1007/978-3-540-24851-4_...