[beagle3]

Indeed. But ... do not confuse your model with reality.

There's a folk story - I don't remember where I read it - about a genealogy database that made it impossible to e.g. have someone be both the father and the grandfather of the same person. Which worked well until they had to put in details about a person who had fathered a child with his own daughter - and was thus both the father and the grandfather of that child. (Sad as it might be, it is something that can, in fact, happen in reality, and unfortunately does).

While that was probably just database constraints of some sort which could easily be relaxed, and not strictly "unrepresentable" like in the example in the article - it is easy to paint yourself into a corner by making a possible state of the world, which your mental model dims impossible, unrepresentable.

[louthy]

Your example doesn’t validate your point. That’s a valid state made unrepresentable, not an invalid state made unrepresentable. Your example simply demonstrates a poorly architected set of constraints.

The critical thing with state and constraints is knowing at what level the constraint should be. This is what trips up most people, especially when designing relational database schemas.

[jacquesm]

Any assumption made in order to ship a product on time will eventually be found to have been incorrect and will cause 10x the cost that it would have taken to properly design the thing in the first place. The problem is that if you do that proper design you never survive to the stage where have that problem.

I think the solution to that is to continuously refactor, and to spell out very clearly what your assumptions are when you are writing the code (which is an excellent use for comments).

[watt]

I do see how it does, in a way. That something the designer thought is "invalid state" turns out a valid and possible state in real world. In terms or UI/UX, it's the uncomfortable long pause before something happens and screen renders (lack of feedback, feeling that system hangs). Or, content flicker when window is resized or dragged. Just because somebody thought "oh, this clearly is invalid state and can be ignored".

The real world and user experience requirements have a way of intruding on these underspecified models of how the world "should" be.

[flir]

> at what level the constraint should be

Hi, can you give an example? Not sure I understand what you're getting at there.

(My tuppence: "the map is not the territory", "untruths programmers believe about...", "Those drawn with a very fine camel's hair brush", etc etc.

All models are wrong, and that's inevitable/fine, as long as the model can be altered without pain. Focus on ease of improving the model (eg can we do rollbacks?) is more valuable than getting the model "right").

[incognito124]

That sounds like this (in)famous stackowerflow question: https://stackoverflow.com/a/6198257