> I'm not saying the nominal approach to types is wrong or bad, I just find my way of thinking is better suited for structural systems. I'm thinking less about the semantics around product_id vs user_id and more about what transforms are relevant - the semantics show up in the domain layer.

But that domain layer should make use of the type system! That's where the type system is most useful!

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RossBencina 2 hours ago | parent [-]

I've seen this debate play out before. Often the type theory people and the domain modeling people end up talking past each other.

I think type-theoretic safety is a completely different thing to the use of types (and names) in software-as-domain-modeling (for example, but not necessarily OO modelling). At different times, for different people, one perspective is more important than the other. It is important not to confuse the perspectives, and to value both of them, but also to recognise their strengths and weaknesses.

One theme that sometimes emerges is that the type theory people don't care about names at all. Not even field names. Taken to the extreme Customer( name: str; age: int) is just a Tuple[str, int]. The words "Customer", "name", "age" have no place in the code base.

My take is that when you are dealing with computer-scientific abstract things, e.g. a List of T, then there is no need to reference the domain entities; placeholder names like T, x, xs make sense. On the other hand, if you're writing an application that models domain semantics (eg. business rules), writing software amounts to modelling the domain, it should be easy to correlate software entities with the real-world entities that they model. To do this we use descriptive words, including names, domain events, activities and so on. e.g. List[Customer] not List[Tuple[str, int]]. Then again, you could replace all of the type names with A, B, C, ... and all the variable names with w, x, y, .... The example would end up as X[Y[Z,W]], the software would work exactly the same, and you might get some insights into the structure of the system. However, if you're in the business of building a user management system in general this is not going to fly for very long with your workmates or your client. You will have trouble onboarding new developers.

	▲	b_e_n_t_o_n 2 hours ago \| parent \| next [-]
		I think this is a superb point. If you look at Clojure and Rich Hickey's justifications for it's design, he talks a lot about designing systems that work with a world that is changing constantly, interfacing with systems where you can't always guarantee a stable contract, solving problems that use non-elegant models, dealing with state and time, and change in ways you can't predict. Eric Normand wrote a great article on this and he comes from a dual Haskel/Clojure background [1]. Nominal static type systems absolutely have their place, especially with closed systems where the abstractions are different. 1: https://ericnormand.me/article/clojure-and-types
	▲	lmm 2 hours ago \| parent \| prev [-]
		I'm not sure which side you're putting me on, because I think named types are important for exactly the same reason that named fields are. A customer should not just be a pair of str, int in the same way that an age should not just be an int and a name should not just be a string (and using an int field called "age" is a poor substitute for using an actual age type).