Comment by 4ad
2 years ago
Invariants need to be encoded one way or another anyway, you can't escape that. If your type system is not sophisticated enough this gets tedious and awkward to do in the type system, so it's easier to do in (dynamic) code. But dependent types simplify this, they make it much easier to express yourself in types compared to, say, all the various fancy extensions to System F. Computing types becomes just like computing terms.
That said, there is a vast design space between Javascript/Python and dependent types. Plain old ADT suffice in 95% of cases, yet the only mainstream language with ADTs is Rust. This is a shame.
(Assuming ADT here refers to Algebraic Data Types, as opposed to abstract data types)
Swift, Kotlin, Scala and Typescript all support forms of ADTs, just with different names.
- Swift calls them "enums" (like Rust)
- Kotlin and Scala has "sealed classes"
- Typescript has "discriminated unions"
I forgot about Swift, that's true (and great).
Kotlin (and even Java 15+ nowadays) can emulate ADTs with sealed classes, but the ergonomics are incredibly bad and the ecosystem is not build around the concept.
Scala has ADTs, but I would put Scala in the same category as Haskell or OCaml. A niche language.
Typescript does not have ADT, they have union types. You can build a discriminated type out of a union type, but you have to do this manually. This misses on the ergonomics of using ADTs plus people do it in different ways.
ADTs in one way or another are becoming more mainstream, but they are very far from being accepted by default.
Agree with you on the ergonomics of sealed classes in kotlin/scala/java. It works, but it's clunky.
I quite like Typescript's approach. You do get exhaustive switch-case matching, so that's like 80% of what I want out of sum types. Typescript also lets you enforce that a type is one of the variants of a sum type, something which e.g. Haskell doesn't let you do. I assume this is a function of Typescript doing union types, but it's pretty convenient.
Definitely see ADTs becoming mainstream, because they are genuinely useful. Biggest gap to adoption imo is that no SQL database supports sum types or anything equivalent.
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I definitely agree that ADTs are a big step forward; when I learned about them from Haskell, I was kind of baffled that they weren't more common, because the concept of "this value is always either type A or type B" is pretty simple and occurs a lot. I really wish Go had included them, because it doesn't seem like a terribly complex concept, the type switch syntax would work decently for basic pattern matching, and having ADTs built into the ecosystem from the start would be much smoother.