Names are not type safety (2020)
20 comments
·August 3, 20255pl1n73r
My peers and I work on a language centered around "constructive data modeling" (first time I hear it called that). We implement integers, and indeed, things like non empty lists using algebraic data types, for example. You can both have a theory of values that doesn't rely on trapdoors like "int32" or "string", as well as encode invariants, as this article covers.
As I understand it, the primary purpose of newtypes is actually just to work around typeclass issues like in the examples mentioned at the end of the article. They are specifically designed to be zero cost, because you want to not pay when you work around the type class instance already being taken for the type you want to make an instance for. When you make an abstract data type by not exporting the data constructors, that can be done with or without newtype.
eru
The alternative to newtypes is probably to go the same route as OCaml and have people explicitly bring their own instances for typeclasses, instead of allowing each type only one instance?
I think OCaml calls these things modules or so. But the concepts are similar. For most cases, when there's one obvious instance that you want, having Haskell pick the instance is less of a hassle.
andyferris
These are possibly situations where I’d resort to a panic on the extra branch rather than complicate the return type.
Providing a proof of program correctness is pretty challenging even in languages that support it. In most cases careful checking of invariants at runtime (where not possible at compile time) and crashing loudly and early is sufficient for reliable-enough software.
nixpulvis
In Rust I find myself gaining a good bit of type safety without losing ergonomics by wrapping types in a newtype then implementing Deref for them. At first it might seem like a waste, but it prevents accidentally passing the wrong type of thing to a function (e.g. a user UUID as a post UUID).
lmm
IME this is exactly backwards: type safety is mostly about names, everything else is a nice-to-have. Yes, you can bypass your name checks if you want to, but you can bypass any type check if you want to. Most relevant type relationships in most programming are business relationships that would be prohibitively expensive to express in a full formalism if that was even possible. But putting names on them is cheap, easy, and effective. The biggest win from typed languages comes from using these basic techniques.
b_e_n_t_o_n
Hmm, IME the preferred type systems are structural - a function shouldn't care what the name is of the struct passed to it, it should just work if it has the correct fields.
lmm
I think that's backwards - ultimately everything on a computer is just bytes, so if you push that philosophy to the limit then you would write untyped functions and they can "just work" on any input (just not necessarily giving results that are sensible or useful if the input is wrong). The point of a type system is to help you avoid writing semantically wrong code, to bring errors forward, and actually the most important and valuable use case is distinguishing values that are structurally identical but semantically different (e.g. customer ID vs product ID, x coordinate vs y coordinate, immutable list vs read view of mutable list, sorted vs unsorted...).
seanmcdirmid
Structural type systems mostly don’t support encapsulation (private members that store things like account numbers) without some sort of weird add on, while nominal type systems support encapsulation directly (because the name hides structure). The canonical example is a cowboy and picture that both have a draw method.
b_e_n_t_o_n
Both Go and TS are structural and support encapsulation fine, I'm not sure why that would be an issue.
o11c
The critical problem with structural typing is that it requires weird and arbitrary branding when dealing with unions of singletons.
b_e_n_t_o_n
You mean like if you have two types which are identical but you want your type system to treat them as distinct? To me that's a data modelling issue rather than something wrong with the type system, but I understand how it can sometimes be unavoidable and you need to work around it.
I think it also makes more sense in immutable functional languages like clojure. Oddly enough I like it in Go too, despite being very different from clojure.
andyferris
If I understand you correctly - in popular structurally typed languages, sure.
It seems ok in upcoming languages with polymorphic sum types (eg Roc “tags”) though?
stirfish
> should just work if it has the correct fields.
Correct fields by...name? By structure? I'm trying to understand.
b_e_n_t_o_n
By name, type, and structure. In typescript for example:
let full_name = (in: { first: string, last: string }) => in.first + " " + in.last
b_e_n_t_o_n
Perhaps it's because I'm not a haskeller but I'm not sure if I'm sold on encoding this into the type system. In go (and other languages for example), you would simply use a struct with a hidden Int, and receiver methods for construction/modification/access. I'm not sure I see the benefit of the type ceremony around it.
the_af
Isn't the whole article a discussion of the kind of guarantees such an approach (which can also be done in Haskell) cannot provide?
b_e_n_t_o_n
Right, I'm just unsure how valuable those guarantees really are. Especially if I'm extracting an Int out of the type to interface with other code.
skybrian
The author seems concerned about compile-time range checking: did you handle the full range of inputs?
Range checking can be very annoying to deal with if you take it too seriously. This comes up when writing a property testing framework. It's easy to generate test data that will cause out of memory errors - just pass in maximum-length strings everywhere. Your code accepts any string, right? That's what type signature says!
In practice, setting compile-time limits on string sizes for the inputs to every internal function would be unreasonable. When using dynamically allocated memory, the maximum input size is really a system property: how much memory does the system have? Limits on input sizes need to be set at system boundaries.
kazinator
What if I want a type called MinusIntMaxToPlusIntMax?
In other words the full range of Int?
Is newtype still bad?
In other words how much of this criticism has to do with newtype not providing sub-ranging for enumerable types?
It seems that it could be extended to do that.
Title should been "names are not ENOUGH for type-safety" but then no one would have read it I guess...