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Comment by frenchy | Hacker News Reader

> Branding makes structural typing work like nominal typing for the branded type only.

That's not quite true. Branding doesn't exist at run time, where as nominal typing usually does at some level. Classes exist at runtime, but most typescript types don't, so unless there's something specific about the shape of the data that you can check with a type guard, it's impossible to narrow the type.

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> Classes exist at runtime

Not necessarily, depending on the language. Functional languages and system languages such as OCaml, Haskell, Rust, but also C (painfully) and C++ can represent wrapper types within a nominal type system at no runtime cost.

Haskell implements type classes via dictionary passing that don’t always get optimized away by the compiler so it does have a slight runtime cost:

https://okmij.org/ftp/Computation/typeclass.html#dict

In Rust, using trait objects also generates a vtable for dynamic dispatch so in that case traits are not fully erased:

https://web.mit.edu/rust-lang_v1.25/arch/amd64_ubuntu1404/sh...

My quotation was not good - I intended to reply to the argument that "nominal type do [exist at runtime] to some level". The newtype pattern in either Haskell or Rust is fully transparent at runtime.

As others have said, types don't necessarily exist at runtime. Types allow reasoning about the program source without executing it. Java is more the exception than the rule here; conventionally compiled languages such as C don't usually have types at runtime.

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