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points by eru 2 days ago | 0 comments
> [...] because the success type and error type are the same.

That's a bit weird? In Rust, if you have an thing that returns Result<String, String> that's still type safe, even though success and error types are the same.

I think I understand what you want to express, maybe. You might say it's in-band vs out-of-band error signalling?

[–] itishappy 2 days ago
> In Rust, if you have an thing that returns Result<String, String> that's still type safe, even though success and error types are the same.

These types are not the same:

    Ok<String>
    Err<String>
[–] markasoftware 2 days ago
Those are not types at all. Ok and Err are type constructors for the Result<A, B> type that take arguments of type A and B respectively.
[–] SkiFire13 2 days ago
> type constructors

Nit: the type constructor is `Result`: given two types `A` and `B` (not instances of those types, but actual types) it gives you a new type `Result<A, B>`.

`Ok` and `Err` are called "variants" and can be used to construct a new _instance_ of `Result<A, B>`.

[–] markasoftware 1 day ago
Ah, thanks! I'd remembered the term "data constructor" from Haskell and called it a type constructor instead.
[–] kazinator 1 day ago
Ok and Err are "thingies" which are in the type system, which give rise to distinctions that allow for compile-time diagnostics. They are part of the type.

Ok and Err are not just type constructors because they appear on the left hand side in pattern matching: they "do" matching and destructuring also.

[–] kazinator 1 day ago
The success and error types are not the same, because they are in different positions in the type, and so distinguished by different patterns.

If you have literally the above convention where a valid result is a String, and an error result is also a String (e.g. error message) that is still type safe.

  match (whatever()) {
    Ok(s) => println!("good result string: {s}"),
    Err(s) => println!("error string: {s}"),
  }
The success type and error type having to be different is not a good way to express the idea; rather that success and error cases are distinguished by type (even if the success and error data units are of the same type).

In other words, the "Ok" and "Err" entities are an ingredient of the type and that effectively makes the result and error be distinguished at the type level.

But we could also argue that Rust still allows you to write an integer-returning function such that -1 means error.

[–] eru 1 day ago
Basically, you need the whole 'success and error types are different' in a language like Python if you make the return type something like "int | string", where 'string' stands for your error message.

Of course that breaks down, when the success type is also an error message, eg when you are looking up something in a catalogue of error messages.

Rust has no trouble with this, for exactly the reasons you line out.