Comment by icen | Hacker News Reader

Comment by icen | Hacker News Reader

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Yes, the actual code will always tell you exactly what it does.

There is always a necessary distinction between specification and implementation: if there weren't, one of them would have no value. We often suggest that the intent of a piece of code should be specified in comments, and this is in that category. This way, the compiler can read your comment and figure out if it needs updating! Languages without dependent types work this way too - but you have to try very hard indeed to be eloquent in type signatures, and being able to talk about values lets you say a bit more.

Coming onto your second point: if the function is only valid for some set of values of the given type, why is it 'not really called for' to create specialised types for it? If you call it with something else and it runs, it will be a bug. Newtypes or wrappers are common patterns.

I think that most mainstream typed languages have common constructs to offer that 'A or a B' value you want: this is solved via inheritance, `Either`, `Result`, or `std::variant`.

Types don't generally specify intent as well as comments do. They still can only tell you what the code does, not really why. They do clarify some assumptions the implementation makes, essentially defining pre- and post-conditions, which you'd normally write in comments if you didn't have types. But at some point, it does get much easier to say 'performs a stable sort' then to define the dependent types to express that the list is sorted and that the order of equal elements doesn't change.

My point about newtypes and wrappers is that they are almost pure overhead for helper functions. There is a reason why we don't define precise types for each of our intermediate calculations in general, and this doesn't change when we move those computations to a separate function.

Finally, inheritance is not a real solution for returning A or B, with inheritance I can only return a C that both A and B derive from, and this C often doesn't exist and can't be added post-hoc. What you normally do is go for Object or void*, which is much closer to dynamic typing than inheritance. And most mainstream typed languages don't have Either or Result. C++ with std::variant is the only exception, I forgot that it was added. BTW, I should be more explicit - the way I see it, the mainstream typed languages are Java, C#, C, C++, and maybe Go.

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