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Comment by Animats | Hacker News Reader

Pascal worked that way all the time, and it was hated. You could have "inch" and "meter" version of integer, and they were not interchangeable. This was sometimes called "strong typing"

It's interesting that in Rust, "type" does not work that way. I kind of expected that it would. But no, "type" in Rust is just an alternate name, like "typedef" in C.

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Both approaches are useful at different times. For example you wouldn't want to accidentally multiple a meter by a centimeter but you may want to provide std::io::Result<T> which is equivalent to Result<T, std::io::Error> but just a bit nicer to type.

For example in Rust you can do:

    type Foo = Bar;

Which is just an alias, interchangeable with Bar.

Or you can do:

    struct Foo(Bar);

Which is a completely new type that just so happens to contain a Bar.

NASA might have some thoughts on mixing inch and meter types :-)

https://en.wikipedia.org/wiki/Mars_Climate_Orbiter

It is a form of strong typing because integer could be the length of your toe nail, a temperature or the seconds since the unix epoch.

Sometimes you really want to make sure someone is not going to introduce billion dollar bugs, by making the type different from the underlying representation. In Haskell that would be sth like

     newtype Temperature = Int

At other times, you just want to document in stead of forcing semantics. A contrived example:

    type AgeMin = Int
    type AgeMax = Int

    isAdmissible :: AgeMin -> AgeMax -> Bool   
    isAdmissible :: Int -> Int -> Bool          // less clear

The AgeMin/AgeMax example seems more of a deficiency due to a lack of named function parameters; it would be equally clear if it had a type signature (using OCaml as an example) of

    val is_admissible : min_age:int -> max_age:int -> bool

There are other places were you use types as well.

Also, if you later decide that an Age should not be int, but a string, you wont miss it in refactoring whereas in your example you don't have that facility.

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