[mixedCase]

Does your argument extend to automated tests? If not, why not?

I see types as mere tests. More robust, yet more limited in scope. FWIW it's much easier to see them that way once you introduce dependent types into the discussion, but it applies to the simplest type systems just as well.

[friendzis]

Types are tests. On top of being unit tests that are forcibly run, they are formal and apply everywhere a datum is used.

[12thwonder]

sort of. I don't write tests at early stages, do you?

I don't see types as serious tests and I don't think they are robust. let's say that some integer must be between 10 and 100, do you use type checks for this?

[ebingdom]

> I don't write tests at early stages, do you?

I definitely do. In my experience, it's much easier to adopt a discipline of testing (and static typing) early on than it is to try to retroactively add that to an existing system, which may or may not be written in a way that is even testable.

But I do appreciate that viewpoints can differ on this topic. Regarding types, I studied type theory academically, so types are natural to me and don't really add any extra cognitive work (and perhaps they eliminate some). So I might as well use and benefit from them if they basically cost me nothing. But for someone who thinks of static typing as just trying to make the compiler happy (perhaps because they don't really understand the type system or because the type system is not ergonomic), I can see why they might have a more pessimistic view of it.

[chriswarbo]

> let's say that some integer must be between 10 and 100, do you use type checks for this?

Yep. In particular, I would use:

- A "wrapper" type around an unsigned byte (we don't need negatives, or a whole machine word)

- A "newtype" feature, to replace the wrapper with a Byte after type-checking (Haskell calls this "newtype"; Scala calls this "opaque type aliases").

- A private/unexported/scoped constructor, to prevent arbitrary Byte values getting wrapped

- A "smart constructor" which checks the bounds of a given Byte, returning a 'Maybe MyBoundedIntType' or some other type-checked error mechanism (Scala's 'Try[MyBoundedIntType]' works well).

- Polymorphism/overloading to call that smart constructor of various numeric types (char, int, long, signed, unsigned, etc.)

In Scala that would look something like:

    opaque type MyBoundedIntType = Char

    object MyBoundedIntType {
      def apply(c: Char): Try[MyBoundedIntType] =
        if (c >= 10 && c <= 100)
          Success(c)
        else
          Failure(new IllegalArgumentException(s"Value ${c.toInt} outside range [10, 100]"))

      def apply(i: Int ): Try[MyBoundedIntType] = Try(i.toChar).flatMap(MyBoundedIntType(_))
      def apply(l: Long): Try[MyBoundedIntType] = Try(l.toChar).flatMap(MyBoundedIntType(_))
    }

In Haskell:

    module MyModule (MyBoundedIntType(), toByte, MakeBounded(..)) where

    newtype MyBoundedIntType = MBIT { toByte :: Word8 }

    class MakeBounded t where
      mkBounded :: t -> Either String MyBoundedIntType

    instance MakeBounded Word8 where
      mkBounded b | b >= 10 && b <= 100 = Right (MBIT b)
      mkBounded b | otherwise           = Left ("Value " ++ show b ++ " not in range [10, 100]")

    instance MakeBounded Int where
      mkBounded i = toWord8 i >>= mkBounded

    instance MakeBounded Integer where
      mkBounded i = toInt i >>= mkBounded

[Jtsummers]

You can with Ada. Though not many other languages.

[the_duke]

You can do it in most languages by just using a wrapper type with fallible constructors.

A lot more awkward than dependent types, but no popular language has those.

[mixedCase]

> let's say that some integer must be between 10 and 100, do you use type checks for this?

Yes. I explicitly mentioned dependent types for this reason.

This is also expressable in a more limited fashion in a language like TypeScript, although some may argue it also employs a form of dependent typing.