[jasim]

Passing a string where you expect a number is not the only kind of type errors modern type systems are about. They can catch a particular class of domain-level bugs, and more importantly, it acts as a pair programmer who tells us about all the logical edge cases that we forgot to think about, as we program.

This is done using what is called "sum types" - we can tell the compiler that say a user can be "Premium" or "Regular". This distinction will affect policies across the codebase - eg. Premium customers get a discount on their shipping costs. We'll have robust unit tests and integration test that cover all these policies.

But even with the tests, we run into trouble when the feature request to create a new type of User comes in - "Semi-Premium" who gets only their _domestic_ shipping free. We now have to hunt and peck across the codebase, and change all our policies so that it handles this possibility well. Our tests are of no help here - they are meant to verify existing facts about the system, and can't tell when this user fall through certain policies because we forgot to handle it there. The type system on the other hand knows exactly the places where we decide things based on the kind of user. It will then realize that a new type of user has come in, and our policies don't handle them. This turns what usually is a high-risk and difficult task into an almost mechanical one.

What's happening here is that we're telling the compiler more things about our domain. This means the compiler can then remind us as we go about our business writing code, to handle every case, to never pass a null, and to help model our data structures that prevents inconsistent states from ever happening. The more we can encode in types, the more powerful this becomes.

[conanbatt]

> ...We now have to hunt and peck across the codebase, and change all our policies so that it handles this possibility well.

A stellar case about Object design, not strong typing. I don't find type solves this issue, in fact, it even might be the opposite. Its way looser and more flexible to be able to say how things actually behave than to define the exact type they are to see how they behave.

The static analysis is definitely a huge plus, but it's a bit tangential: if a static analysis tool were able to point to the same issues, the reason would evaporate. (It's like saying a language is better because it has a better community, which is practical but not intrinsic).

[jasim]

Objects don't solve this. Untyped programming is more flexible, but are able to express less things about the domain explicitly than typed languages (not Java/C#, but rather OCaml/Haskell etc.).

> if a static analysis tool were able to point to the same issues, the reason would evaporate.

This static analysis tool is called the type checker. The experience of using a type checker is best when the underlying language has sound types. Sorbet, TypeScript, Typed Racket etc. bolt a type system on top of an existing language, and so don't have a choice but to be unsound, and so their qualitative experience isn't as nice as it could be. Even so, they're highly recommended for large codebases.

> It's like saying a language is better because it has a better community, which is practical but not intrinsic

Not at all. This is an intrinsic feature of programming languages and has a huge effect on how we think of programs.

It is however not a silver bullet and when building web applications, practicality - tooling, ecosystem etc. trumps everything else. However if you run into difficult problems which require deep attention to data modelling, and wish for a better tool to help frame the problem, then Typed Functional Programming might be a good try.