> And how do you know you've constricted your data enough?

Types are composable. Sum-types and product-types allow the composition of smaller types into larger ones. They come in the form of discriminated-unions and records in FP languages. So when it comes to the question of how do I know when I've constricted my data enough, it's when I know all the components of all types have been constricted enough.

You don't have to design some massive schema for your entire application. You can absolutely do it as you go. Just create a type that represents the smallest set of values that are needed for whatever concept it reprsents. Then compose those.

> Logic bugs is a giant category of bugs and are what most bugs belong to.

I think logic bugs are relatively rare once you constrain the possible states your code can be in - and they usually come about because of some cyclomatic complexity issue (i.e. putting the if/else branches the wrong way around). Logic just happens to be the bit of your codebase that can't be captured in types (well, not with trivial types anyway). It's the bit of what we write that we interpret from the human world. That's our job, to translate the requirements into code, and the logic is that bit and therefore more prone to human error. I still find I have fewer of them when I have good constraining types that limit the scope for fuckups.

The current state of the world can absolutely be captured in types, then the logic is simply a fold over a sequence of events that take the old state of the world and transforms it into a new one. This is the best way of modeling time also, which is a major source of bugs in imperative code. Mutating data structures kills any effective modeling of time, so cheats like locks are needed. Even with locks there's no effective history and no understanding of now, which makes code difficult to reason about.

That is the idealised end goal of pure functional programming. The reality is most people will still use an `int` when they should use a constrained value (say [1..10] rather than [int.min..int.max]); and so their program can still get into a bad-state™. The more you constrain though, the fewer bugs you will have.

This is the lot of most devs, working out how much risk you're willing to take.

* Those using dynamic languages take the most risk. They do it because they believe they can write code quicker. Their trade off is lots of runtime errors, or runtime checks, or building lots of unit tests.

* Those using languages like Java or C# take fewer risks, as the compiler can help them out, but they don't tend to create types like `Month` that can only hold the constrained set of ints [1..12]. So they run the risk of accepting 13. They also have extensible type-hierarchies (inheritance) that means its impossible to know the full scope of any 'abstracted' interface reference passed around. They must cross their fingers and hope nobody screwed up the implementation. They're also not composable, which leads to additional logic problems as they're glued together manually.

* Those using languages like Haskell don't have the inheritance problem. Everything is sum-types, product-types, or exponential-types (functions) and is therefore composable. This creates an opportunity to create the idealised program (as I mention above). Logic bugs still exist and usually people don't constrain their types completely, but the situation is often much less risky and turns into more robust code.

> On a small scare this is possible but once you start working on a larger team or if you have to integrate your code with constantly changing systems you will want to be testing your code.

I run two development teams. But I also write a lot of research prototypes, this tends to be where I will just have some open ended project that I am playing around with - often programming with types to see what 'level ups' I can get from doing so. But even in a day-to-day project I could easily spend a few days writing code before I need to check it. For example, I recently wrote a whole language virtual-machine: AST, type-system, type-inference, type-checking, and evaluation without needing to run it - all the types lined-up and each subsystem was clear from a logic point-of-view. Currently, I'm building a C# monad-transformer system for language-ext [1] based around transducers, this is pure abstract type-level programming - I haven't run it yet. If I can run it in my head and it type-checks, that's usually good enough.

Of course at some point I do actually run the thing ;)

[1] https://github.com/louthy/language-ext