I have thought about this a lot (I'm writing a static typechecking library for Elixir) and really the problem is the dialyzer typesystem. Almost all of the problems stem from the root fact that there isn't a granular concept of how to treat functions in the type system. What does `(int, int) -> int` mean? Dialyzer does not have a good answer for that question.
Mind you I am not a "PL theory" type of guy, I do have a math degree, but my PhD is in the sciences, so I am more approaching this issue from a "discovering what would be truly useful" perspective more than "applying an existing typesystem to the BEAM", which is I think in line with the traditional pragmatism of the BEAM ecosystem.
This is an interesting question. Do you have any thoughts on what a better answer to that question might mean? Or languages you think do better here? Thank you
let's go with a simpler one: int -> int
"I guarantee that if you send me an int then i will return you an int without crashing"
In elixirland: `IO.inspect/1` would satisfy this, as would `Kernel.-/1`. The interesting thing is that `any -> int` is then a subtype of `int -> int` and we need a new type `_ -> int` which is "any one-arity function that emits int".
Then types become a statement of "resistance to crashing" in the BEAM vm, which I think is the useful thing that people are looking for.
I don't have answers to everything; I don't know how to deal with a function that generates random numbers from 1 to 20 and crashes on 13, for example.
We should have a discussion about this! I'll reach out on another channel.
> "I guarantee that if you send me an int then i will return you an int without crashing"
This is a much stronger guarantee than you get in any language except very specialized ones. Coq, for example (not just not crashing, but also guaranteed termination). But even in Haskell an Int -> Int function can crash or decide not to terminate. Reading more into a type opens up cans of worms that are hard to solve.
An effects tracking system can be used to determine things such as use of FFI, `assert`, recursion, and side effects. With these statements like "this function does not crash" is easy, and "this function does not diverge" is also possible, though it will not detect all functions that do not diverge.
The only no_returns otherwise are loops and hibernates. Hibernates are explicit. You might have a hard time analyzing the tail-calls to figure out an ultimate type that captures a no_return loop, but I think ignoring that condition would not be horrible for a BEAM type system (along the same lines as the random number crash scenario that I described).
I'm not sure what you're saying. Crashing "miscodes" (like pattern match failures) are a thing, no? A "well defined" thing, but not one that a type system that doesn't include proper theorem proving will save you from.
Most languages don't have a way to type that, I'm pretty sure it requires dependent types.
The types don't prove the logic doing the right thing or that nothing can break while the function runs, but they prove that you are using it mostly correctly in what you give it and what you do with what it'll return.