Help/recommendation working with generics and Psalm

[alincota]

I'm trying out psalm to get generics for a project of mine.

I started off with the examples and once I thought I understood the concepts I attempted to implement an Option monad because conceptually it feels like an easy generic.

I stumbled accross issues when I was having some static functions and was failing to get 100% coverage so I've tried different ways (interface, one object etc).

Based on my understanding from #8142 psalm templates don't work with static functions.

After some more tryouts, this is the solution I got to:
https://psalm.dev/r/49b76986dc

It works, is expressive and should allow me to have the confidence when mixing functions using them that they'll match correctly.

I'm now wondering if:-

1. There's a better way to achieve this while maintaining 100% type inference?
2. The option function always returns Option<T>. Is there a way to make it return/distinguish the subtypes? e.g Some<5> or None<null>. Or should I even care?
3. How can I track Some<T> but let None have no type? It's a little weird having None<null>. (It's OK as is for me but some purists might argue None shouldn't hold a value which I get.)

Less important, but how is it that it reports 100% yet it always output it's unable to infer types? Even if I have the most basic file, it still says that.

Psalm was unable to infer types in the codebase
100.000% tests/examples/psalm-generics.php (0 mixed)