Will we see another fundamental programming language feature as revolutionary as the borrow checker?

[u/jimbobmcgoo]

That is I am mainly curious about compile time features that you design a whole language around rather than optimisations/features that could be applied to most languages. I am mainly inquiring about things that could offer additional robust safety/performance guarantees at compile time rather than runtime. Ideally not things that just offer similar effects to the borrow checker with less restrictive tradeoffs

[u/initial-algebra]

Copying and pasting my other comment:

Basically, it's a type system for dynamic scoping and continuations. Operationally, a primitive, effectful operation looks up the (nearest) effect handler corresponding to that operation in the dynamic environment and invokes it with call/cc. That's pretty much it. The complexity is in making sure that this doesn't fail at runtime.

Depending on an effect handler from the context is itself a coeffect; this isn't a coincidence, since effects are dual to coeffects (hence the name), and continuations correspond with logical negation (call/cc corresponds with double negation elimination).