Definitely, that's a pattern in many programming languages (including, increasingly, in Python). And in dependently-typed languages, you actually can obtain a p

Definitely, that's a pattern in many programming languages (including, increasingly, in Python). And in dependently-typed languages, you actually can obtain a proof of validity that gets erased at runtime, leaving nothing but basic unboxed data to be executed. However that's not always an option in all languages or situations, and it's hard to encode conditions like "must only be called while an event loop is running" that way. I'm envisioning this partly as kind of a catch-all/fallback mechanism that can either be bolted on a language that doesn't have these kinds of features. But it's not always the case that you can effectively communicate contextual requirements through the types of function parameters, and this would cover that case as well. If you want to reduce this to anything, it's a workaround for not having monads in your language, with some tradeoffs around composability and dynamic extent.