Hey thanks so much for writing this up. This is a great post! I've only had time to skim the article, so my apologies if you covered this and I missed it: have you investigated whether specifying expression/function return type affects performance? I work on something of a large codebase, and I wonder if whether we annotated our returns, type checking would be faster.

Yeah! Making explicit return types, especially of public functions, is a good practice to follow. I'd say the main reason isn't performance, though, but rather to ensure you have a stable public API for your module.

How much it actually speeds up the type checker depends on how hard it is for the type checker to infer the return type. And that depends on the return expression, but I don't think there is a single hard and fast rule here. But, if you already have a named type for the return value of the expression, I would absolutely annotate it explicitly when possible. Sometimes the inferred type won't be really the type you intend, and there might just be a more clear type you want to use for communication/documentation purposes.