[evmar]

This is the point under discussion though -- humans think they're reasoning the same way as the inferencer but can't get it right in the limit, so they're not really equipped to know where they ought to have added type annotations. See e.g. the comment here about the inferencer working 'backwards': https://github.com/ReactiveX/rxjs/issues/4959#issuecomment-5... .

[lonelappde]

You add the types on every function definition, and almost never have a problem.

Many people forget that the point of type inference is to eliminate uselessly redundant clutter, not to completely eliminate types from source code. A function is usually called many times, so keeping types on public API definitions and eliminating types from call sites and throwaway REPL code eliminates a huge amount of wasted typing (hah!) with near no loss of clarity.

[WorldMaker]

In the case of an RxJS pipeline, as one example, you often have a huge variety of lambda functions that may indeed be called many times, but often exist themselves as "one off" pieces in a pipeline. Leaving the types to be inferred and/or generic, is sometimes even necessary for the most code-reuse of functions between pipelines. It's also rare for such functions to feel like they are a part of a public API definition, because they feel so huge a part of internal and boring plumbing.

As someone that has done a lot of RxJS (and kindred libraries) programming over the last few years, I have a lot of sympathy for anyone hurt by type inference changes in the middle of those pipelines. Some of my ugliest rats nests of explicit any types and/or worse have been buried in RxJS pipelines next to // TODO and // HACK marker warnings to future spelunkers (myself included) about how much I was fighting the inferencing engine and losing in that particular combo of Typescript version and available typings. It's usually great when I can fix those later, and so far Typescript continues to move in that direction where those rats nests only get cleaner when I get a chance to revisit them. (But again, I sympathize with that existential fear that Typescript inferencing magic can sometimes take an unexpected step back until you understand why the change was made and how it benefits you elsewhere and/or once you figure out the root cause.)