Comment by yakubin

Yes. That's thanks to Hindley-Milner type inference which makes the compiler infer which None you mean. But in longer code snippets full Hindley-Milner type inference can cause hard-to-understand type errors. Here specifically the compiler infers the type of a value in your function body based on the type of the function it belongs to, which is in the opposite direction than what a programmer typically thinks in. It leads to spooky actions at a distance.