Whilst I'm not going to argue the proposal is a bad thing, I think you've just summarized why some are giving it the side eye pretty well.

To me it's just, "the dot hugs a field, dot by itself is composition, and module names are capitalized."

While I wouldn't write f. g, if I see it, it's not hugging the field name so it's composition. I don't think I'd have trouble reading it.

But... it will probably trip up newbies and people with odd spacing styles. Hopefully GHC can give a useful error like "In the field selection expression f.g, g is a function defined on XX. Did you mean f . g?"

(.g) us a field selector, is (. g) then a function :: (a -> b) -> c -> b?

Yep! You could check this in GHCi (the repl), for example:

  > :t (. id)
  > (. id) :: (b -> c) -> b -> c

(:t prints the type of an expression, and id is the "identity function" that's b -> b here.)

(EDIT: Maybe you know the above and you're asking if it's being changed in the proposal. It doesn't seem so to me.)

I'm just pointing out that the difference between (.g) and (. g) is potentially easy to miss.