Can't find anything? that isn't a type variable?

[herkhinah]

I've tried to search for types like (a -> Bool) but apparently searching for any? type signature with a concrete type instead of a type variable doesn't get any results even if you type in the exact type signature of an existing and indexed function. I have the same issue with the https://hoogle.haskell.org/ database and a locally generated (version 5.0.18.3) one.