This Atom package suggests standard library function names as you are typing.
Register Every Module- (Ctrl + Shift + H) by default
starts suggesting function names from all standard library modules. This is toggle-able so turning it on twice will turn it off.
I couldn't find an atom package that did this so I made one.
This uses the autocomplete-plus package (it's built in there's no need to install it) to suggest function names that you might use.
The data that this uses has been scraped off of Hackage and formatted with python. If a function had signatures like Bool -> Bool -> Bool and Int -> Int -> Int it was converted to a -> a -> a because this plug-in is too simple to tell which one you would need. This also means that not every signature is 100% accurate.