Explain Haskell empty import list ()

Question

I saw much lines like import HSP.ServerPartT() - list of imports is empty. Why is this done? What difference with just not importing this module?

Answer 1

It imports only typeclass instances from the module. With -Wall, GHC issues a warning for modules that are imported but from which no definitions are used:

foo.hs:1:1:
    Warning: The import of `M' is redundant
               except perhaps to import instances from `M'
             To import instances alone, use: import M()

The empty import list silences this warning and serves as documentation of the purpose of the import.

Answer 2

That form imports nothing but the instances from that module. And that's the reason of that form, you want to have the instances in scope, but nothing else.