Matthias-Christian Ott writes:

That's true. This is a big advantage when compiling programmes. But as far as I know type inference is not always decidable in Haskell. Am I right?

Decidability of type inference depends on features you use (GADTs, type classes etc). Type inference in Haskell doesn't mean you avoid providing type signatures at all costs. It is good programming practice to provide type signatures, as elaborate design of ADTs, type classes you use in your program gives you possibility to encode specification of how program behaves in static, that is type signatures help you to document your code.