A function has ONE parameter and ONE return value. The trick is that these parameters and returned values may be tuples or functions themselves.

So we can have a function which takes a single parameter which is a tuple :

    function_name :: (Param1Type, Param2Type) -> ReturnType

Or we can have a function which takes a single parameter and returns another function:

    function_name :: Param1Type -> (Param2Type -> ReturnType)

Using implied operator precedence, the later is written:

    function_name :: Param1Type -> Param2Type -> ReturnType

And has to be distinguished from a function which takes a function as parameter:

    function_name :: (Param1Type -> Param2Type) -> ReturnType