It's my understanding that we have to involve a function call as a fallback, as a function call is the only way we have of producing a value of type c in the general case.
But we could potentially have some list of exceptions.
So one valid implementation would be: If c is string we always return hello world elseif a, b and c are integers we add the two ints. Elseif a and b are the same type we call the function with the argument order swapped. And finally, if none of these are the case, we call call the function with the arguments.
In a type system like Haskell's, a function with that signature has to be unconditionally polymorphic in its variable types, so you can't have a list of exceptions.
But we could potentially have some list of exceptions.
So one valid implementation would be: If c is string we always return hello world elseif a, b and c are integers we add the two ints. Elseif a and b are the same type we call the function with the argument order swapped. And finally, if none of these are the case, we call call the function with the arguments.