What's the difference between Unit and Nothing?
Unit is a type that has exactly one value ‒ see Unit type. On the other hand, Nothing has no possible value - see Bottom type.
A function that doesn't return anything must have the return type Unit. If it were Nothing then the function could not return a result. The only way to exit the function would be by an exception.
Nothing is used in a different way. It is characterized by two properties:
Nothingis a subtype of every other type (includingNull).- There exist no instances of this type.
When is this useful? Consider None:
object None extends Option[Nothing]
Because Option is covariant in its type parameter and Nothing is a subtype of everything, Option[Nothing] is a subtype of Option[A] for every type A. So, we can make one object None which is a subtype of Option[A] for every A. This is reasonable, since Nothing cannot be instantiated so Option[Nothing] will always be without a value. Similarly
object Nil extends List[Nothing]
Unit corresponds to logical true and Nothing corresponds to logical false under the Curry-Howard isomorphism, where we view types as propositions and functions as proofs, .
Unit means that (a) function has side effects like input and output, (b) these side effects are the main goal of the function. Of course, a function can have side effects even if its type is different from Unit.
Nothing is a special type in Scala, because (a) it has no values (Unit has exactly one value - ()), so you cannot return a value of type Nothing, and (b) it is a subtype of every other type. That means that if something has the type Nothing, it can be used instead of any other type (via subtyping), but it won't produce any result. This is useful for dealing with exceptions - the throw expression has a type of Nothing, so it can be used everywhere in a program.
Simply, Nothing means that there was an error or termination of a program and nothing was returned, while Unit means there were side effects, but execution ended normally with no result.
Programming in Scala has a nice explanation of that.
To add one aspect to Petr's reply: Nothing plays an important role in the type hierarchy. It is a bottom type. That means that it is a subtype of every other type, which is like the opposite of Any, which is a supertype of everything. You can find a nice explanation here.