Question

Both types Unit and Nothing indicate a function that does not return anything. What’s the difference between them?

Answer 1

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.

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Nothing is used in a different way. It is characterized by two properties:

1. Nothing is a subtype of every other type (including Null).
2. 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]

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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, .