Define a type that allows a specified subset of case objects?

Question

I'm using case objects to define an enumeration thusly:

sealed trait Fruit
case object Apple extends Fruit
case object Orange extends Fruit
case object Banana extends Fruit

This is in-line with this official example for case objects.

Is it possible to define a type that is either an Apple or an Orange, but never a Banana?

val appleOrOrange: Either[Apple, Orange]

This ^ doesn't work, because Apple and Orange themselves are not types, but singleton case objects. Is it possible to constrain this type to only accept one or the other?

https://stackoverflow.com/questions/3508077/how-to-define-type-disjunction-union-types — Dmytro Mitin, Sep 30 '20 at 16:19

Cory Klein · Answer 1 · 2020-09-30T16:24:17.997

1

When referring to the types of Apple and Orange you can use type to access the singleton type:

val appleOrOrange: Either[Apple.type, Orange.type]

edited Sep 30 '20 at 16:24

answered Sep 29 '20 at 16:58

Cory Klein

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I'm not sure `type` can be referred to as a "method" – user Sep 29 '20 at 17:08
What is it then? I actually was trying to figure out where the documentation was on `type`. – Cory Klein Sep 29 '20 at 17:09
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[This answer](https://stackoverflow.com/a/30983781/11882002) calls it a singleton type annotation – user Sep 29 '20 at 17:14
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https://scala-lang.org/files/archive/spec/2.13/03-types.html#singleton-types "A singleton type is of the form .type. " – Dmytro Mitin Sep 30 '20 at 16:23

Define a type that allows a specified subset of case objects?

1 Answers1