Sealed trait
A sealed trait is a Scala trait whose permitted subtypes are restricted to declarations known to the compiler.
Akka in Action, Second Edition
A sealed trait is a Scala trait whose permitted direct subtype cases are restricted so the compiler can check pattern-match exhaustiveness.
- How it is used
- Structures typed actor protocols so messages form an explicit, compiler-checkable hierarchy.
- How deep it goes
- Defines sealed traits and exhaustiveness warnings, then applies them in a wallet actor without detailing compiler internals.
- Examples
- Uses a wallet with Increase and Decrease commands, bounded state, and stopping after an excessive increase.
- What you can do after
- Build typed actor protocols whose pattern matches receive compiler warnings for omitted command cases.
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Functional Programming in Scala, Second Edition
A sealed trait hierarchy is a Scala type hierarchy whose permitted subtypes are restricted to declarations known to the compiler.
- How it is used
- Defines closed algebraic data types whose constructors support reliable recursive operations and exhaustive pattern matching.
- How deep it goes
- Explains traits, same-file constructors, companion objects and list implementation, but defers minor trait differences and broader abstraction details.
- Examples
- Uses List with Cons and Nil, plus recursive size operations, to demonstrate a closed constructor set.
- What you can do after
- Build sealed Scala data types with case-based operations and compiler-checkable exhaustive matches.
Read how Functional Programming in Scala, Second Edition explains sealed trait →
Related concepts
- enumalternative to · An enum is a type or language construct whose values are selected from a finite set of named alternatives, which may carry associated data.
- algebraic data typeused by · An algebraic data type is a composite type whose values are formed from product structures, finite alternatives, or both through type constructors.
- actor protocolpart of · An actor protocol is the set of message types and communication rules that defines how an actor can be addressed.
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