Sealed interfaces + records as ADT representation

Context

Algebraic types (Result, Try, Option, Either, Validated) need to represent exactly two variants without allowing external extension.

Decision

Implement each type as a sealed interface with record variants (Ok/Err, Success/Failure, Some/None, etc.).

Consequences

Positive:

• The compiler verifies exhaustiveness in switch — no risk of missing a variant.
• Records eliminate boilerplate (constructor, equals, hashCode, toString).
• Pattern matching (case Ok<V, E>(V v) ->) extracts values in a single expression.
• No third variant can be created externally — the contract is unbreakable.

Negative / tradeoffs:

• Requires Java 17+ for sealed, Java 21+ for full pattern matching in switch.
• Records do not support state inheritance — all logic must live on the interface as default methods.

Alternatives considered

• Abstract class + subclasses: no exhaustiveness guarantee, more boilerplate.
• Enum with generic payload: impossible with generic types in Java.