Arvoluokat

Arvoluokat, literally “value classes” in Finnish, are a programming concept used to create distinct types that wrap a single underlying value. They provide stronger type safety by distinguishing different kinds of values at the type level, while aiming to avoid runtime overhead by allowing the wrapper to be inlined or erased by the compiler. In practice, arvoluokat are designed to behave like their underlying value from the programmer’s point of view, yet enforce correctness by preventing mixing values of unrelated types.

The term arvo means value and luokka means class or type, reflecting the idea that the new type represents a value with a dedicated semantic meaning. In many languages, value classes are implemented as lightweight wrappers around a primitive or existing value type. They are intended to be zero-cost abstractions: operations on the value class are designed to be compiled away or specialized so that no extra allocation or indirection is required.

Common characteristics include wrapping a single value field and avoiding additional mutable state. Depending on the language, value classes may have restrictions such as limits on inheritance, memory layout, or how they interact with generics, serialization, and reflection. Semantics around equality, hashing, and ordering can also differ from the underlying value type, which is a design consideration for API authors.

Use cases for arvoluokat span domain modeling and API design. Examples include defining distinct types for identifiers (such as UserId or OrderId) or for units of measure (Meters, Dollars) to prevent inadvertent mixing of concepts. By providing intent-revealing types, arvoluokat help reduce errors while preserving performance through zero-cost abstractions.