Typalias

Typalias, or type alias, is a programming construct that defines a new name for an existing type. It is used to simplify complex type expressions, improve readability, and express domain concepts without creating a new type.

In most languages, a typalias is a synonym rather than a distinct type. This means values of the alias are interchangeable with values of the underlying type, and the alias does not add constraints or behavior by itself. Some languages treat aliasing as a purely cosmetic rename, while others allow aliasing of generic types or parameterized forms.

Common language implementations include:

- TypeScript: type ID = number; which creates ID as an alias for number.

- Rust: type UserId = u64; providing a new name for the 64-bit unsigned integer type.

- Swift: typealias UserID = UInt64; a generated alias for the unsigned 64-bit integer.

- Kotlin: typealias UserId = Long; enabling a domain-specific name without new type semantics.

In addition, many languages provide alternatives to type aliases that do create distinct types, such as typedef in C/C++ or using in modern C++, which differ from aliases in how they interact with the type system and type-checking.

Use cases include reducing verbosity of complex generic types, modeling domain concepts (for example, distinguishing a UserId from a plain integer while keeping the same runtime representation), and aiding cross-platform code by abstracting underlying platform types.

Caveats include that aliases do not enforce type safety beyond the underlying type, and overuse can obscure meaning or lead to confusion if aliases proliferate. When designing APIs, aliases should clarify intent without masking important distinctions between concepts.