Typealias

Typealias is a programming language feature that introduces an alias for an existing type. The alias provides a new name for the same underlying type and does not create a distinct type or change its runtime behavior. It is evaluated at compile time and typically incurs no performance cost.

Common purposes include improving readability, reducing long or complex type expressions, and enabling easier API evolution or platform-specific adaptations. By presenting a more expressive name, type aliases can make code that uses generic or deeply nested types easier to understand and maintain.

Language examples illustrate its usage. In Swift, one can write:

typealias Meter = Double

let distance: Meter = 3.0

typealias CompletionHandler = (Result<Bool, Error>) -> Void

In Kotlin, a type alias can be declared as:

typealias UserList = List<User>

val users: UserList = listOf(User("Alice"), User("Bob"))

Semantics and limitations include that aliases do not create distinct types; you can’t use an alias to enforce new type constraints or to create type-safe separations where a real new type is required. They are purely alternate names. Some languages restrict where you can use aliases (for example, with mutability or generics), and overusing aliases can obscure the actual types.

Best practices suggest using type aliases to clarify complex types, to adapt public APIs to platform differences, and to expose meaningful domain concepts, while avoiding excessive or opaque aliases that hide the underlying type. See also typedef and type aliases in other languages.