Type aliasing
Type aliasing is a feature in some programming languages that allows creating a reference to a type using another name. It does not create a new type hence does not increase type safety. It can be used to shorten a long name. Languages allowing type aliasing include: C++, C# Crystal, D, Dart, Elixir, Elm, F#, Go, Hack, Haskell, Julia, Kotlin, Nim, OCaml, Python, Rust, Scala, Swift and TypeScript.
Example
C++
C++ features type aliasing with the using keyword.
using Distance = int;
C#
C# version 12 and higher supports type aliasing with the using keyword. Earlier versions restrict its use to file-local scope or specific import contexts..[1]
using Distance = int;
Crystal
Crystal features type aliasing using the alias keyword.[2]
alias Distance = Int32;
D
D features type aliasing using the alias keyword.[3]
alias Distance = int;
Dart
Dart features type aliasing using the typedef keyword.[4]
typedef Distance = int;
Elixir
Elixir features type aliasing using @type.[5]
@type Distance :: integer
Elm
Elm features type aliasing using type alias.
type alias Distance = Int
F#
F3 features type aliasing using the type keyword.
type Distance = int
Go
Go features type aliasing using the type keyword and =.
type Distance = int
Hack
Hack features type aliasing using the newtype keyword.[6] Functionally, this creates a new, distinct type that is incompatible with its underlying type (int). This is stricter than a simple alias, which is generally transparent and interchangeable with the original type.
newtype Distance = int;
Haskell
Haskell features type aliasing using the type keyword.[7]
type Distance = Int;
Julia
Julia features type aliasing.[8] The use of const is best practice (though not strictly required for aliasing). It prevents the alias from being rebound to a different type later in the program, ensuring the alias is stable.
const Distance = Int
Kotlin
Kotlin features type aliasing using the typealias
typealias Distance = Int
Nim
Nim features type aliasing.[10]
type
Distance* = int
OCaml
OCaml features type aliasing.[11]
type distance = int
Python
Python features type aliasing.[12]
Vector = list[float]
Type aliases may be marked with TypeAlias to make it explicit that the statement is a type alias declaration, not a normal variable assignment. The use of : TypeAlias (from PEP 613) is not required for the alias to function, but it explicitly tells static type checkers (like Mypy) that the assignment is a type declaration, not a runtime variable assignment.
from typing import TypeAlias
Vector: TypeAlias = list[float]
Rust
Rust features type aliasing using the type keyword.[13]
type Point = (u8, u8);
Scala
Scala can create type aliases using opaque types.[14]
object Logarithms:
opaque type Logarithm = Double
Swift
Swift features type aliasing using the typealias keyword.
typealias Distance = Int;
TypeScript
TypeScript features type aliasing using the type
type Distance = number;
Zig
Zig features type aliasing by assigning a data type to a constant.[16]
const distance = u32;
References
- ↑ "Alias any type - C# 12.0 draft feature specifications" (in en-us). 16 August 2023. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/using-alias-types.
- ↑ "alias - Crystal". https://crystal-lang.org/reference/1.11/syntax_and_semantics/alias.html.
- ↑ "Alias Alias - D Programming Language". https://dlang.org/library/std/meta/alias.html.
- ↑ "Typedefs" (in en). https://dart.dev/language/typedefs.
- ↑ "Typespecs and behaviours" (in en). https://elixir-lang.org/getting-started/typespecs-and-behaviours.html.
- ↑ "Types: Type Aliases". https://docs.hhvm.com/hack/types/type-aliases.
- ↑ "Type synonym - HaskellWiki". https://wiki.haskell.org/Type_synonym.
- ↑ "Types · The Julia Language". https://docs.julialang.org/en/v1/manual/types/#Type-Aliases-1.
- ↑ "Type aliases | Kotlin". https://kotlinlang.org/docs/type-aliases.html.
- ↑ "Nim by Example - Types". https://nim-by-example.github.io/types/.
- ↑ "OCaml reference manual". https://ocaml.org/docs/basic-data-types#type-aliases.
- ↑ "typing — Support for type hints". Python Software Foundation. https://docs.python.org/3/library/typing.html#type-aliases.
- ↑ "Type aliases - The Rust Reference". https://doc.rust-lang.org/reference/items/type-aliases.html.
- ↑ "Opaque Types". https://docs.scala-lang.org/scala3/book/types-opaque-types.html.
- ↑ "Documentation - Everyday Types" (in en). https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#type-aliases.
- ↑ "Documentation - The Zig Programming Language". https://ziglang.org/documentation/master/#Examples.
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