Expressionsthattype

Expressionsthattype refers to a class of syntactic forms in programming languages whose main purpose is to influence, specify, or compute types. These expressions are often evaluated at compile time or checked by the type checker, and may produce a type, a type constraint, or a type-level value rather than a direct runtime result. The concept helps separate concerns between value computation and type formation, and is used in discussions of type inference, generics, and dependent types.

In practice, expressions that type appear in several forms. These include type queries that extract a type from a value, literals that serve as type-level constants, and generic or dependent constructs that parameterize code by types. In real languages, notable examples include type-level usages such as TypeScript's type-level typeof, and languages with robust type-level programming such as Haskell (DataKinds, TypeFamilies) and Rust-style generics that guide compilation through type constraints. An expression is said to be type-bearing if its evaluation contributes to the determination of a type, either directly (producing a type) or indirectly (constraining type variables).

Examples

- TypeScript: typeof used in a type context to query the type of a symbol

- Haskell: DataKinds and TypeFamilies for type-level computation

- Rust: type annotations and trait bounds that influence compilation

See also: Type systems, Type inference, Type safety.