typeand

Typeand is a term used in discussions of type systems to denote the conjunction of two types or type constraints, producing an intersection type. It is not a formal operator in all languages, but in many contexts it is used informally to describe the idea of a value that belongs to both A and B.

In formal terms, if a value x has type A and type B, one can describe its type as A and B, or A ∧ B in some notations. The set of inhabitants of A ∧ B is the intersection of the sets of inhabitants of A and B. This is distinct from a product type A × B, which is a pair consisting of one value of type A and one value of type B, whereas A ∧ B denotes a single value that satisfies both type constraints.

Languages that support intersection or constraint-based typing provide syntax that resembles typeand. For instance, in TypeScript, an intersection type A & B denotes a value that conforms to both A and B. In other language ecosystems, intersection types appear as constraints or type-class mixes. The practical use of typeand is to express that a function or data structure requires a value that meets multiple interfaces or capabilities without forcing duplication of values.

Because the term typeand is informal, its exact syntax and rules vary by language or literature. Some sources treat it as synonymous with intersection types; others treat it as a mnemonic for expressing multiple constraints in documentation.

See also: intersection type, product type, type constraint, type class, dependent type, TypeScript.