Enkeltype

Enkeltype is a term used in type theory and programming language design to denote a basic, indivisible type that serves as a primitive building block in a type system. Enkeltypes are the leaves of the type hierarchy and are not defined by applying type constructors such as function, product, or sum types. In many languages and texts, enkeltypes are also called base types, primitive types, or atomic types.

In practical terms, an enkeltype represents values with a fixed, well-understood representation and semantics. Common examples include Boolean values, integers, floating-point numbers, and strings. These types form the foundation from which more complex types are constructed using type constructors. For instance, if A and B are types, then A -> B (a function type) and A × B (a product type) are built from them; yet A and B themselves may be enkeltypes.

In the context of the simply typed lambda calculus and related type systems, the set of types is defined inductively: start from a chosen set of atomic (enkel) types, and then close under type constructors to form function, product, or other composite types. The distinction between enkeltypes and composite types helps with type inference, type checking, and the semantics of programs.

Etymology and usage: the term derives from the idea of being "simple" or "atomic" within the type landscape. In English discussions, similar concepts are referred to as base types, primitive types, or atomic types. The exact set of enkeltypes is language-specific, reflecting the primitive concepts a language exposes to programmers. See also base type, primitive type, atomic type, simply typed lambda calculus.