pertype

Pertype is a term used in discussions of type systems to denote data types that carry per-instance metadata governing allowed operations. The concept aims to express fine-grained capabilities alongside data values, enabling policy enforcement at either compile time or runtime. The term is not canonical and appears in various design notes and speculative proposals rather than in standardized language specifications.

Definition and structure

A pertype consists of a base type T and a per-element policy P that maps each value of T to a set of permissions. Operations on a pertype value consult P to determine legality; performing an unsupported operation may result in a type error or runtime fault. Some formulations keep the policy separate from the data value by using a capability object that carries permissions alongside the value.

Examples

File handle pertype: a file handle value includes metadata specifying read or write permissions, constraining which operations can be performed. Cloud resource handle: a handle includes region, project, and role information that gates actions such as modification or deletion.

Variants and design choices

Policies can be static (checked at compile time) or dynamic (checked at runtime). Policies may be monotonic (permissions can only increase or stay the same) or can adapt based on context. Some approaches use dependent or refinement types to encode policy directly in the type.

Relation to other concepts

Pertypes relate to capability-based security, effect systems, and dependent types. They differ by attaching operational permissions to individual values rather than solely to types, enabling fine-grained control within a program’s data flow.

Implementation notes

In programming languages, pertypes can be realized through dependent types, phantom types, or capability-safe abstractions. Enforcement can occur via compiler checks, runtime checks, or a combination of both.

Notes

The term remains a proposal concept rather than a widely adopted standard. Its utility lies in modeling per-value permissions and supporting modular, secure composition of components. See also: type system, capability-based security, dependent types.