modulestype

Modulestype is a formal construct used in some modular programming languages to describe the interface that a module must implement. It specifies the types, values, submodules, and possibly parameter requirements that a module exposes, while excluding the details of how those elements are implemented. As a contract, a module declared to implement a modulestype is expected to provide concrete definitions for the declared entities and to satisfy any type constraints included in the modulestype.

A modulestype typically includes declarations for types (abstract or concrete), values, and submodules. It may also express constraints on type parameters, enabling polymorphism while preserving encapsulation. In systems that support module parameterization, modulestypes can be used to describe families of modules produced by applying a parameter, a mechanism similar to functors or generics in other languages.

Relation to other module systems: modulestype mirrors the concept of a signature or interface found in many programming languages. The language’s type checker or module loader uses modulestype definitions to verify that modules conform to the declared interface before linking, instantiating, or composing them. This facilitates separate compilation, abstraction, and safer composition of software components.

Implementation and semantics: modulestypes can be static, with checks performed at compile time, or dynamic in languages that support runtime module loading. Some designs allow modules to expose opaque types or hidden representations, thereby preserving abstraction even when the implementation details are known internally.

Examples: a modulestype for a simple stack might declare a polymorphic type 'a stack and operations such as empty, push, pop, and is_empty. A module implementing this modulestype would provide a concrete type and definitions for those operations, ensuring their types align with the signature.

See also: module system, signature, interface, OCaml, SML.