OCaml Module Signatures and First-Class Modules: Static Type Safety vs. Dynamic Selection

Dependency Injection via Functors and First-Class Modules

OCaml provides two primary mechanisms for implementing dependency injection: static functors and first-class modules. Functors allow for compile-time specialization of components based on a provided signature, ensuring strict type safety and optimization through static resolution.

In contrast, first-class modules enable the treatment of modules as values, allowing a program to select an implementation at runtime. While this provides flexibility, it introduces a boundary where the static guarantees of the module system must be reconciled with the dynamic nature of value-based selection.

When integrating these two approaches—specifically when a functor is designed to accept a module that is being passed as a first-class value—the resolution of module paths and the preservation of private type abstractions can become ambiguous.

• How does the OCaml compiler maintain type inference consistency when a first-class module is unpacked into a functor's signature?
• What are the specific constraints on using sealed modules when they are transitioned from first-class values back into static module instances?