The `module strengthening` extension

What is the type of module M in this tiny program?

module type S = sig type t end

module M : S

If you said S then you are only partially correct: we also need to keep track of the fact that all type declarations inside it (just t in this example) come from M. So the type that the compiler infers is actually stronger:

sig type t = M.t end

We call this type “S strengthened with M”. It can be written out explicitly as above and this is, in fact, the only way to write it without -extension module_strengthening. The new extension allows this type to be written as S with M.

The main motivation for this work is compiler performance improvements.

Topics

The module strengthening extension

The `module strengthening` extension