ReScript module signatures with abstract types

ReScript version: bs-platform@8.4.2

Suppose we declare a module signature:

module type SHOW = {
  type t
  let show: t => string
}

Here we have an abstract type t and it's up to the implmentation module to provide the type.

Let's declare a person type:

type person = {name: string}

And a ShowPerson: SHOW module:

module ShowPerson: SHOW = {
  type t = person
  let show = (person: t) => person.name
}

Now we can make use of our ShowPerson module:

let joe: person = {name: "Joe"};

ShowPerson.show(joe);

Unfortunately on that last line the reference to joe is causing an error:

This has type: person
Somewhere wanted: ShowPerson.t

To fix this we need to add a with type annotation to our ShowPerson module:

module ShowPerson: SHOW with type t = person = {
  type t = person
  let show = person => person.name
}

References

Unfamiliar syntax in documentation for Show
https://github.com/reazen/relude/issues/193