You can extract `MyFunctorReturnType` in a separate module to avoid the repetition

test.re

module type MyFunctorReturnType = {let myFunc: unit => string;};

module MyFunctor (Impl: {type myType;}) => {
  let myFunc () => "hello";
};

module MyExample =
  MyFunctor {
    type myType = string;
  };

test.rei

module type MyFunctorReturnType = {let myFunc: unit => string;};

module MyFunctor: (Impl: {type myType;}) => MyFunctorReturnType;

module MyExample:  MyFunctorReturnType;

You can extract MyFunctorReturnType in a separate module to avoid the repetition