What happens if the caller writes
f 0 : int t option? The caller has chosen'a = int, but the function implementation mandates'a = string. That’s why this definition is ill-typed.
While I agree with this, this is a type error even without abstract types. I guess the question here is what should the type argument to t be, which is why the module method works (the type is Returned_module.t). I didn’t consider this, which I guess is an answer to my question.
I thought about using a wrapper GADT as well, which is more elegant than the modules but still a bit less than I would like. Oh, well – it makes sense 
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Thanks!