This is a guess since you didn't include the definition of
stream_declarations, but I suspect that:

> (* I have the following function, which seems to type check *)
> let stream_declarations (type a)
> : (internal_result,a) result_stream = ...

is not polymorphic. The `(type a)` annotation ensures that `a` is
abstract in the body of `stream_declarations` but does not enforce
that the result is polymorphic -- in particular I suspect that you
are running up against the value restriction. An easy way to check
would be to change the definition to:

  let stream_declarations : 'a. (internal_result,'a) result_stream =
  ...

or

  let stream_declarations : type a. (internal_result,a) result_stream =
  ...

both of which enforce polymorphism, and so will give an error if your
definition is actually monomorphic.

If `stream_declarations` does have the monomorphic type
`(internal_result, '_a) result_stream` then any attempt to use it as
`(internal_result, b) result_stream` would cause `b` to be unified with
`'_a` and thus escape it's scope.

Regards,

Leo