On Thu, Aug 27, 2015 at 10:59 AM, Christoph Höger <
christoph.hoeger@tu-berlin.de> wrote:

> Obviously, 'a is bound by its first occurrence in the type annotation,
> here. Can someone explain this behavior?


​In a type annotation, the scope of variables is only the type annotation.
So in your example the ['a] in the annotation of [a_to_opt] and the ['a] in
the annotation of [None] are distinct. To refer to the same type, you need
to use a local abstract type:

    let rec a_to_opt : type a. (a -> a option) = fun x -> (None : a
​ option​
)​​

-- 
Jeremie