From: nakata keiko <keiko@kaba.or.jp>

> > > This does not answer to me why this works,
> > >
> > > #type t = { t : 'a. 'a -> 'a}
> > >
> > > #let v = let module M = struct let t x = x end in {t = M.t} in (v.t 5, 
> > > v.t true)
> 
> I have the impression that if the above code is type checked,
> then, something like the following code could be type checked.
> What is the differences?
> 
> > > #let v : 'a. 'a -> 'a = let module M = struct let t x = x end in  M.t 
> > > in (v 5, v true)

These two examples use different kinds of polymorphism.
In the first case, the data structure ensures the polymorphism,
meaning that even if the type of the data structure is made
non-generalizable, you can still extract a polymorphic value from it
later.
That's why you can pass such values to functions, and use them
polymorphically inside (which you cannot do with normal variables.)

A simple example of the difference is
   (fun v -> (v.t 5, v.t true)) {t = fun x -> x}
vs.
   (fun v -> (v 5, v true)) (fun x -> x)

There is currently no way to make a first-class polymorphic value in
ocaml without using either record or object.
(There is no theoretical limitation, just we could never come up with
the right syntax for introduction and elimination.)

Jacques Garrigue