I'm surprised noone has pointed out the new type annotation syntax:

let id : type s. s -> s = fun x -> x


On Fri, Oct 25, 2013 at 4:32 PM, Yaron Minsky <yminsky@janestreet.com>wrote:

> Changing the semantics of this will, I think, break a _lot_ of code.
> I for one am not excited to do so.
>
> For what it's worth, I suspect that most people who are surprised by
> this are people who were trained on Standard ML.  At Jane Street we've
> had a lot of people learn the language, and the complaints I've heard
> about this feature are, I think, mostly from that group.
>
> I also don't find Andreas suggestion particularly intuitive.  I would
> have guessed that (x: '_a) would constrain x to be a weakly
> polymorphic value, which is at odds with the proposal.
>
> y
>
> On Fri, Oct 25, 2013 at 10:24 AM, Andreas Rossberg <rossberg@mpi-sws.org>
> wrote:
> > On 10/25/2013 01:09 PM, Gabriel Scherer wrote:
> >>
> >> However, I think that the current syntax of implicitly-introduced
> >> variables with heuristically-defined scoping rules is bad in any case.
> >> My own toy experiment with explicit syntaxes always use an explicit
> >> binding syntax for both rigid and flexible variables (eg. "forall a b c
> >> in ..." and "some a b c in ..."). In this regard, the ('a 'b . ty) or
> >> (type a) syntaxes are definite improvements -- if only we had applied
> >> those explicit binding forms to GADT constructor types as well... So I
> >> think that even with Andreas' proposed change, people would still be
> >> surprised by things like the following:
> >>
> >>    let id : 'a -> 'a = fun x -> x
> >>
> >>    let dup (x : 'a) ('a * 'a) =
> >>      let result = (x, x) in
> >>      (id : 'a -> 'a) result  (* fails, while (id : 'b -> 'b) works *)
> >
> >
> > Yes, I agree that implicit scoping is a bit of an ugly hack. That said, I
> > don't expect anybody to be truly surprised about the example above. At
> least
> > I never heard this being an issue for SML programmers. People probably
> > hardly ever write anything like the above, or avoid shadowing for clarity
> > anyway.
> >
> > /Andreas
> >
> >
> >
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