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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