Re: [Caml-list] Equality between abstract type definitions

[Yaron Minsky <yminsky@janestreet.com>]

But isn't this the exact opposite of Andreas' proposal?  He was
proposing using '_a as a unification variable, which may very well be
generalized.  It is exactly this use case for '_a that seems at odds
with Andreas' proposal.

y

On Sun, Oct 27, 2013 at 10:28 AM, Gabriel Scherer
<gabriel.scherer@gmail.com> wrote:
>> > Note how OCaml already uses '_a for a sort of flexible variable in its
>> > output.
>> Where?
>
> '_a is used for type variables that cannot be generalized.
>
> # let x = ref None;;
> val x : '_a option ref = {contents = None}
> # let id x = x in id id;;
> - : '_a -> '_a = <fun>
>
>
>
> On Sun, Oct 27, 2013 at 1:56 PM, Yaron Minsky <yminsky@janestreet.com>
> wrote:
>>
>> On Sun, Oct 27, 2013 at 8:16 AM, Andreas Rossberg <rossberg@mpi-sws.org>
>> wrote:
>> > On Oct 25, 2013, at 22:32 , Yaron Minsky <yminsky@janestreet.com> wrote:
>> >> Changing the semantics of this will, I think, break a _lot_ of code.
>> >
>> > Interesting. Do you have specific examples in mind?
>>
>> I know that I've seen many examples come up in my code.  One common
>> use is to partially specify a type.  For example, if I wanted to
>> ignore a return value that is a Tcp.Server.t from Async, I would
>> probably write it like this:
>>
>>     (ignore server : ('a,'b) Tcp.Server.t)
>>
>> without specifying the sometimes rather complicated details of those
>> types.  Similarly, if I were to ignore a Map, I might write
>>
>>     (ignore map : (int,string,'a) Map.t
>>
>> since it's not helpful here to specify the comparator type, which is
>> what goes into the third slot here.
>>
>> Nowadays, I would probably use an underscore in these cases rather
>> than an explicit type variable, but our codebase has plenty of old
>> examples of this kind of thing.  If a change like the one you propose
>> is changed, I presume that _ would keep its current meeting, which
>> would address many use cases.
>>
>> Given the existence of such use-cases, I would hope that we could
>> avoid making the change in a way that would non-optionally break lots
>> of code.  If people agree this change should be made, perhaps it
>> should be done in the mode of -strict-sequence.  That change was added
>> as a flag, so users could take it at their own pace.
>>
>> >> 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.
>> >
>> > Maybe, but it's not my impression that this is true for most people I
>> > see asking related questions here on the list or on SO.
>>
>> To be clear, my guess above is less than scientific.
>>
>> >> 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.
>> >
>> > Now, _that_ is something I would only expect from programmers trained on
>> > SML -- ancient SML'90 to be precise. ;)
>> >
>> > Note how OCaml already uses '_a for a sort of flexible variable in its
>> > output.
>>
>> Where?
>>
>> > /Andreas
>> >
>>
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>