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

[Yaron Minsky <yminsky@janestreet.com>]

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
>