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

[Gabriel Scherer <gabriel.scherer@gmail.com>]

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> > 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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