[darcs-users] [PATCH, RESEND] Eliminate HopefullyPrivate (rename over Hopefully)

[David Roundy]

On Sun, Aug 10, 2008 at 2:03 AM, Jason Dagit <dagit at codersbase.com> wrote:
> I think the idea behind the patch sounds fine.  We don't seem to use the
> separation between Hopefully and HopefullyPrivate, so it just seems
> supurfluous at the moment.  We could always add it back at a later time if
> it seems useful then.
>
> More comments below.
>
> On Sat, Aug 9, 2008 at 2:26 PM, Nathaniel W Filardo <nwf at cs.jhu.edu> wrote:
>>
>> On Sat, Aug 09, 2008 at 06:51:24PM +0200, Nicolas Pouillard wrote:
>> > Excerpts from Simon Michael's message of Sat Aug 09 18:41:33 +0200 2008:
>> > > Second thoughts!
>> > >
>> > > I had trouble understanding and applying this patch. It complained
>> > > about
>> > > removing an non-empty file, but then did it anyway. I obliterated the
>> > > patch and then noticed my my repo was inconsistent, with Hopefully.lhs
>> > > containing 0 bytes. darcs check confirmed, darcs repair fixed it.
>> > > That's
>> > > a bad patch.
>> >
>> > Nathaniel did you use the --look-for-adds option? If so, you've probably
>> > it a
>> > known bug (to lazy to look at the tracker). I traditionally workaround
>> > this
>> > by killing some/all parts of the _darcs/patches/pending: DO IT
>> > CAREFULLY, or
>> > by using darcs repair.
>>
>> I don't think so, but just in case, I've regenerated the patch (attached)
>> --
>> this one passes the visual smoke test.
>>
>> Thinking more about it, though, I don't really understand why Hopefully
>> exists.  The data type,
>>
>> > data Hopefully a C(x y) = Hopefully (SimpleHopefully a C(x y))
>> >                         | Hashed  String (SimpleHopefully a C(x y))
>>
>> seems equivalent to
>>
>> > SimpleHopefully a C(x y) * Maybe String
>
> We have:
> data SimpleHopefully a C(x y) = Actually (a C(x y)) | Unavailable String
>
> Ignoring for a moment the phantom types, it's this:
> data SimpleHopefully a = Actually a | Unavailable String
>
> Which looks much closer to "Either String a".  Oh, but maybe I don't
> understand your notation?  Does the * mean product and thus you would take
> that to mean the Either type in this case?
>
> Now it seems that we have this same structure again with the Hopefully
> type.  So in terms of existing types we could do this:
>
> type Hopefully (a C(x y)) = Either (Either String (a C(x y))) (Either
> (String, (Either String (a C(x y))))
>
> It should be obvious why we created a new type.  Even if we only create
> SimpleHopefully, it's not so pretty:
>
> type Hopefully (a C(x y)) = Either (SimpleHopefully (a C(x y))) (String,
> SimpleHopefully (a C(x y)))
>
> Given how easily we can construct our own custom type and then give the
> corresponding instances that Either would have, it seems like making a new
> data type is best.
>
> Although, it turns out, due to our phantom types, we can't give an instance
> of Monad, so we defined something that behaves the same way as the Monad
> instance if we didn't have the phantom types.
>
>> which should be
>>
>> > Maybe (a C(x y)) * Maybe String
>>
>> But, AFAICT, the only reason for the Maybe String seems to be because
>> PatchInfo structures don't store the hashed-repository ("new") hash value
>> --
>> the root complication with issue861.
>
> I can't really comment on this without digging into it.  Maybe it was done
> this way to avoid putzing around with all the existing PatchInfo code,
> derived instances and what not.
>
>>
>> Is it silly to think that PatchInfos should, even for old repositories,
>> store (potentially unevaluated thunks -- hooray laziness) their new hash
>> value?  Am I missing something obvious?
>
> The hash could be a Maybe String.  (Hopefully David) will comment is
> (Hopefully (Actually ..)) and not (Hopefully (Unavailable ..)).  :-)

I'm afraid I don't  have much time  (or rather if I stay at my
computer longer today my back will cause trouble tomorrow).
Eliminating HopefullyPrivate is a good idea.  It used to be used
outside of Hopefully, which was its only reason for existence.

As far as eliminating the Hopefully type, I'm certain it'd be a lot of
work, and am not sure it'd be worth it.  The trouble is that this type
shows up in bits of code where we have some very tricky laziness
required (or at least it was tricky to me).  I'm not sure it would be
worthwhile (or beneficial).

David