Here's my view.

1a. I don't know how to answer this question because I have no idea
what "good enough job" means.
1b. Unequivocally yes, we should do our best to support this
expectation to the best of our ability.  However we can't state it as
strongly as "no matter what happens".  I think we can say "as long as
all of his dependencies are well-behaved, the package should be pretty
likely to build..."

1c. If "always run in the same way" means that it will always be built
with the same set of transitive dependencies, then no.
2a is a bit stickier.  I want to say yes, but right now I'll leave
myself open to convincing.  My biggest concern in this whole debate is
that users of the foo package should specify some upper bound, because
as Greg has said, if you don't, the probability that the package
builds goes to ZERO (not epsilon) as t goes to infinity.  Personally I
like the safe and conservative upper bound of <1.3 because I think
practically it's difficult to make more granular contracts work in
practice and it follows the PVP's clear meaning.  If you're committing
to support the same API up to 2.0, why can't you just commit to
supporting that API up to 1.3?  The author of foo still has the
flexibility to jump to 2.0 to signal something to the users, and when
that happens, the users can change their bounds appropriately.

On Wed, Apr 9, 2014 at 3:22 PM, Michael Snoyman <michael@snoyman.com> wrote:
> I have to say, the responses on this thread are truly confusing. Let's
> ignore point (3) of my proposal (since it can essentially be subsumed under
> (4)). Point (2) is clearly a change in the PVP, and boils down to "users get
> cabal error messages" or "users get GHC error messages." I understand
> (though strongly disagree with) those opposed to the change I'm proposing
> there. So let's ignore it.
>
> For (1) and (4), the responses vary from support, to opposition, to "that's
> what the PVP already says." So there's clearly a problem here, and I don't
> think the problem is in my proposal: people have very different ideas of
> what the PVP actually expects of us.
>
> So forget my proposal for the moment, I want to engage in a thought
> experiment. What does the current PVP say about the following scenarios:
>
> 1. A user is writing an application based on a number of Hackage libraries.
> He places version bounds following the PVP, e.g. `text >= 1.0 && < 1.1,
> aeson >= 0.7 && < 0.8`.
>     a. Has he done a good enough job of writing his application?
>     b. Should he have an expectation that, no matter what happens, his
> software will always build when running `cabal clean && cabal install`
> (assuming same GHC version and OS)?
>     c. Should he have an expectation that the code will always run in the
> same way it did when first built?
> 2. I author a package called foo and release version 1.2 with the statement:
> "I guarantee that the Foo module will exist and continue to export the foo1
> and foo2 functions, with the same type signature, until version 2.0."
>     a. If a user of the foo package only uses the foo1 and foo2 functions,
> is he "in violation" of the PVP by using a bound on the foo package of `foo
>>= 1.2 && < 2`?
>
>
>
> On Wed, Apr 9, 2014 at 11:47 AM, Michael Snoyman <michael@snoyman.com>
> wrote:
>>
>> I would like to propose the following changes to the PVP. These are the
>> same changes that I recently published on the Yesod blog[1]. For more
>> information on the motivations behind these changes, please see that blog
>> post.
>>
>> 1. The goal of the PVP needs to be clarified. Its purpose is not to ensure
>> reproducible builds of non-published software, but rather to provide for
>> more reliable builds of libraries on Hackage. Reproducible builds should be
>> handled exclusively through version freezing, the only known technique to
>> actually give the necessary guarantees.
>>
>> 2. Upper bounds should not be included on non-upgradeable packages, such
>> as base and template-haskell (are there others?). Alternatively, we should
>> establish some accepted upper bound on these packages, e.g. many people
>> place base < 5 on their code.
>>
>> 3. We should be distinguishing between mostly-stable packages and unstable
>> packages. For a package like text, if you simply import Data.Text (Text,
>> pack, reverse), or some other sane subset, there's no need for upper bounds.
>> (Note that this doesn't provide a hard-and-fast rule like the current PVP,
>> but is rather a matter of discretion. Communication between library authors
>> and users (via documentation or other means) would be vital to making this
>> work well.)
>>
>> 4. For a package version A.B.C, a bump in A or B indicates some level of
>> breaking change. As an opt-in approach, package authors are free to
>> associated meaning to A and B beyond what the PVP requires. Libraries which
>> use these packages are free to rely on the guarantees provided by package
>> authors when placing upper bounds. (Note that this is very related to point
>> (3).)
>>
>> Discussion period: 3 weeks.
>>
>> [1] http://www.yesodweb.com/blog/2014/04/proposal-changes-pvp
>
>
>

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