The PVP ought to mention dependency bounds (Was: Growing Haskell Platform)

Johan Tibell johan.tibell at
Tue Dec 11 17:58:20 CET 2012

On Tue, Dec 11, 2012 at 8:34 AM, Simon Marlow <marlowsd at> wrote:
> On 11/12/12 16:14, Johan Tibell wrote:
>> The problem is one of two things, depending on how you look at it. On
>> the one hand, if we take "C-1.0 depends on A-1.0.0.*" to mean that C
>> states that it can work with any version of A-1.0.0, that is a lie (as
>> per the scenario in my original email) and the PVP needs to be
>> adjusted.
> If users expect this interpretation, then I think we should tackle that with
> better documentation. It's important that the dependencies on a package are
> a *local* statement about the APIs that *this* package requires, and don't
> need to take into account constraints imposed elsewhere.
> The consequences of the alternative are pretty severe: every package author
> basically has to do Cabal's job and perform a backtracking search, solving
> the constraints so they can put the results in their build-depends.
> Furthermore they would have to re-do this every time a new constraint is
> added to the system (somebody uploads a new package that might be a
> dependency of one of yours).

It's still a local statement: if we changed the PVP (which I don't
suggest we do at this point), it would be to require that every time
you bump a lower version bound on a dependency you have to bump your
own major version number. You don't have to recursively look at all
your dependencies as we assume they also follow the PVP.

> Right, better diagnostics is the solution.
>   I couldn't use B- because it depends on A-2.0
>     and I couldn't use A-2.0 because C requires A-1.0.0.*
> Although, listing all the solutions that *don't* work will quickly get very
> verbose.  This is a tricky UI problem.  Perhaps there should be a way to ask
> questions - "why didn't you use B-".

Better diagnostic is probably they way forward for now. I feel that
cabal used to be better at telling me why it made certain dependency
choices. Now it tells me very little (i.e. basically a list of
packages it tried, without information about why they were tried).

-- Johan

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