[Haskell-cafe] Curious why "cabal upgrade parsec" not installing latest version

[Peter Schmitz]

Jason,
Thank you for the insights into Cabal. I am new to Parsec, and
relatively new to Cabal, and was not aware of the info that you (and
Thomas and Ivan) posted. Great help; thanks again.

I will probably stick with Cabal's default for the time being because
I don't have a compelling reason not to, but it's good to know how
Cabal works.

I am finding developing in Haskell + Gtk2Hs + Glade + Parsec a lot of
fun. Parser combinators are so cool.
-- Peter


On Wed, Sep 15, 2010 at 8:15 PM, Jason Dagit <dagit at codersbase.com> wrote:
> On Wed, Sep 15, 2010 at 7:47 PM, Peter Schmitz <ps.haskell at gmail.com> wrote:
>> Not that I'm having any problem with parsec 2.1.0.1, but I guess I
>> would like to install the latest (3.1.0), unless there is a reason
>> not to.
>>
>> I can't seem to get Cabal to do so; thanks in advance for any help.
>>
>> I don't understand part of the output from "cabal install --dry-run
>> --reinstall -v parsec" at the end below, which includes:
>> "selecting parsec-2.1.0.1 (hackage) and discarding parsec-2.0,
>> 2.1.0.0, 3.0.0, 3.0.1 and 3.1.0".
>>
>> (http://hackage.haskell.org/package/parsec seems to point to 3.1.0.)
>
> I consider this a misfeature of cabal-install.  Cabal-install will check here:
> http://hackage.haskell.org/packages/archive/preferred-versions
>
> Before deciding out how to handle an 'install' request.  You'll notice
> that it says:
> parsec < 3
>
> Sadly cabal-install doesn't warn you or ask what version you want.
> There is an unwritten policy that cabal knows better than you do about
> what packages you want.  And then it silently enforces it's point of
> view on you unless you specify a conflicting constraint.  And unless
> the package is base, in which case it only listens if you either a)
> give an upper bound on base, or b) provide a preference (instead of a
> constaint).
>
> You can work around these (broken) global policies by editing your
> local cabal config.  Should be in $HOME/.cabal/config (or the similar
> place on windows).  You can add a line like:
> preference: base >= 4, parsec >= 3
>
> That should help to nullify the global policy.
>
> Cabal bugs not withstanding, this comes about because cabal is not a
> package manager.  Unlike distributions like Debian there is no way to
> subscribe to 'stable', 'experimental', or 'developer only' package
> classifications.  Unfortunately, the current 'solution' has been to
> enforce a 'one size fits all' policy on users with an essentially
> undocumented way to override it (which has to be applied for each
> cabal-install installation).  I assume this strategy was picked
> because it was easy to implement and developer time can be scarce.
>
> Jason
>