cabal install problems

[Duncan Coutts]

On Mon, 2009-02-02 at 23:56 +0100, Jürgen Nicklisch-Franken wrote:
> I recently had some (well actually a lot of) troubles with cabal install
> which are not over. I installed ghc-6.10 on Windows with Cygwin and the
> fetched cabal-install 0.6.0. 
> 
> When I then tried to install a package it had regex-posix as dependency
> it fails to install the newest version package. This was because it
> doesn't find the c header files. So I tried to use the
>  --extra-include-dirs=PATH flag, but this didn't help, and I guess it
> was because it was the c2hs preprocessor which didn't get it. 

Do you have any more details on that? It's the 'hsc2hs' pre-processor
btw. Confusingly there is a similar pre-processor called 'c2hs'.

> So I did a manual install of the package and somehow did it.

That's interesting. It should be the same either way. More details would
be interesting.

> But although ghc-pkg listed the package, when I next time called cabal
> install it started with installing the package that was already
> installed. When called with a verbose flag I saw that cabal-install
> detected that the package was not installed by cabal install. However
> why does it try to reinstall an already installed package? How does it
> know, which packages are installed with cabal install?

Actually it does not track which agent installed the package. If you run
with --dry-run -v it gives some explanation for what it is doing and why
it might think it needs to reinstall the package. If you paste that
output we might be able to see what is going on. There is one specific
problem with these symptoms that's been fixed in the development
version, but there are some other reasons why the solver might choose to
reinstall a package. 

> I then changed the version of the required regex-posix package to the
> exact version of the lib pre-installed with ghc on Windows. Yet I can
> never use cabal upgrade again, because it will always fail.

cabal upgrade isn't such a good idea anyway. I'd stick to the install
command. The only difference is that upgrade tries to install the latest
version all dependencies too, and that tends to lead to problems. I'm
considering disabling the feature for now.

> Is their any way to exclude a package from the cabal install process?

There is a flag --constraint="foo == 1.0" which is quite handy. The next
version may or may not have a way to specify a constraint to always pick
an installed version of a particular package. It depends if there is
enough time.

> I realised as well that there is no way to ever un-install a package
> installed with cabal-install, as far as I see. This is not satisfactory,
> right?

Right. Currently only by using ghc-pkg to unregister and the file
manager to delete the files. For some reason uninstall never seems to be
as high on users priorities as other bugs and features. At least that is
my impression. Of course if people help out we'll get round to it
sooner.

> I regret as well that no option exist to have the source code expanded
> (I mean from tar.gz), which can be useful if you want to look up some
> library code later, which I happen to do frequently.

There is in the development version

cabal unpack foo

> I regret as well that there is no way to adopt a cabal file to my needs
> when using cabal install, which I find sometimes necessary. 

unpack is great for that.

> As cabal install is supposed to become a standard way of installing
> Haskell packages, at least for developers, it would be necessary to have
> some documentation on it, other then just explaining command line
> parameters.

I would very much appreciate some help with the documentation. We
already have a Cabal user guide and there are not that many extra things
in the command line interface of cabal-install.

> E.g. if you install some packages as user, which it does by default, it
> will later not be possible to install other packages global, which
> anyone using cabal install should know. (Or am I wrong)

Global packages cannot depend on user ones. So installing something
globally will also install all of its dependencies globally.

> So please help us from spending our time with stupid install problems,
> and make the road for new Haskell users as smooth as possible, and
> installation is a big issue for them.

That is what we're trying to do :-). That is what this whole project is
about. I can assure you it used to be much harder than it is now. We're
making progress but with more people helping we could make progress
quicker :-)

Duncan