On Sun, 2011-02-27 at 18:24 +0100, Henning Thielemann wrote:

I am lost. Cabal-install says 'there is no available version of parsec that satisfies ==3.1.1', but ghc-pkg says I have this parsec version installed.

mypkg$ cabal install --constraint=parsec==3.1.1

So you are installing the package in the current directory.

Resolving dependencies... cabal: There is no available version of parsec that satisfies ==3.1.1

So what is happening is that the package in the current dir does not depend (directly or indirectly) on parsec. As an optimisation, Cabal's dep resolver trims out all packages that are not in the transitive closure of the dependencies of the package(s) you're installing, and as parsec is not in that set then it gets ignored too. Then the dep solver comes along and tries to apply the constraint parsec == 3.1.1 which fails because there are no available instances of parsec left (as they were all trimmed out earlier since they will never be needed). Strictly speaking this constraint failure is spurious because it's ok for parsec to be over-constrained since it is not needed. Ideally what we would like to happen here is for the constraint to simply be ignored, perhaps with a warning that it was unnecessary. As it happens, I've been working on the dep resolver recently and the changes I'm making will mean that superfluous constraints will indeed be ignored. I'll think about whether we can easily warn too. Duncan