[Haskell-beginners] Why am I having such weird dependency issues?

[Andres Löh]

>> cabal: dependencies conflict: ghc-7.0.3 requires containers ==0.4.0.0
>> however
>> containers-0.4.0.0 was excluded because ghc-7.0.3 requires containers
>> ==0.4.2.0
>
>
> You have multiple versions of containers installed, and yi wants to use both
> of them.  Unfortunately, cabal-install loves to report the conflict this way
> (I think it's bubbling the actual dependency backwards to ghc-7.0.3 instead
> of leaving it where it happened?).

I can't reproduce this one easily on my system currently. I can only
say that I can produce a valid install plan for yi with ghc-7.0.3.

>> cabal: cannot configure HPath-0.0.2. It requires base >=2 && <=4
>> For the dependency on base >=2 && <=4 there are these packages:
>> base-3.0.3.1
>> and base-3.0.3.2. However none of them are available.
>> base-3.0.3.1 was excluded because of the top level dependency base -any
>> base-3.0.3.2 was excluded because of the top level dependency base -any
>
>
> "base -any" here can be read as "base that came with your compiler"; it
> cannot be replaced.  HPath apparently is not known to work with the version
> of base that came with your ghc, so cabal tries the versions of base that it
> will work with; but it can't replace the base package, and has that rather
> unfortunate way of notifying you so.

Right.

> I'm given to understand the unreleased version of cabal-install in the
> repository does a better job of explaining these kinds of things.  As it is,
> it is *very* hair-tearing....

This is what you get for the HPath case when you use the modular
solver. It's admittedly still not as polished as it could be, but I do
think it's an improvement:

$ cabal install --solver=modular HPath
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: HPath-0.0.2 (user goal)
next goal: base (dependency of HPath-0.0.2)
rejecting: base-3.0.3.2, 3.0.3.1 (global constraint requires installed
instance)
rejecting: base-4.3.1.0/installed (conflict: HPath => base>=2 && <=4)
rejecting: base-4.5.0.0, 4.4.1.0, 4.4.0.0, 4.3.1.0, 4.3.0.0, 4.2.0.2, 4.2.0.1,
4.2.0.0, 4.1.0.0, 4.0.0.0 (global constraint requires installed instance)

So this tells you it's trying to find a version of base because HPath
depends on it. It then tells you that all versions but base-4.3.1.0
are rejected because you can only use an already installed instance of
base. But version base-4.3.1.0 is also rejected, because HPath depends
on a version of base between 2 and 4.

Cheers,
  Andres