[Haskell-beginners] Cabal complains that cabal-dev “indirectly depends on multiple versions of the same package”, even though it (apparently) does not

[Brent Yorgey]

On Wed, Feb 15, 2012 at 09:55:32PM +0000, James Fisher wrote:
> 
> I guess another thing I don't understand is: why is there both a version
> number and a hash?  I'm told by Hackage that "Re-uploading a package with
> the same version number is not permitted" (
> http://hackage.haskell.org/packages/upload.html), so how could I have a
> package with two different hashes?  Unless one or both came from somewhere
> other than Hackage, perhaps?
> 
> Yet another thing I don't properly understand is the relationship between
> all these Haskell packaging organizations and programs.  cabal seems to
> have some pseudo-official relationship with Hackage.  And how do cabal and
> ghc-pkg interact?  Where are things on my system apart from in ~/.cabal?

For answers to all these questions (and more), see

  http://www.vex.net/~trebla/haskell/sicp.xhtml

-Brent