Cabal and GHC

[Simon Peyton-Jones]

Michael Snoyman wrote a blog post about Solving Cabal Hall, which came across via the peerless Haskell Weekly News. http://www.yesodweb.com/blog/2012/11/solving-cabal-hell

But I'd be unlikely to return there so I'm posting this to the libraries and cabal-devel lists.

I'm not deep in Cabal lore, but it seems to me that there is a fairly easy way to do a Lot Better than we are now. Let's go back to "Identifying the Problem".  Michael doesn't say, but I'm guessing that Fay depends on yesod-platform.  Very well, so Cabal sees

  I'm installing Fay, 
  which depends on yesod-platform, which dpends on data-default
  and depends directly on data-default

If the package database was empty, Cabal would try to figure out a version of data-default that is acceptable to both yesod-platform and to Fay.  And that's what we want!

Suppose that Cabal figures out that yesod-platform-2.7 and data-default-0,4 would work.  Then, in an empty package database, it could just go ahead and install those.  But in your example, the database isn't empty; we have already isntalled yesod-platform-2.7, dependin gon data-default-0.5.  And (here's the rub) you can only have yesod-platform-2.7 installed once.

One solution might be 
 1. *un* install yesod-platform-2.7 (depending on data-default-0.5) and 
 2. *re* install it (depending on data-default-0.4).  
But that would break any programs that use both yesod-platform-2.7 and data-default-0.5.

The solution is obvious: we should make it possible to instally yesod-platform-2.7 twice, 
  once version depending on data-default-0.4 
  once version depending on data-default-0.5.  

Now, if Cabal can figure out a plan based on an empty database, it can deliver on that plan even in a non-empty database, without messing up any existing installations.

Of course, if Fay depends exclusively on data-default-0.4, and yesod-platform depends exclusively on data-default-0.5, then they genuinely are incompatible, and Cabal can and should say so. That's not Cabal Hell.  That's just saying that the package authors have *specified* that they are incompatible.

Summary.  I may be way off beam here, but I think we can easily make things way better than they are.  By "easily" I mean with just design and implementation work.  That's not free, and no one has any time.... but it's well within reach. I'd even been wondering about trying to crowd-fund this development so that Well Typed can do it.  But let's first see if it'd solve the problem.

There must be a wiki page somewhere that articulates the challenge, points to blog posts about it, and discusses solutions. 

Simon