cabal-install always finds conflicting dependencies between base 3 and 4 on ghc 6.9 and 6.10

[ganesh]

Built GHC from HEAD, and cabal install (built with a previous GHC) hangs eating CPU and producing no output after "Resolving dependencies...", apparently for any package.

e.g.

cabal install -v rmonad --global --root-cmd=sudo

Global package.conf file attached. dcoutts suspects that the base3/base4 thing might be the cause.

ganesh@defoe:~$ cabal --version cabal-install version 0.5.2 using version 1.4.0.2 of the Cabal library

[ganesh]

Global package.conf file in use when cabal install hung

[guest]

minimal package.conf exhibiting the problem

[guest]

I tested with the attached package.conf.min and a minimal package without any modules that just depends on base, and without downloading the hackage.org package database.

It is indeed the base4/base3 thing; if the dependence of base-3.0.3.0 on base-4.0 is removed, everything works fine (at least as fine as it can with a broken package config like this).

-- int-e

[duncan]

We need to fix this one. Unfortunately it's a lot harder than just resolving this infinite loop. Just fixing that would give us the result that instead of looping it'd fail to find a solution, more or less every time. That's because the current resolver is looking for a solution that uses each package at most once and the base 3 - 4 thing breaks that assumption.

[Deewiant]

Affects GHC 6.10 as well, of course.

[duncan]

The current darcs version of cabal-install fixes the hanging bug, thanks to Saizan.

Thu Sep 25 11:14:41 PDT 2008  Andrea Vezzosi <sanzhiyan@gmail.com>
  * Fix infinite loop in the TopDown dependency resolver
  The loop occurred only if a package depended on another one
  with the same name, e.g. base-3.0.3.0 <- base-4.0.0.0

Of course the resolver is still broken with base 3 and 4. I've been working on that part yesterday and today and I think it's fixable. Changing the summary to reflect the remainder of the problem.

[duncan]

Fixed.

It no longer finds that it's a conflict. However to have it actually pick base 3 for most packages you need to modify your 00-index.tar file to contain a file preferred-versions that looks like:

base < 4

Eventually this will be in the standard index downloaded from hackage.