On Mon, 2009-02-02 at 13:18 +0100, Sean Leather wrote:
Either I'm doing something wrong or this doesn't work for cabal-install and GHC 6.8.3. I used the "flag newer-th" approach in EMGM:
https://svn.cs.uu.nl:12443/viewvc/dgp-haskell/EMGM/tags/emgm-0.2/emgm.cabal?...
[...]
flag th23 description: Define a CPP flag that enables conditional compilation for template-haskell package version 2.3 and newer.
Library
[...]
build-depends: base >= 3.0 && < 4.0, template-haskell < 2.4
-- Include deriveRep for Loc. This was introduced with -- template-haskell-2.3, included with GHC 6.10. if flag(th23) build-depends: template-haskell >= 2.3 cpp-options: -DTH_LOC_DERIVEREP else build-depends: template-haskell < 2.3
[...]
When I run cabal install emgm (with GHC 6.8.3 and either Cabal 1.2 or 1.6), it tries (and fails) to install template-haskell-2.3. That's exactly what I don't want. Any suggestions?
No immediate workarounds, though you could perhaps try reversing the condition or the default of the flag. Though that might just reverse the problem case. Presumably the template-haskell-2.3 package does not build with ghc-6.8 but fails to correctly specify the version of base or ghc that it requires. If it did then we would have a better chance to get this right. In general this is going to be hard. The solver is not that smart and it deliberately does no backtracking (as a defence against searching too much given that it has no smart methods for pruning the search space). This is just the kind of situation where we need a proper solver which is probably a couple months work. Another possible workaround... If you avoid using the flag hack then the solver would not have to choose so early which version of template-haskell to commit to and it would be able to pick the right version later. Though then you need an alternative method of finding which version of template-haskell you ended up with. You could do it with some custom code in Setup.hs. Duncan