Daniel Fischer schrieb:
On Wednesday 05 May 2010 15:45:38, Henning Thielemann wrote:
Surprisingly using plain Cabal (runhaskell Setup configure; runhaskell Setup build; runhaskell Setup install) often works in these cases.
That's not surprising. runhaskell ./Setup.hs configure can only go by what the .cabal file in the current directory and ghc-pkg say. If you edit the .cabal file to install e.g. happstack-util-0.5.0 with parsec-3, the changed .cabal file and the ghc-pkg output are consistent, so Cabal sees no reason to reinstall.
cabal-install presumably looks at the downloaded index from Hackage (it must recursively follow the dependencies to see which of them must be installed before [and in which order]). The happstack-util .cabal file there says parsec < 3. But ghc-pkg says - there is no parsec < 3 installed or - happstack-util-0.5.0 was built with parsec-3.* (don't know which check comes first if both are made). That is inconsistent with what cabal-install knows from other sources, hence it assumes it's broken.
This makes sense. It would be certainly better if cabal-install would alert about the found inconsistency instead of trying to fix it. Then there might an additional cabal-install flag, that makes cabal-install prefer the output of ghc-pkg over the hackage database 00-index.tar in case of inconsistencies. Could the problem also be solved by changing the version of packages with locally modified cabal files from a.b.c.d to a.b.c.d+1 ?