On Wednesday 05 May 2010 15:45:38, Henning Thielemann wrote:
Maciej Piechotka schrieb:
On Wed, 2010-05-05 at 01:09 +0200, Daniel Fischer wrote:
On Mittwoch 05 Mai 2010 00:55:38, Maciej Piechotka wrote:
I try to configure happstack with parsec 3.1. It seems to fail due to cabal:
happstack-util.cabal says parsec < 3, so --constraint="parsec > 3" and the given dependencies are incompatible, hence it can't be configured.
Probably parsec < 3 was specified because parsec-3.0 was significantly slower than parsec-2.*.
I updated local copy, as shown, but cabal wants to rebuild it anyway. My question was rather why the repo is considered at all when the package is installed.
Maciej, if you need to edit further .cabal files, you could make a minor- minor version bump, x.y.z -> x.y.z.1 e.g., maybe that would convince cabal- install not to reinstall.
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.
Cabal-install is somehow too clever and if it cannot resolve the dependencies it thinks this must be impossible. If it finds a package, that it could not have installed by itself, it tries to install it by itself.
First I thought that this due to cabal-install trying to find appropriate flag assignments by itself. However, Duncan Coutts told me that plain Cabal tries this as well. I have no idea, what the key difference between cabal-install and plain Cabal is.