On Sat, 2008-11-15 at 12:39 +0100, Thomas Hartman wrote:
When I specify
Build-Depends: base, parsec, HaXml >= 1.19.4
in xml-parsec.cabal
it does install correctly.
Yes, saying what version it needs is a good thing. It's all guesses otherwise.
I guess what happens is that cabal install takes the earliest version of a package registered to try to build. I guess that's a reasonable default.
Actually it does that for specific packages, there's a file in the hackage index called 'referred-versions': base < 4 parsec < 3 HaXml == 1.13.* QuickCheck < 2 These are all cases where there are large numbers of packages that fail to specify an upper version constraint but break when built with the later version of the package. In the case of HaXml it is also because the 1.13 series is the one considered stable by its author.
What seems unreasonable to me is that cabal continued to take the earlier version even after I hid it with ghc-pkg hide.
We cannot in general consider the hidden status of packages or we'd never be able to use packages like 'ghc' which are always hidden. Perhaps they could be used as soft preferences. If so should those come before or after the preferences from the command line or from the hackage index?
My advice would be to improve the error message to read
"... Could not find module `Text.XML.HaXml.Posn': it is a member of package HaXml-1.19.4, which is hidden It can be unhidden by adding
Build-Depends: HaXml >= 1.19.4
to xml-parsec.cabal"
Unfortunately we cannot easily do that. It is ghc that produces the message about hidden packages but it is cabal that told ghc to hide all the packages (other than those specified in the .cabal file). Some people suggest that we should try parsing the ghc error messages to be able to provide such hints but I don't think that's sensible.
That leaves the issue of cabal upgrade not upgrading to the latest version available on hackage, which still feels like a real bug to me.
That's again down to the preferred versions in the hackage index. It only comes into effect when you specify no version when asking for a package. If you cabal install 'haxml >= 1.14' then it'll pick the latest version. When you "cabal install haxml", do you mean "install the default version" or "install the latest experimental version"? Currently it means the former. If there is general agreement it should mean the latter then we can switch it. Or perhaps it just needs an informational message to tell the user whenever the default version is not the latest version. cabal install haxml Downloading HaXml-1.13.3... Note: the recommended version is HaXml-1.13.3 but latest version is HaXml-1.19, use cabal install haxml-1.19 if you want that version. Or something like that. Duncan