On Wed, 2008-08-20 at 13:53 -0500, Nicolas Frisby wrote:
I have a question about cabal's behavior for the build command. When using the build command on a cabalized project, any version changes for installed packages go unnoticed - the necessary modules in the project are not re-compiled.
Yes. That's a ghc bug.
If however, you run the configure command (though the .cabal file for the project has not changed) and then the build command, the appropriate modules (and only the appropriate modules) are re-compiled.
Really? I didn't think that made any difference. I've no idea what configure would be doing that would get ghc to notice when it simply lacks the ability to do so in general.
Not knowing that the configure command is necessary to detect changes in package that the current project depends on and proceeding only with the build command has led to BusErrors and GHC incurring the impossible in my exploration.
Yes, it's a pretty annoying ghc bug. You'll be glad to know that it is fixed in ghc-6.10: http://hackage.haskell.org/trac/ghc/ticket/1372 You'll notice a lot of people have commented on this one. It's tripped up a lot of people.
Is there a reason that the build command does not check the packages for version changes? It seems fair to expect package-sensitivity of the process that determines if modules need to be re-compiled. This process, I think, is part of the build command and not the configure command.
There's really not much we can do in Cabal itself. It has to be fixed in ghc and fortunately it now is fixed. That's because Cabal just uses ghc --make for builds and rebuilds, so if it doesn't realise that it has to rebuild after dependent packages change then we can't do much. As I say, I've no idea why configure should make any difference. If it does it's only by fluke and not design. Duncan