[Haskell-cafe] Re: problem with cabal for syb-with-class

[Duncan Coutts]

On Tue, 2008-08-26 at 12:06 +0200, Andrew U. Frank wrote:
> in fairness, i have to add that i did inadvertetly install version 0.3.
> of syb-with-class and got the error i still cannot understand.
> 
> installing version 0.4 did work flawlessly!
> 
> nevertheless, i would be interested to understand the problem i
> encountered.

The error message unfortunately refers to the mechanism and not the
cause. When Cabal builds a package it tells ghc to hide every package
and then use only the packages listed in the build-depends field:

ghc --make -hide-all-packages -package base-3.0.1.0 ... etc

So when ghc finds that one of your modules needs to import something
that is not in one of the given packages it says that it's in another
package that is 'hidden'. Of course it's only hidden because Cabal told
ghc to hide them.

So what the error message really means is that you're missing a package
from the build-depends field in the .cabal file.

The error message will improve when Cabal does it's own dependency
chasing, but don't hold your breath.

Duncan