Added RPM generation capability to Cabal

[Bryan O'Sullivan]

I spent a few hours over the past couple of days hacking on Cabal to add 
the ability to build RPM packages. You can fetch my darcs repository 
from here:

darcs get --partial http://darcs.serpentine.com/cabal-rpm

This new capability is easy to use. It adds a single new Cabal command, 
called rpm.

runhaskell Setup.*hs rpm

This generates a spec file, and builds source and binary RPMs.

Here’s a quick example, of trying to build the Haskell XML-RPC library:

~/src/darcs/haxr $ runghc Setup.*hs rpm
Source tarball created: dist/SOURCES/haxr-3000.0.0.tar.gz
error: Failed build dependencies:
     HaXml-ghc66 >= 1.13 is needed by haxr-3000.0.0-1.i386
     HaXml-ghc66 < 1.14 is needed by haxr-3000.0.0-1.i386
Setup.lhs: rpmbuild failed with status 1

The rpm command has converted the dependencies in the haxr.cabal file 
into build-time and runtime dependencies in the haxr.spec file that it 
generated, but rpmbuild can’t find the HaXml package.

Having earlier built a HaXml package using the rpm command, I can 
install it with the system’s rpm command.

# rpm -i haxml-ghc66-1.13.2-1.i386.rpm
Reading package info from stdin ... done.
Saving old package config file... done.
Writing new package config file... done.

The RPM’s post-install scriptlet informs GHC’s package manager about the 
package’s availability:

# ghc-pkg list --simple | tr ' ' '\n' | grep -i haxml
HaXml-1.13.2

Now if I try to build haxr again, it will succeed.

By default, the rpm command builds both normal and profiling-enabled 
libraries. It also uses Haddock to generate library documentation. It’s 
possible to control these behaviours from the command line.

The command also provides a --gen-spec option, which only generates a 
spec file. You can use this >spec file as a basis for crafting one of 
your own.

I’ve used the rpm command to build about a third of the packages listed 
in the Hackage package database, with no problems.