The patch

  Sat May  3 17:11:30 BST 2014  Owen Stephens <darcs@owenstephens.co.uk>
    * Use custom replHook to fix cabal repl
  
breaks the build for me:

  darcs.reviewed$ cabal build
  [1 of 1] Compiling Main             ( Setup.lhs, dist/setup/Main.o )
  
  Setup.lhs:37:58:
      Module `Distribution.Simple.Setup' does not export `replVerbosity'

Backing out just that from the current reviewed repo lets it build.
I don't know what installed stuff might be relevant, but I have:

  darcs.reviewed$ ghc --version
  The Glorious Glasgow Haskell Compilation System, version 7.4.1
  darcs.reviewed$ cabal --version
  cabal-install version 0.14.0
  using version 1.14.0 of the Cabal library

Owen, we probably have to rollback your patch, or can it be fixed?

We'd need to special case on the version of Cabal, so that only versions
>1.18 add the replHook.

I tried to use 
#if MIN_VERSION_Cabal(1,18,0)
replHook = ...
#endif

and similar around the import...

but that didn't seem to work - probably easiest to rollback the patch
instead.

There was a suggestion on IRC about fixing this using Template Haskell - 
I'll check into the details.

This was the idea: http://irclog.perlgeek.de/darcs/2014-05-09#i_8702735

Do we think it's worth the ugliness? I think it probably is because 'cabal 
repl' is very worth having.

patch1168 seems to work, but is it worth it?

My feeling is yes but keep a close eye to see if any problems show up.