[Haskell-beginners] Cabal install fails to download packages

[Cal Paterson]

Hello,

I am new to haskell, and I'm trying to get cabal to download and
install packages for me.  However, it seems to fail to download
packages.  Having cabal act on already installed packages seems to
work fine.  Here is the end of the output:

cal at romeo:~$ cabal install haddock
...
selecting unix-2.3.0.0 (installed)
selecting ghc-paths-0.1.0.5 (hackage) and discarding ghc-paths-0.1,
0.1.0.1,
0.1.0.2, 0.1.0.3 and 0.1.0.4
In order, the following would be installed:
ghc-paths-0.1.0.5 (new package)
haddock-2.4.2 (new package)
Downloading ghc-paths-0.1.0.5...
GET
http://hackage.haskell.org/packages/archive/ghc-paths/0.1.0.5/ghc-paths-0.1.0.5.tar.gz
Sending:
GET /packages/archive/ghc-paths/0.1.0.5/ghc-paths-0.1.0.5.tar.gz
HTTP/1.1
User-Agent: cabal-install/0.6.2
Host: hackage.haskell.org
proxy uri host: , port:
Creating new connection to
cabal: Error: some packages failed to install:
ghc-paths-0.1.0.5 failed while downloading the package.
haddock-2.4.2 depends on ghc-paths-0.1.0.5 which failed to install.

I first guessed that cabal was tripping up over some environment
variable, but it seems that $http_proxy is unset:

cal at romeo:~$ set | grep -i http | xclip
http_proxy=

Can anyone provide some advice as to what is likely to be going
wrong?

Thanks,
Cal