I ran "darcs get --lazy --tag=2.3.1 http://darcs.net" and then followed the 
README, which said to run "cabal install".  I did so and got this error:

Resolving dependencies...
cabal: cannot configure darcs-2.3.1. It requires hashed-storage >=0.4.0 &&
<0.5 && >=0.4.0 && <0.5, haskeline >=0.6.1 && <0.7 && >=0.6.1 && <0.7 and zlib
>=0.5.1.0 && <0.6.0.0 && >=0.5.1.0 && <0.6.0.0 && >=0.5.1.0 && <0.6.0.0
There is no available version of hashed-storage that satisfies >=0.4.0 && <0.5
&& >=0.4.0 && <0.5
There is no available version of haskeline that satisfies >=0.6.1 && <0.7 &&
>=0.6.1 && <0.7
There is no available version of zlib that satisfies >=0.5.1.0 && <0.6.0.0 &&
>=0.5.1.0 && <0.6.0.0 && >=0.5.1.0 && <0.6.0.0

By asking on IRC, I was told that I needed to run "cabal update" to get past 
this problem.  Perhaps the README should suggest running "cabal update".  
(Presumably people who are cabal experts will know that they can skip this step 
in most cases.)

Zooko wrote:

 > [...etc...]
> 
> By asking on IRC, I was told that I needed to run "cabal update" to get past 
> this problem.  Perhaps the README should suggest running "cabal update".  
> (Presumably people who are cabal experts will know that they can skip this step 
> in most cases.)

I would suggest that *cabal* should suggest to run "cabal update"-- that 
seems like the Right Thing To Do (TM).  (And for current versions of 
cabal which don't, yes, please update the README.)

I'll submit a patch for this.

As for the cabal update suggestion, that would be
http://hackage.haskell.org/trac/hackage/ticket/497

The following patch updated the status of issue1652 to be resolved:

* Resolve issue1652: suggest cabal update before cabal install. 
Ignore-this: de39107c4c324ee83d1a63d950e83a1c