This seems wrong, as Context.hs is autogenerated.

    # cabal build
    Preprocessing library darcs-2.1.98.2...
    Preprocessing executables for darcs-2.1.98.2...
    Building darcs-2.1.98.2...
    /usr/bin/ar: creating dist/build/libHSdarcs-2.1.98.2.a
    Linking dist/build/witnesses/witnesses ...
    [  1 of 134] Compiling Context          ( src/Context.hs, dist/build/darcs/darcs-tmp/Context.o )

    src/Context.hs:3:55:
        lexical error in string/character literal at character '/'

    # nl -ba src/Context.hs | sed -n 2,4p
    2  context :: String
    3  context ="# configured Thu Jan 15 17:13:21 EST 2009\n\
    4  \./configure /usr/local/share/config.site /usr/local/etc/config.site\n\

Note that "make witnesses" works fine:

    # make witnesses
    perl release/determine_release_state.pl 2.2.0pre2 > release/STATE
    make: Nothing to be done for `witnesses'.

10:41:56 | morn@eri:~/tmp -> cat foobar.hs
context :: String
context ="# configured Thu Jan 15 17:13:21 EST 2009\n\
\./configure /usr/local/share/config.site /usr/local/etc/config.site"
10:42:01 | morn@eri:~/tmp -> cat foobar.cpp.hs
# 1 "foobar.hs"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "foobar.hs"
context :: String
context ="# configured Thu Jan 15 17:13:21 EST 2009\n\./configure /usr/local/
share/config.site /usr/local/etc/config.site"
10:42:03 | morn@eri:~/tmp -> ghc foobar.hs

foobar.hs:1:0: The function `main' is not defined in module `Main'
10:42:12 | morn@eri:~/tmp -> ghc foobar.cpp.hs

foobar.hs:2:55:
    lexical error in string/character literal at character '/'

Here comes the problem. Doing make maintainer-clean should solve the problem.

I confirm that doing "make maintainer-clean" after hitting this error, then "cabal 
build" again, works. It's not very good that this is required. People might think 
to try try cabal clean, make clean, maybe even make distclean, but not this.

Just deleting src/Context.hs works for me.  This may be something where the best
fix is just documentation.  Anyway, the makefile is going away in July.

This is not using the makefile, but just a cabal build. Documentation  
should not be required for that to work. :)

As I understand it, this is an issue which affects people that have (i) used
make to build darcs followed by (ii) trying the cabal based build

The issue as I understand it is that the context file generated via the make
build method is for one reason or another not compatible with the cabal build. 
So the context file is just make detritus, and the problem of make detritus will
go away by itself once the makefile is gone.

Of course, fixing how Context.hs is generated by the makefile in the meantime
would probably be good since at the time of this writing, we'll still be dealing
with this for another 4 months.  But once we stop building darcs with make, it
becomes moot.

Fixed by dumping autoconf.

Actually, this was resolved even before dropping autoconf by this change to
Setup.lhs:
  -- Autoconf may have generated a context file.  Remove it before
  -- building, as its existence inexplicably breaks Cabal.
  removeFile "src/Context.hs"
    `catch` (\e -> unless (isDoesNotExistError e) (ioError e))

Bah, silly roundup.