Darcs 1.0.0-rc1's upstream distribution erroneously distributes .hi files. This causes compilation errors if building darcs with a different version of ghc than the author used. I am able to build successfully if I 'make clean' between the 'unpack' and 'compile' stages. I will also report this to the author.
Thanks for spotting this. I have added "make clean" to the ebuild. ks