darcs has been disabled in the current stackage nightly due to not supporting the newer sandi that is required 
with GHC 8. See https://github.com/fpco/stackage/issues/1399#issuecomment-242946911 .

darcs is still in the current lts snapshot (lts-6.14), but the nightly will become the new lts before too much 
longer (and will then be the one used by default for users doing "stack install darcs").

Moving to or adding support for sandi 0.4 will also make it easier (possible ?) to build darcs with GHC 8.

Couple of notes on the current status of this:

- sandi 0.4 is API incompatible with sandi 0.3, so darcs will either 
need to just support sandi 0.4, or use CPP
- sandi 0.4 doesn't support GHC 7.6, which darcs 2.12.x does support 
(HEAD/darcs 2.13 will probably drop this support, but I'm not sure 
it's appropriate for a release branch)

- sandi 0.3.6 does work ok with GHC 8 if you use --allow-newer, so 
darcs does currently build with GHC 8.0 if you use --allow-newer --
constraint 'sandi<0.4' (I only tested HEAD but I think 2.12 is the 
same)

This has been fixed by Sergei's patch in HEAD and bracn 2.12.