Possible explanation: you have upgraded GHC to 7.10, which does things a bit differently I gather, but not upgraded cabal-install. If this is the problem, running
cabal install cabal-install -w ghc-7.8.4 # or whatever other version of GHC you have
may fix the problem.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…