Control/IMonad/Trans.hs:50:9:
Could not deduce (IMonad k (U k * m)) arising from a use of `liftI'
from the context (Monad m, IMonadTrans k t)
bound by the type signature for
liftU :: (Monad m, IMonadTrans k t) =>
m a -> t (U k * m) ((:=) k a i) i
I fixed index-core to build on GHC 7.2 and 7.4 by disabling the Polykinds extension for base-4.5.* and older. The fix is up on Hackage as index-core-1.0.4.
GHC 7.4
GHC 7.2
Made a revision on Hackage with
base >=4.6