Resolving dependencies...
Configuring stm-2.1.2.1...
Preprocessing library stm-2.1.2.1...
Building stm-2.1.2.1...
[1 of 7] Compiling Control.Sequential.STM ( Control/Sequential/STM.hs, dist/build/Control/Sequential/STM.o )
[2 of 7] Compiling Control.Concurrent.STM.TChan ( Control/Concurrent/STM/TChan.hs, dist/build/Control/Concurrent/STM/TChan.o )
[3 of 7] Compiling Control.Concurrent.STM.TMVar ( Control/Concurrent/STM/TMVar.hs, dist/build/Control/Concurrent/STM/TMVar.o )
[4 of 7] Compiling Control.Concurrent.STM.TVar ( Control/Concurrent/STM/TVar.hs, dist/build/Control/Concurrent/STM/TVar.o )

Control/Concurrent/STM/TVar.hs:22:8:
    Ambiguous occurrence `readTVarIO'
    It could refer to either `Control.Concurrent.STM.TVar.readTVarIO', defined at Control/Concurrent/STM/TVar.hs:35:0
                          or `GHC.Conc.readTVarIO', imported from GHC.Conc at Control/Concurrent/STM/TVar.hs:29:0-14
cabal: Error: some packages failed to install:
haskell-platform-2010.2.0.0 depends on stm-2.1.2.1 which failed to install.
stm-2.1.2.1 failed during the building phase. The exception was:
ExitFailure 1