I'm using GHC 7.2.1 and cabal-install 0.8 (Cabal 1.8.0.2), and when "cabal install rsa" I got the error $ cabal install rsa Resolving dependencies... Configuring RSA-1.0.6.2... Preprocessing library RSA-1.0.6.2... Preprocessing executables for RSA-1.0.6.2... Building RSA-1.0.6.2... [1 of 1] Compiling Codec.Crypto.RSA ( Codec/Crypto/RSA.hs, dist/build/Codec/Crypto/RSA.o ) Codec/Crypto/RSA.hs:580:10: Duplicate instance declarations: instance Random Word8 -- Defined at Codec/Crypto/RSA.hs:580:10-21 instance Random Word8 -- Defined in System.Random cabal: Error: some packages failed to install: RSA-1.0.6.2 failed during the building phase. The exception was: ExitFailure 1 Apparently it's an instance being declared twice. However RSA hackage page states that it compiles under GHC 7.2: http://hackage.haskell.org/package/RSA