Resolving dependencies... Configuring splay-0.0.3... Building splay-0.0.3... Preprocessing library splay-0.0.3... [1 of 2] Compiling Data.Splay.Internal ( src/Data/Splay/Internal.hs, dist/build/Data/Splay/Internal.o ) [2 of 2] Compiling Data.Splay ( src/Data/Splay.hs, dist/build/Data/Splay.o ) src/Data/Splay.hs:46:14: Illegal type signature in instance declaration: mappend :: Measured s a => Splay s a -> Splay s a -> Splay s a (Use InstanceSigs to allow this) In the instance declaration for ‘Monoid (Splay s a)’ Failed to install splay-0.0.3 cabal: Error: some packages failed to install: splay-0.0.3 failed during the building phase. The exception was: ExitFailure 1