Resolving dependencies... Configuring split-0.2.3.1... Building split-0.2.3.1... Installed split-0.2.3.1 Configuring libroman-3.0.0... Building libroman-3.0.0... Failed to install libroman-3.0.0 Build log ( /home/builder/.cabal/logs/libroman-3.0.0.log ): cabal: Entering directory '/tmp/cabal-tmp-12915/libroman-3.0.0' Configuring libroman-3.0.0... Building libroman-3.0.0... Preprocessing library libroman-3.0.0... [1 of 2] Compiling Data.Roman ( src/Data/Roman.hs, dist/build/Data/Roman.o ) src/Data/Roman.hs:35:5: error: • Constraint ‘Roman r’ in the type of ‘fromRoman’ constrains only the class type variables Use ConstrainedClassMethods to allow it • When checking the class method: fromRoman :: forall r. Roman r => forall b. (Roman r, Integral b) => r -> b In the class declaration for ‘Roman’ cabal: Leaving directory '/tmp/cabal-tmp-12915/libroman-3.0.0' cabal: Error: some packages failed to install: libroman-3.0.0 failed during the building phase. The exception was: ExitFailure 1