I tried to follow the instructions at https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid and I got a <<loop>> when my code runs with ghc 8.2.2. My code looked like this: import Data.Monoid import Prelude data InfoVal v = NoInfoVal | InfoVal v deriving (Show) instance Monoid (InfoVal v) where mempty = NoInfoVal mappend _ v@(InfoVal _) = v mappend v NoInfoVal = v Following the "recommended variant" for compatible code, I changed that too: {-# LANGUAGE CPP #-} import Data.Monoid import qualified Data.Semigroup as Sem import Prelude data InfoVal v = NoInfoVal | InfoVal v deriving (Show) instance Monoid (InfoVal v) where mempty = NoInfoVal #if !(MIN_VERSION_base(4,11,0)) mappend = (<>) #endif instance Sem.Semigroup (InfoVal v) where _ <> v@(InfoVal _) = v v <> NoInfoVal = v This loops because <> comes from Data.Monoid not from Data.Semigroup, so mappend = mappend. Note that I diverged slightly from the instructions to get to this wrong code; I imported Data.Semigroup qualified. Without the qualification, the above code fails to compile, with Ambiguous occurrence ‘<>’ So, the transition instructions don't produce code that ghc 8.2.2 can build, and when the obvious fix is made to get it to compile, it compiles into a loop, that is *not* a loop when compiled with newer versions of ghc. -- see shy jo