I wanted to define a simple Monad instance for (Bool,) as instance Monad ((,) Bool) where return x = (False, x) (True, x) >>= k = (True, snd (k x)) (False, x) >>= k = k x -- or: (b, x) >>= k = let (c, y) = k x in (b||c, y) The idea was to keep track of whether functions change their input value or not. To my astonishment I found that this definition overlaps with an existing one. GHC tells me x.hs:2:10: error: • No instance for (Monoid Bool) arising from the superclasses of an instance declaration • In the instance declaration for ‘Monad ((,) Bool)’ | 2 | instance Monad ((,) Bool) where | ^^^^^^^^^^^^^^^^ x.hs:2:10: error: • Overlapping instances for Monad ((,) Bool) arising from a use of ‘GHC.Base.$dm>>’ Matching instances: instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’ instance Monad ((,) Bool) -- Defined at x.hs:2:10 • In the expression: GHC.Base.$dm>> @((,) Bool) In an equation for ‘>>’: (>>) = GHC.Base.$dm>> @((,) Bool) In the instance declaration for ‘Monad ((,) Bool)’ | 2 | instance Monad ((,) Bool) where | ^^^^^^^^^^^^^^^^ [one more similar overlap error elided] But I could not find the instance Monoid a => Monad ((,) a) documented anywhere in the base package. Should I look harder? Or is this instance accidentally leaking from GHC.Base? Eventually, through some experimenting I found that if I replace Bool with Any, then the existing instance seems to do what I want. Cheers Ben