> To fix that, you need to start supporting error polymorphism. > map :: List a -> (a -> b -[err]) -> List b -[err] FWIW, Haskell has this already; it's cal

> To fix that, you need to start supporting error polymorphism. > map :: List a -> (a -> b -[err]) -> List b -[err] FWIW, Haskell has this already; it's called[0] Traversable: ghci> :t mapA mapA :: (Traversable t,Applicative f) => (a -> f b) -> t a -> f (t b) ghci> :t mapA @(Error ||) @[] mapA :: (a -> Error || b) -> [a] -> Error || [b] 0: The default prelude calls it `traverse` instead of `mapA` (which is terrible for reasons that should be obvious[1]) and `Either Error a` instead of `Error || a` (which is a matter of taste). 1: Especially if you rename the method of Functor from `fmap` to: map :: Functor t => (a -> b) -> t a -> t b -- for comparison: mapA :: ... => (a -> f b) -> t a -> f (t b)