InvalidNewtypeInstance Error

Example

newtype ManyErrors e a = ManyErrors (Either e (Maybe a))

derive newtype instance manyErrorsFunctor :: Functor (ManyErrors e)

Cause

While ManyErrors is a valid newtype, its underlying representation is not a type for which a Functor instance already exists. Most causes of this error are due to the newtype being some composition of other types (in our case, Either and Maybe) instead of just one type.

Fix

• In many cases you can use existing higher-order newtypes that already have the relevant instances defined. For example, we could have done the following without getting an error:

import Data.Functor.Compose (Compose)

newtype ManyErrors e a = ManyErrors (Compose (Either e) Maybe a)

derive newtype instance manyErrorsFunctor :: Functor (ManyErrors e)

This works because Compose f g is a functor whenever f and g are functors.

Otherwise, we would need to explicitly write the instance ourselves.

Notes

• If the underlying type does not have an instance to derive, you will instead get a NoInstanceFound error.