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T15290b.hs:28:49: error: [GHC-18872]
• Couldn't match representation of type: f (m b)
with that of: f (T1 m b)
arising from the coercion of the method ‘traverse'’
from type ‘forall (f :: * -> *) a b.
Applicative' f =>
(a -> f b) -> m a -> f (m b)’
to type ‘forall (f :: * -> *) a b.
Applicative' f =>
(a -> f b) -> T1 m a -> f (T1 m b)’
NB: We cannot know what roles the parameters to ‘f’ have;
we must assume that the role is nominal
• When deriving the instance for (Traversable' (T1 m))
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