Constrained Type Families?

[Ryan Ingram]

I think it's more like the non-keyworded default definitions of class
methods, for the same reasons; the default definition has to potentially be
valid for all instances of the class​.

It's the difference between

class Applicative m => Monad m where
    return :: a -> m a
    return = pure  -- always valid, but can be overridden in instance
declarations

and

class Fuctor f => Applicative f where
    (<*>) :: f (a -> b) -> f a -> f b
    default (<*>) :: Monad f => f (a -> b) -> f a -> f b
    (<*>) = ap -- only valid if matches the type signature above
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